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  <title>Reflection for C++26</title>
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<h1 class="title" style="text-align:center">Reflection for C++26</h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2996R1</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2023-12-18</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      EWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Wyatt Childers<br>&lt;<a href="mailto:wcc@edg.com" class="email">wcc@edg.com</a>&gt;<br>
      Peter Dimov<br>&lt;<a href="mailto:pdimov@gmail.com" class="email">pdimov@gmail.com</a>&gt;<br>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>&gt;<br>
      Andrew Sutton<br>&lt;<a href="mailto:andrew.n.sutton@gmail.com" class="email">andrew.n.sutton@gmail.com</a>&gt;<br>
      Faisal Vali<br>&lt;<a href="mailto:faisalv@gmail.com" class="email">faisalv@gmail.com</a>&gt;<br>
      Daveed Vandevoorde<br>&lt;<a href="mailto:daveed@edg.com" class="email">daveed@edg.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a></li>
<li><a href="#introduction"><span class="toc-section-number">2</span> Introduction<span></span></a>
<ul>
<li><a href="#notable-additions-to-p1240"><span class="toc-section-number">2.1</span> Notable Additions to P1240<span></span></a></li>
<li><a href="#why-a-single-opaque-reflection-type"><span class="toc-section-number">2.2</span> Why a single opaque reflection type?<span></span></a></li>
<li><a href="#implementation-status"><span class="toc-section-number">2.3</span> Implementation Status<span></span></a></li>
</ul></li>
<li><a href="#examples"><span class="toc-section-number">3</span> Examples<span></span></a>
<ul>
<li><a href="#back-and-forth"><span class="toc-section-number">3.1</span> Back-And-Forth<span></span></a></li>
<li><a href="#selecting-members"><span class="toc-section-number">3.2</span> Selecting Members<span></span></a></li>
<li><a href="#list-of-types-to-list-of-sizes"><span class="toc-section-number">3.3</span> List of Types to List of Sizes<span></span></a></li>
<li><a href="#implementing-make_integer_sequence"><span class="toc-section-number">3.4</span> Implementing <code class="sourceCode cpp">make_integer_sequence</code><span></span></a></li>
<li><a href="#getting-class-layout"><span class="toc-section-number">3.5</span> Getting Class Layout<span></span></a></li>
<li><a href="#enum-to-string"><span class="toc-section-number">3.6</span> Enum to String<span></span></a></li>
<li><a href="#parsing-command-line-options"><span class="toc-section-number">3.7</span> Parsing Command-Line Options<span></span></a></li>
<li><a href="#a-simple-tuple-type"><span class="toc-section-number">3.8</span> A Simple Tuple Type<span></span></a></li>
<li><a href="#a-simple-variant-type"><span class="toc-section-number">3.9</span> A Simple Variant Type<span></span></a></li>
<li><a href="#struct-to-struct-of-arrays"><span class="toc-section-number">3.10</span> Struct to Struct of Arrays<span></span></a></li>
<li><a href="#parsing-command-line-options-ii"><span class="toc-section-number">3.11</span> Parsing Command-Line Options II<span></span></a></li>
<li><a href="#a-universal-formatter"><span class="toc-section-number">3.12</span> A Universal Formatter<span></span></a></li>
<li><a href="#implementing-member-wise-hash_append"><span class="toc-section-number">3.13</span> Implementing member-wise <code class="sourceCode cpp">hash_append</code><span></span></a></li>
<li><a href="#converting-a-struct-to-a-tuple"><span class="toc-section-number">3.14</span> Converting a Struct to a Tuple<span></span></a></li>
<li><a href="#compile-time-ticket-counter"><span class="toc-section-number">3.15</span> Compile-Time Ticket Counter<span></span></a></li>
</ul></li>
<li><a href="#proposed-features"><span class="toc-section-number">4</span> Proposed Features<span></span></a>
<ul>
<li><a href="#the-reflection-operator"><span class="toc-section-number">4.1</span> The Reflection Operator (<code class="sourceCode cpp"><span class="op">^</span></code>)<span></span></a></li>
<li><a href="#splicers"><span class="toc-section-number">4.2</span> Splicers (<code class="sourceCode cpp"><span class="op">[:</span></code>…<code class="sourceCode cpp"><span class="op">:]</span></code>)<span></span></a>
<ul>
<li><a href="#range-splicers"><span class="toc-section-number">4.2.1</span> Range Splicers<span></span></a></li>
</ul></li>
<li><a href="#stdmetainfo"><span class="toc-section-number">4.3</span> <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code><span></span></a></li>
<li><a href="#metafunctions"><span class="toc-section-number">4.4</span> Metafunctions<span></span></a>
<ul>
<li><a href="#error-handling-in-reflection"><span class="toc-section-number">4.4.1</span> Error-Handling in Reflection<span></span></a></li>
<li><a href="#synopsis"><span class="toc-section-number">4.4.2</span> Synopsis<span></span></a></li>
<li><a href="#name_of-display_name_of-source_location_of"><span class="toc-section-number">4.4.3</span> <code class="sourceCode cpp">name_of</code>, <code class="sourceCode cpp">display_name_of</code>, <code class="sourceCode cpp">source_location_of</code><span></span></a></li>
<li><a href="#type_of-parent_of-dealias"><span class="toc-section-number">4.4.4</span> <code class="sourceCode cpp">type_of</code>, <code class="sourceCode cpp">parent_of</code>, <code class="sourceCode cpp">dealias</code><span></span></a></li>
<li><a href="#template_of-template_arguments_of"><span class="toc-section-number">4.4.5</span> <code class="sourceCode cpp">template_of</code>, <code class="sourceCode cpp">template_arguments_of</code><span></span></a></li>
<li><a href="#members_of-static_data_members_of-nonstatic_data_members_of-bases_of-enumerators_of-subobjects_of"><span class="toc-section-number">4.4.6</span> <code class="sourceCode cpp">members_of</code>, <code class="sourceCode cpp">static_data_members_of</code>, <code class="sourceCode cpp">nonstatic_data_members_of</code>, <code class="sourceCode cpp">bases_of</code>, <code class="sourceCode cpp">enumerators_of</code>, <code class="sourceCode cpp">subobjects_of</code><span></span></a></li>
<li><a href="#substitute"><span class="toc-section-number">4.4.7</span> <code class="sourceCode cpp">substitute</code><span></span></a></li>
<li><a href="#value_oft"><span class="toc-section-number">4.4.8</span> <code class="sourceCode cpp">value_of<span class="op">&lt;</span>T<span class="op">&gt;</span></code><span></span></a></li>
<li><a href="#test_type-test_types"><span class="toc-section-number">4.4.9</span> <code class="sourceCode cpp">test_type</code>, <code class="sourceCode cpp">test_types</code><span></span></a></li>
<li><a href="#other-singular-reflection-predicates"><span class="toc-section-number">4.4.10</span> Other Singular Reflection Predicates<span></span></a></li>
<li><a href="#reflect_value"><span class="toc-section-number">4.4.11</span> <code class="sourceCode cpp">reflect_value</code><span></span></a></li>
<li><a href="#nsdm_description-define_class"><span class="toc-section-number">4.4.12</span> <code class="sourceCode cpp">nsdm_description</code>, <code class="sourceCode cpp">define_class</code><span></span></a></li>
<li><a href="#data-layout-reflection"><span class="toc-section-number">4.4.13</span> Data Layout Reflection<span></span></a></li>
<li><a href="#other-type-traits"><span class="toc-section-number">4.4.14</span> Other Type Traits<span></span></a></li>
</ul></li>
</ul></li>
<li><a href="#bibliography"><span class="toc-section-number">5</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>Since <span class="citation" data-cites="P2996R0">[<a href="#ref-P2996R0" role="doc-biblioref">P2996R0</a>]</span>:</p>
<ul>
<li>added links to Compiler Explorer demonstrating just about all of the examples</li>
<li>respecified <code class="sourceCode cpp">synth_struct</code> to <code class="sourceCode cpp">define_class</code></li>
<li>respecified a few metafunctions to be functions instead of function templates</li>
<li>introduced section on error handling mechanism and our preference for exceptions (removing invalid reflections)</li>
<li>added ticket counter and variant examples</li>
<li>collapsed <code class="sourceCode cpp">entity_ref</code> and <code class="sourceCode cpp">pointer_to_member</code> into <code class="sourceCode cpp">value_of</code></li>
</ul>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>This is a proposal for a reduced initial set of features to support static reflection in C++. Specifically, we are mostly proposing a subset of features suggested in <span class="citation" data-cites="P1240R2">[<a href="#ref-P1240R2" role="doc-biblioref">P1240R2</a>]</span>:</p>
<ul>
<li>the representation of program elements via constant-expressions producing <em>reflection values</em> — <em>reflections</em> for short — of an opaque type <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code>,</li>
<li>a <em>reflection operator</em> (prefix <code class="sourceCode cpp"><span class="op">^</span></code>) that produces a reflection value for its operand construct,</li>
<li>a number of <code class="sourceCode cpp"><span class="kw">consteval</span></code> <em>metafunctions</em> to work with reflections (including deriving other reflections), and</li>
<li>constructs called <em>splicers</em> to produce grammatical elements from reflections (e.g., <code class="sourceCode cpp"><span class="op">[:</span> <em>refl</em> <span class="op">:]</span></code>).</li>
</ul>
<p>This proposal is not intended to be the end-game as far as reflection and compile-time metaprogramming are concerned. Instead, we expect it will be a useful core around which more powerful features will be added incrementally over time. In particular, we believe that most or all the remaining features explored in P1240R2 and that code injection (along the lines described in <span class="citation" data-cites="P2237R0">[<a href="#ref-P2237R0" role="doc-biblioref">P2237R0</a>]</span>) are desirable directions to pursue.</p>
<p>Our choice to start with something smaller is primarily motivated by the belief that that improves the chances of these facilities making it into the language sooner rather than later.</p>
<h2 data-number="2.1" id="notable-additions-to-p1240"><span class="header-section-number">2.1</span> Notable Additions to P1240<a href="#notable-additions-to-p1240" class="self-link"></a></h2>
<p>While we tried to select a useful subset of the P1240 features, we also made a few additions and changes. Most of those changes are minor. For example, we added a <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>test_type</code> interface that makes it convenient to use existing standard type predicates (such as <code class="sourceCode cpp">is_class_v</code>) in reflection computations.</p>
<p>One addition does stand out, however: We have added metafunctions that permit the synthesis of simple struct and union types. While it is not nearly as powerful as generalized code injection (see <span class="citation" data-cites="P2237R0">[<a href="#ref-P2237R0" role="doc-biblioref">P2237R0</a>]</span>), it can be remarkably effective in practice.</p>
<h2 data-number="2.2" id="why-a-single-opaque-reflection-type"><span class="header-section-number">2.2</span> Why a single opaque reflection type?<a href="#why-a-single-opaque-reflection-type" class="self-link"></a></h2>
<p>Perhaps the most common suggestion made regarding the framework outlined in P1240 is to switch from the single <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code> type to a family of types covering various language elements (e.g., <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>variable</code>, <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>type</code>, etc.).</p>
<p>We believe that doing so would be a mistake with very serious consequences for the future of C++.</p>
<p>Specifically, it would codify the language design into the type system. We know from experience that it has been quasi-impossible to change the semantics of standard types once they were standardized, and there is no reason to think that such evolution would become easier in the future. Suppose for example that we had standardized a reflection type <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>variable</code> in C++03 to represent what the standard called “variables” at the time. In C++11, the term “variable” was extended to include “references”. Such an change would have been difficult to do given that C++ by then likely would have had plenty of code that depended on a type arrangement around the more restricted definition of “variable”. That scenario is clearly backward-looking, but there is no reason to believe that similar changes might not be wanted in the future and we strongly believe that it behooves us to avoid adding undue constraints on the evolution of the language.</p>
<p>Other advantages of a single opaque type include:</p>
<ul>
<li>it makes no assumptions about the representation used within the implementation (e.g., it doesn’t advantage one compiler over another),</li>
<li>it is trivially extensible (no types need to be added to represent additional language elements and meta-elements as the language evolves), and</li>
<li>it allows convenient collections of heterogeneous constructs without having to surface reference semantics (e.g., a <code class="sourceCode cpp">std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>meta<span class="op">::</span>info<span class="op">&gt;</span></code> can easily represent a mixed template argument list — containing types and nontypes — without fear of slicing values).</li>
</ul>
<h2 data-number="2.3" id="implementation-status"><span class="header-section-number">2.3</span> Implementation Status<a href="#implementation-status" class="self-link"></a></h2>
<p>Lock3 implemented the equivalent of much that is proposed here in a fork of Clang (specifically, it worked with the P1240 proposal, but also included several other capabilities including a first-class injection mechanism).</p>
<p>EDG has an ongoing implementation of this proposal that is currently available on Compiler Explorer (thank you, Matt Godbolt). Nearly all of the examples below have links to compiler explorer demonstrating them.</p>
<p>Note that the implementation is not complete (notably, for debugging purposes, <code class="sourceCode cpp">name_of<span class="op">(^</span><span class="dt">int</span><span class="op">)</span></code> yields an empty string and <code class="sourceCode cpp">name_of<span class="op">(^</span>std<span class="op">::</span>optional<span class="op">&lt;</span>std<span class="op">::</span>string<span class="op">&gt;)</span></code> yields <code class="sourceCode cpp"><span class="st">&quot;optional&quot;</span></code>, neither of which are what we want), and does not include some of the other language features we would like to be able to take advantage of (like expansion statements) but will be regularly be updated as this paper progresses. Notably, it does not currently support expansion statements - and so a workaround that will be used in the linked implementations of examples is the following facility:</p>
<blockquote>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">namespace</span> __impl <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">auto</span><span class="op">...</span> vals<span class="op">&gt;</span></span>
<span id="cb1-3"><a href="#cb1-3"></a>  <span class="kw">struct</span> replicator_type <span class="op">{</span></span>
<span id="cb1-4"><a href="#cb1-4"></a>    <span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> F<span class="op">&gt;</span></span>
<span id="cb1-5"><a href="#cb1-5"></a>      <span class="kw">constexpr</span> <span class="dt">void</span> <span class="kw">operator</span><span class="op">&gt;&gt;(</span>F body<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb1-6"><a href="#cb1-6"></a>        <span class="op">(</span>body<span class="op">.</span><span class="kw">template</span> <span class="kw">operator</span><span class="op">()&lt;</span>vals<span class="op">&gt;()</span>, <span class="op">...)</span>;</span>
<span id="cb1-7"><a href="#cb1-7"></a>      <span class="op">}</span></span>
<span id="cb1-8"><a href="#cb1-8"></a>  <span class="op">}</span>;</span>
<span id="cb1-9"><a href="#cb1-9"></a></span>
<span id="cb1-10"><a href="#cb1-10"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">auto</span><span class="op">...</span> vals<span class="op">&gt;</span></span>
<span id="cb1-11"><a href="#cb1-11"></a>  replicator_type<span class="op">&lt;</span>vals<span class="op">...&gt;</span> replicator <span class="op">=</span> <span class="op">{}</span>;</span>
<span id="cb1-12"><a href="#cb1-12"></a><span class="op">}</span></span>
<span id="cb1-13"><a href="#cb1-13"></a></span>
<span id="cb1-14"><a href="#cb1-14"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> R<span class="op">&gt;</span></span>
<span id="cb1-15"><a href="#cb1-15"></a><span class="kw">consteval</span> <span class="kw">auto</span> expand<span class="op">(</span>R range<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-16"><a href="#cb1-16"></a>  std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>meta<span class="op">::</span>info<span class="op">&gt;</span> args;</span>
<span id="cb1-17"><a href="#cb1-17"></a>  <span class="cf">for</span> <span class="op">(</span><span class="kw">auto</span> r <span class="op">:</span> range<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-18"><a href="#cb1-18"></a>    args<span class="op">.</span>push_back<span class="op">(</span>reflect_value<span class="op">(</span>r<span class="op">))</span>;</span>
<span id="cb1-19"><a href="#cb1-19"></a>  <span class="op">}</span></span>
<span id="cb1-20"><a href="#cb1-20"></a>  <span class="cf">return</span> substitute<span class="op">(^</span>__impl<span class="op">::</span>replicator, args<span class="op">)</span>;</span>
<span id="cb1-21"><a href="#cb1-21"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Used like:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>With expansion statements</strong>
</div></th>
<th><div style="text-align:center">
<strong>With <code class="sourceCode cpp">expand</code> workaround</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> E<span class="op">&gt;</span></span>
<span id="cb2-2"><a href="#cb2-2"></a>  <span class="kw">requires</span> std<span class="op">::</span>is_enum_v<span class="op">&lt;</span>E<span class="op">&gt;</span></span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="kw">constexpr</span> std<span class="op">::</span>string enum_to_string<span class="op">(</span>E value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-4"><a href="#cb2-4"></a>  <span class="kw">template</span> <span class="cf">for</span> <span class="op">(</span><span class="kw">constexpr</span> <span class="kw">auto</span> e <span class="op">:</span> std<span class="op">::</span>meta<span class="op">::</span>members_of<span class="op">(^</span>E<span class="op">))</span> <span class="op">{</span></span>
<span id="cb2-5"><a href="#cb2-5"></a>    <span class="cf">if</span> <span class="op">(</span>value <span class="op">==</span> <span class="op">[:</span>e<span class="op">:])</span> <span class="op">{</span></span>
<span id="cb2-6"><a href="#cb2-6"></a>      <span class="cf">return</span> std<span class="op">::</span>string<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>name_of<span class="op">(</span>e<span class="op">))</span>;</span>
<span id="cb2-7"><a href="#cb2-7"></a>    <span class="op">}</span></span>
<span id="cb2-8"><a href="#cb2-8"></a>  <span class="op">}</span></span>
<span id="cb2-9"><a href="#cb2-9"></a></span>
<span id="cb2-10"><a href="#cb2-10"></a>  <span class="cf">return</span> <span class="st">&quot;&lt;unnamed&gt;&quot;</span>;</span>
<span id="cb2-11"><a href="#cb2-11"></a><span class="op">}</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> E<span class="op">&gt;</span></span>
<span id="cb3-2"><a href="#cb3-2"></a>  <span class="kw">requires</span> std<span class="op">::</span>is_enum_v<span class="op">&lt;</span>E<span class="op">&gt;</span></span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="kw">constexpr</span> std<span class="op">::</span>string enum_to_string<span class="op">(</span>E value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb3-4"><a href="#cb3-4"></a>  std<span class="op">::</span>string result <span class="op">=</span> <span class="st">&quot;&lt;unnamed&gt;&quot;</span>;</span>
<span id="cb3-5"><a href="#cb3-5"></a>  <span class="op">[:</span>expand<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>enumerators_of<span class="op">(^</span>E<span class="op">)):]</span> <span class="op">&gt;&gt;</span> <span class="op">[&amp;]&lt;</span><span class="kw">auto</span> e<span class="op">&gt;{</span></span>
<span id="cb3-6"><a href="#cb3-6"></a>    <span class="cf">if</span> <span class="op">(</span>value <span class="op">==</span> <span class="op">[:</span>e<span class="op">:])</span> <span class="op">{</span></span>
<span id="cb3-7"><a href="#cb3-7"></a>      result <span class="op">=</span> std<span class="op">::</span>meta<span class="op">::</span>name_of<span class="op">(</span>e<span class="op">)</span>;</span>
<span id="cb3-8"><a href="#cb3-8"></a>    <span class="op">}</span></span>
<span id="cb3-9"><a href="#cb3-9"></a>  <span class="op">}</span>;</span>
<span id="cb3-10"><a href="#cb3-10"></a>  <span class="cf">return</span> result;</span>
<span id="cb3-11"><a href="#cb3-11"></a><span class="op">}</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="examples"><span class="header-section-number">3</span> Examples<a href="#examples" class="self-link"></a></h1>
<p>We start with a number of examples that show off what is possible with the proposed set of features. It is expected that these are mostly self-explanatory. Read ahead to the next sections for a more systematic description of each element of this proposal.</p>
<p>A number of our examples here show a few other language features that we hope to progress at the same time. This facility does not strictly rely on these features, and it is possible to do without them - but it would greatly help the usability experience if those could be adopted as well:</p>
<ul>
<li>expansion statements <span class="citation" data-cites="P1306R1">[<a href="#ref-P1306R1" role="doc-biblioref">P1306R1</a>]</span></li>
<li>non-transient constexpr allocation <span class="citation" data-cites="P0784R7">[<a href="#ref-P0784R7" role="doc-biblioref">P0784R7</a>]</span> <span class="citation" data-cites="P1974R0">[<a href="#ref-P1974R0" role="doc-biblioref">P1974R0</a>]</span> <span class="citation" data-cites="P2670R1">[<a href="#ref-P2670R1" role="doc-biblioref">P2670R1</a>]</span></li>
</ul>
<h2 data-number="3.1" id="back-and-forth"><span class="header-section-number">3.1</span> Back-And-Forth<a href="#back-and-forth" class="self-link"></a></h2>
<p>Our first example is not meant to be compelling but to show how to go back and forth between the reflection domain and the grammatical domain:</p>
<blockquote>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> r <span class="op">=</span> <span class="op">^</span><span class="dt">int</span>;</span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="kw">typename</span><span class="op">[:</span>r<span class="op">:]</span> x <span class="op">=</span> <span class="dv">42</span>;       <span class="co">// Same as: int x = 42;</span></span>
<span id="cb4-3"><a href="#cb4-3"></a><span class="kw">typename</span><span class="op">[:^</span><span class="dt">char</span><span class="op">:]</span> c <span class="op">=</span> <span class="ch">&#39;*&#39;</span>;  <span class="co">// Same as: char c = &#39;*&#39;;</span></span></code></pre></div>
</blockquote>
<p>The <code class="sourceCode cpp"><span class="kw">typename</span></code> prefix can be omitted in the same contexts as with dependent qualified names. For example:</p>
<blockquote>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">using</span> MyType <span class="op">=</span> <span class="op">[:</span><span class="kw">sizeof</span><span class="op">(</span><span class="dt">int</span><span class="op">)&lt;</span><span class="kw">sizeof</span><span class="op">(</span><span class="dt">long</span><span class="op">)?</span> <span class="op">^</span><span class="dt">long</span> <span class="op">:</span> <span class="op">^</span><span class="dt">int</span><span class="op">:]</span>;  <span class="co">// Implicit &quot;typename&quot; prefix.</span></span></code></pre></div>
</blockquote>
<p><a href="https://godbolt.org/z/13anqE1Pa">On Compiler Explorer</a>.</p>
<h2 data-number="3.2" id="selecting-members"><span class="header-section-number">3.2</span> Selecting Members<a href="#selecting-members" class="self-link"></a></h2>
<p>Our second example enables selecting a member “by number” for a specific type:</p>
<blockquote>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">struct</span> S <span class="op">{</span> <span class="dt">unsigned</span> i<span class="op">:</span><span class="dv">2</span>, j<span class="op">:</span><span class="dv">6</span>; <span class="op">}</span>;</span>
<span id="cb6-2"><a href="#cb6-2"></a></span>
<span id="cb6-3"><a href="#cb6-3"></a><span class="kw">consteval</span> <span class="kw">auto</span> member_number<span class="op">(</span><span class="dt">int</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-4"><a href="#cb6-4"></a>  <span class="cf">if</span> <span class="op">(</span>n <span class="op">==</span> <span class="dv">0</span><span class="op">)</span> <span class="cf">return</span> <span class="op">^</span>S<span class="op">::</span>i;</span>
<span id="cb6-5"><a href="#cb6-5"></a>  <span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>n <span class="op">==</span> <span class="dv">1</span><span class="op">)</span> <span class="cf">return</span> <span class="op">^</span>S<span class="op">::</span>j;</span>
<span id="cb6-6"><a href="#cb6-6"></a><span class="op">}</span></span>
<span id="cb6-7"><a href="#cb6-7"></a></span>
<span id="cb6-8"><a href="#cb6-8"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb6-9"><a href="#cb6-9"></a>  S s<span class="op">{</span><span class="dv">0</span>, <span class="dv">0</span><span class="op">}</span>;</span>
<span id="cb6-10"><a href="#cb6-10"></a>  s<span class="op">.[:</span>member_number<span class="op">(</span><span class="dv">1</span><span class="op">):]</span> <span class="op">=</span> <span class="dv">42</span>;  <span class="co">// Same as: s.j = 42;</span></span>
<span id="cb6-11"><a href="#cb6-11"></a>  s<span class="op">.[:</span>member_number<span class="op">(</span><span class="dv">5</span><span class="op">):]</span> <span class="op">=</span> <span class="dv">0</span>;   <span class="co">// Error (member_number(5) is not a constant).</span></span>
<span id="cb6-12"><a href="#cb6-12"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This example also illustrates that bit fields are not beyond the reach of this proposal.</p>
<p><a href="https://godbolt.org/z/vT4rbva7M">On Compiler Explorer</a></p>
<h2 data-number="3.3" id="list-of-types-to-list-of-sizes"><span class="header-section-number">3.3</span> List of Types to List of Sizes<a href="#list-of-types-to-list-of-sizes" class="self-link"></a></h2>
<p>Here, <code class="sourceCode cpp">sizes</code> will be a <code class="sourceCode cpp">std<span class="op">::</span>array<span class="op">&lt;</span>std<span class="op">::</span><span class="dt">size_t</span>, <span class="dv">3</span><span class="op">&gt;</span></code> initialized with <code class="sourceCode cpp"><span class="op">{</span><span class="kw">sizeof</span><span class="op">(</span><span class="dt">int</span><span class="op">)</span>, <span class="kw">sizeof</span><span class="op">(</span><span class="dt">float</span><span class="op">)</span>, <span class="kw">sizeof</span><span class="op">(</span><span class="dt">double</span><span class="op">)}</span></code>:</p>
<blockquote>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a><span class="kw">constexpr</span> std<span class="op">::</span>array types <span class="op">=</span> <span class="op">{^</span><span class="dt">int</span>, <span class="op">^</span><span class="dt">float</span>, <span class="op">^</span><span class="dt">double</span><span class="op">}</span>;</span>
<span id="cb7-2"><a href="#cb7-2"></a><span class="kw">constexpr</span> std<span class="op">::</span>array sizes <span class="op">=</span> <span class="op">[]{</span></span>
<span id="cb7-3"><a href="#cb7-3"></a>  std<span class="op">::</span>array<span class="op">&lt;</span>std<span class="op">::</span><span class="dt">size_t</span>, types<span class="op">.</span>size<span class="op">()&gt;</span> r;</span>
<span id="cb7-4"><a href="#cb7-4"></a>  std<span class="op">::</span>ranges<span class="op">::</span>transform<span class="op">(</span>types, r<span class="op">.</span>begin<span class="op">()</span>, std<span class="op">::</span>meta<span class="op">::</span>size_of<span class="op">)</span>;</span>
<span id="cb7-5"><a href="#cb7-5"></a>  <span class="cf">return</span> r;</span>
<span id="cb7-6"><a href="#cb7-6"></a><span class="op">}()</span>;</span></code></pre></div>
</blockquote>
<p>Compare this to the following type-based approach, which produces the same array <code class="sourceCode cpp">sizes</code>:</p>
<blockquote>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span><span class="op">...&gt;</span> <span class="kw">struct</span> list <span class="op">{}</span>;</span>
<span id="cb8-2"><a href="#cb8-2"></a></span>
<span id="cb8-3"><a href="#cb8-3"></a><span class="kw">using</span> types <span class="op">=</span> list<span class="op">&lt;</span><span class="dt">int</span>, <span class="dt">float</span>, <span class="dt">double</span><span class="op">&gt;</span>;</span>
<span id="cb8-4"><a href="#cb8-4"></a></span>
<span id="cb8-5"><a href="#cb8-5"></a><span class="kw">constexpr</span> <span class="kw">auto</span> sizes <span class="op">=</span> <span class="op">[]&lt;</span><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span><span class="op">...&gt;</span> <span class="kw">class</span> L, <span class="kw">class</span><span class="op">...</span> T<span class="op">&gt;(</span>L<span class="op">&lt;</span>T<span class="op">...&gt;)</span> <span class="op">{</span></span>
<span id="cb8-6"><a href="#cb8-6"></a>    <span class="cf">return</span> std<span class="op">::</span>array<span class="op">&lt;</span>std<span class="op">::</span><span class="dt">size_t</span>, <span class="kw">sizeof</span><span class="op">...(</span>T<span class="op">)&gt;{{</span> <span class="kw">sizeof</span><span class="op">(</span>T<span class="op">)...</span> <span class="op">}}</span>;</span>
<span id="cb8-7"><a href="#cb8-7"></a><span class="op">}(</span>types<span class="op">{})</span>;</span></code></pre></div>
</blockquote>
<p><a href="https://godbolt.org/z/4xz9Wsa8f">On Compiler Explorer</a>.</p>
<h2 data-number="3.4" id="implementing-make_integer_sequence"><span class="header-section-number">3.4</span> Implementing <code class="sourceCode cpp">make_integer_sequence</code><a href="#implementing-make_integer_sequence" class="self-link"></a></h2>
<p>We can provide a better implementation of <code class="sourceCode cpp">make_integer_sequence</code> than a hand-rolled approach using regular template metaprogramming (although standard libraries today rely on an intrinsic for this):</p>
<blockquote>
<div class="sourceCode" id="cb9"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb9-1"><a href="#cb9-1"></a><span class="pp">#include </span><span class="im">&lt;utility&gt;</span></span>
<span id="cb9-2"><a href="#cb9-2"></a><span class="pp">#include </span><span class="im">&lt;vector&gt;</span></span>
<span id="cb9-3"><a href="#cb9-3"></a></span>
<span id="cb9-4"><a href="#cb9-4"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb9-5"><a href="#cb9-5"></a><span class="kw">consteval</span> std<span class="op">::</span>meta<span class="op">::</span>info make_integer_seq_refl<span class="op">(</span>T N<span class="op">)</span> <span class="op">{</span></span>
<span id="cb9-6"><a href="#cb9-6"></a>  std<span class="op">::</span>vector args<span class="op">{^</span>T<span class="op">}</span>;</span>
<span id="cb9-7"><a href="#cb9-7"></a>  <span class="cf">for</span> <span class="op">(</span>T k <span class="op">=</span> <span class="dv">0</span>; k <span class="op">&lt;</span> N; <span class="op">++</span>k<span class="op">)</span> <span class="op">{</span></span>
<span id="cb9-8"><a href="#cb9-8"></a>    args<span class="op">.</span>push_back<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>reflect_value<span class="op">(</span>k<span class="op">))</span>;</span>
<span id="cb9-9"><a href="#cb9-9"></a>  <span class="op">}</span></span>
<span id="cb9-10"><a href="#cb9-10"></a>  <span class="cf">return</span> substitute<span class="op">(^</span>std<span class="op">::</span>integer_sequence, args<span class="op">)</span>;</span>
<span id="cb9-11"><a href="#cb9-11"></a><span class="op">}</span></span>
<span id="cb9-12"><a href="#cb9-12"></a></span>
<span id="cb9-13"><a href="#cb9-13"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> T, T N<span class="op">&gt;</span></span>
<span id="cb9-14"><a href="#cb9-14"></a>  <span class="kw">using</span> make_integer_sequence <span class="op">=</span> <span class="op">[:</span>make_integer_seq_refl<span class="op">&lt;</span>T<span class="op">&gt;(</span>N<span class="op">):]</span>;</span></code></pre></div>
</blockquote>
<p><a href="https://godbolt.org/z/bvPeqvaK5">On Compiler Explorer</a>.</p>
<h2 data-number="3.5" id="getting-class-layout"><span class="header-section-number">3.5</span> Getting Class Layout<a href="#getting-class-layout" class="self-link"></a></h2>
<blockquote>
<div class="sourceCode" id="cb10"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1"></a><span class="kw">struct</span> member_descriptor</span>
<span id="cb10-2"><a href="#cb10-2"></a><span class="op">{</span></span>
<span id="cb10-3"><a href="#cb10-3"></a>  std<span class="op">::</span><span class="dt">size_t</span> offset;</span>
<span id="cb10-4"><a href="#cb10-4"></a>  std<span class="op">::</span><span class="dt">size_t</span> size;</span>
<span id="cb10-5"><a href="#cb10-5"></a><span class="op">}</span>;</span>
<span id="cb10-6"><a href="#cb10-6"></a></span>
<span id="cb10-7"><a href="#cb10-7"></a><span class="co">// returns std::array&lt;member_descriptor, N&gt;</span></span>
<span id="cb10-8"><a href="#cb10-8"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> S<span class="op">&gt;</span></span>
<span id="cb10-9"><a href="#cb10-9"></a><span class="kw">consteval</span> <span class="kw">auto</span> get_layout<span class="op">()</span> <span class="op">{</span></span>
<span id="cb10-10"><a href="#cb10-10"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> members <span class="op">=</span> nonstatic_data_members_of<span class="op">(^</span>S<span class="op">)</span>;</span>
<span id="cb10-11"><a href="#cb10-11"></a>  std<span class="op">::</span>array<span class="op">&lt;</span>member_descriptor, members<span class="op">.</span>size<span class="op">()&gt;</span> layout;</span>
<span id="cb10-12"><a href="#cb10-12"></a>  <span class="cf">for</span> <span class="op">(</span><span class="dt">int</span> i <span class="op">=</span> <span class="dv">0</span>; i <span class="op">&lt;</span> members<span class="op">.</span>size<span class="op">()</span>; <span class="op">++</span>i<span class="op">)</span> <span class="op">{</span></span>
<span id="cb10-13"><a href="#cb10-13"></a>      layout<span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> <span class="op">{.</span>offset<span class="op">=</span>offset_of<span class="op">(</span>members<span class="op">[</span>i<span class="op">])</span>, <span class="op">.</span>size<span class="op">=</span>size_of<span class="op">(</span>members<span class="op">[</span>i<span class="op">])}</span>;</span>
<span id="cb10-14"><a href="#cb10-14"></a>  <span class="op">}</span></span>
<span id="cb10-15"><a href="#cb10-15"></a>  <span class="cf">return</span> layout;</span>
<span id="cb10-16"><a href="#cb10-16"></a><span class="op">}</span></span>
<span id="cb10-17"><a href="#cb10-17"></a></span>
<span id="cb10-18"><a href="#cb10-18"></a><span class="kw">struct</span> X</span>
<span id="cb10-19"><a href="#cb10-19"></a><span class="op">{</span></span>
<span id="cb10-20"><a href="#cb10-20"></a>    <span class="dt">char</span> a;</span>
<span id="cb10-21"><a href="#cb10-21"></a>    <span class="dt">int</span> b;</span>
<span id="cb10-22"><a href="#cb10-22"></a>    <span class="dt">double</span> c;</span>
<span id="cb10-23"><a href="#cb10-23"></a><span class="op">}</span>;</span>
<span id="cb10-24"><a href="#cb10-24"></a></span>
<span id="cb10-25"><a href="#cb10-25"></a><span class="co">/*constexpr*/</span> <span class="kw">auto</span> Xd <span class="op">=</span> get_layout<span class="op">&lt;</span>X<span class="op">&gt;()</span>;</span>
<span id="cb10-26"><a href="#cb10-26"></a></span>
<span id="cb10-27"><a href="#cb10-27"></a><span class="co">/*</span></span>
<span id="cb10-28"><a href="#cb10-28"></a><span class="co">where Xd would be std::array&lt;member_descriptor, 3&gt;{{</span></span>
<span id="cb10-29"><a href="#cb10-29"></a><span class="co">  { 0, 1 }, { 4, 4 }, { 8, 8 }</span></span>
<span id="cb10-30"><a href="#cb10-30"></a><span class="co">}}</span></span>
<span id="cb10-31"><a href="#cb10-31"></a><span class="co">*/</span></span></code></pre></div>
</blockquote>
<p><a href="https://godbolt.org/z/rbbWY99TM">On Compiler Explorer</a>.</p>
<h2 data-number="3.6" id="enum-to-string"><span class="header-section-number">3.6</span> Enum to String<a href="#enum-to-string" class="self-link"></a></h2>
<p>One of the most commonly requested facilities is to convert an enum value to a string (this example relies on expansion statements):</p>
<blockquote>
<div class="sourceCode" id="cb11"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb11-1"><a href="#cb11-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> E<span class="op">&gt;</span></span>
<span id="cb11-2"><a href="#cb11-2"></a>  <span class="kw">requires</span> std<span class="op">::</span>is_enum_v<span class="op">&lt;</span>E<span class="op">&gt;</span></span>
<span id="cb11-3"><a href="#cb11-3"></a><span class="kw">constexpr</span> std<span class="op">::</span>string enum_to_string<span class="op">(</span>E value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb11-4"><a href="#cb11-4"></a>  <span class="kw">template</span> <span class="cf">for</span> <span class="op">(</span><span class="kw">constexpr</span> <span class="kw">auto</span> e <span class="op">:</span> std<span class="op">::</span>meta<span class="op">::</span>members_of<span class="op">(^</span>E<span class="op">))</span> <span class="op">{</span></span>
<span id="cb11-5"><a href="#cb11-5"></a>    <span class="cf">if</span> <span class="op">(</span>value <span class="op">==</span> <span class="op">[:</span>e<span class="op">:])</span> <span class="op">{</span></span>
<span id="cb11-6"><a href="#cb11-6"></a>      <span class="cf">return</span> std<span class="op">::</span>string<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>name_of<span class="op">(</span>e<span class="op">))</span>;</span>
<span id="cb11-7"><a href="#cb11-7"></a>    <span class="op">}</span></span>
<span id="cb11-8"><a href="#cb11-8"></a>  <span class="op">}</span></span>
<span id="cb11-9"><a href="#cb11-9"></a></span>
<span id="cb11-10"><a href="#cb11-10"></a>  <span class="cf">return</span> <span class="st">&quot;&lt;unnamed&gt;&quot;</span>;</span>
<span id="cb11-11"><a href="#cb11-11"></a><span class="op">}</span></span>
<span id="cb11-12"><a href="#cb11-12"></a></span>
<span id="cb11-13"><a href="#cb11-13"></a><span class="kw">enum</span> Color <span class="op">{</span> red, green, blue <span class="op">}</span>;</span>
<span id="cb11-14"><a href="#cb11-14"></a><span class="kw">static_assert</span><span class="op">(</span>enum_to_string<span class="op">(</span>Color<span class="op">::</span>red<span class="op">)</span> <span class="op">==</span> <span class="st">&quot;red&quot;</span><span class="op">)</span>;</span>
<span id="cb11-15"><a href="#cb11-15"></a><span class="kw">static_assert</span><span class="op">(</span>enum_to_string<span class="op">(</span>Color<span class="op">(</span><span class="dv">42</span><span class="op">))</span> <span class="op">==</span> <span class="st">&quot;&lt;unnamed&gt;&quot;</span><span class="op">)</span>;</span></code></pre></div>
</blockquote>
<p>We can also do the reverse in pretty much the same way:</p>
<blockquote>
<div class="sourceCode" id="cb12"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb12-1"><a href="#cb12-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> E<span class="op">&gt;</span></span>
<span id="cb12-2"><a href="#cb12-2"></a>  <span class="kw">requires</span> std<span class="op">::</span>is_enum_v<span class="op">&lt;</span>E<span class="op">&gt;</span></span>
<span id="cb12-3"><a href="#cb12-3"></a><span class="kw">constexpr</span> std<span class="op">::</span>optional<span class="op">&lt;</span>E<span class="op">&gt;</span> string_to_enum<span class="op">(</span>std<span class="op">::</span>string_view name<span class="op">)</span> <span class="op">{</span></span>
<span id="cb12-4"><a href="#cb12-4"></a>  <span class="kw">template</span> <span class="cf">for</span> <span class="op">(</span><span class="kw">constexpr</span> <span class="kw">auto</span> e <span class="op">:</span> std<span class="op">::</span>meta<span class="op">::</span>members_of<span class="op">(^</span>E<span class="op">))</span> <span class="op">{</span></span>
<span id="cb12-5"><a href="#cb12-5"></a>    <span class="cf">if</span> <span class="op">(</span>name <span class="op">==</span> std<span class="op">::</span>meta<span class="op">::</span>name_of<span class="op">(</span>e<span class="op">))</span> <span class="op">{</span></span>
<span id="cb12-6"><a href="#cb12-6"></a>      <span class="cf">return</span> <span class="op">[:</span>e<span class="op">:]</span>;</span>
<span id="cb12-7"><a href="#cb12-7"></a>    <span class="op">}</span></span>
<span id="cb12-8"><a href="#cb12-8"></a>  <span class="op">}</span></span>
<span id="cb12-9"><a href="#cb12-9"></a></span>
<span id="cb12-10"><a href="#cb12-10"></a>  <span class="cf">return</span> std<span class="op">::</span>nullopt;</span>
<span id="cb12-11"><a href="#cb12-11"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>But we don’t have to use expansion statements - we can also use algorithms. For instance, <code class="sourceCode cpp">enum_to_string</code> can also be implemented this way (this example relies on non-transient constexpr allocation):</p>
<blockquote>
<div class="sourceCode" id="cb13"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb13-1"><a href="#cb13-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> E<span class="op">&gt;</span></span>
<span id="cb13-2"><a href="#cb13-2"></a>  <span class="kw">requires</span> std<span class="op">::</span>is_enum_v<span class="op">&lt;</span>E<span class="op">&gt;</span></span>
<span id="cb13-3"><a href="#cb13-3"></a><span class="kw">constexpr</span> std<span class="op">::</span>string enum_to_string<span class="op">(</span>E value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb13-4"><a href="#cb13-4"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> enumerators <span class="op">=</span></span>
<span id="cb13-5"><a href="#cb13-5"></a>    std<span class="op">::</span>meta<span class="op">::</span>members_of<span class="op">(^</span>E<span class="op">)</span></span>
<span id="cb13-6"><a href="#cb13-6"></a>    <span class="op">|</span> std<span class="op">::</span>views<span class="op">::</span>transform<span class="op">([](</span>std<span class="op">::</span>meta<span class="op">::</span>info e<span class="op">){</span></span>
<span id="cb13-7"><a href="#cb13-7"></a>        <span class="cf">return</span> std<span class="op">::</span>pair<span class="op">&lt;</span>E, std<span class="op">::</span>string<span class="op">&gt;(</span>std<span class="op">::</span>meta<span class="op">::</span>value_of<span class="op">&lt;</span>E<span class="op">&gt;(</span>e<span class="op">)</span>, std<span class="op">::</span>meta<span class="op">::</span>name_of<span class="op">(</span>e<span class="op">))</span>;</span>
<span id="cb13-8"><a href="#cb13-8"></a>      <span class="op">})</span></span>
<span id="cb13-9"><a href="#cb13-9"></a>    <span class="op">|</span> std<span class="op">::</span>ranges<span class="op">::</span>to<span class="op">&lt;</span>std<span class="op">::</span>map<span class="op">&gt;()</span>;</span>
<span id="cb13-10"><a href="#cb13-10"></a></span>
<span id="cb13-11"><a href="#cb13-11"></a>  <span class="kw">auto</span> it <span class="op">=</span> enumerators<span class="op">.</span>find<span class="op">(</span>value<span class="op">)</span>;</span>
<span id="cb13-12"><a href="#cb13-12"></a>  <span class="cf">if</span> <span class="op">(</span>it <span class="op">!=</span> enumerators<span class="op">.</span>end<span class="op">())</span> <span class="op">{</span></span>
<span id="cb13-13"><a href="#cb13-13"></a>    <span class="cf">return</span> it<span class="op">-&gt;</span>second;</span>
<span id="cb13-14"><a href="#cb13-14"></a>  <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb13-15"><a href="#cb13-15"></a>    <span class="cf">return</span> <span class="st">&quot;&lt;unnamed&gt;&quot;</span>;</span>
<span id="cb13-16"><a href="#cb13-16"></a>  <span class="op">}</span></span>
<span id="cb13-17"><a href="#cb13-17"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Note that this last version has lower complexity: While the versions using an expansion statement use an expected O(N) number of comparisons to find the matching entry, a <code class="sourceCode cpp">std<span class="op">::</span>map</code> achieves the same with O(log(N)) complexity (where N is the number of enumerator constants).</p>
<p><a href="https://godbolt.org/z/Y5va8MqzG">On Compiler Explorer</a>.</p>
<h2 data-number="3.7" id="parsing-command-line-options"><span class="header-section-number">3.7</span> Parsing Command-Line Options<a href="#parsing-command-line-options" class="self-link"></a></h2>
<p>Our next example shows how a command-line option parser could work by automatically inferring flags based on member names. A real command-line parser would of course be more complex, this is just the beginning.</p>
<blockquote>
<div class="sourceCode" id="cb14"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> Opts<span class="op">&gt;</span></span>
<span id="cb14-2"><a href="#cb14-2"></a><span class="kw">auto</span> parse_options<span class="op">(</span>std<span class="op">::</span>span<span class="op">&lt;</span>std<span class="op">::</span>string_view <span class="kw">const</span><span class="op">&gt;</span> args<span class="op">)</span> <span class="op">-&gt;</span> Opts <span class="op">{</span></span>
<span id="cb14-3"><a href="#cb14-3"></a>  Opts opts;</span>
<span id="cb14-4"><a href="#cb14-4"></a>  <span class="kw">template</span> <span class="cf">for</span> <span class="op">(</span><span class="kw">constexpr</span> <span class="kw">auto</span> dm <span class="op">:</span> nonstatic_data_members_of<span class="op">(^</span>Opts<span class="op">))</span> <span class="op">{</span></span>
<span id="cb14-5"><a href="#cb14-5"></a>    <span class="kw">auto</span> it <span class="op">=</span> std<span class="op">::</span>ranges<span class="op">::</span>find_if<span class="op">(</span>args,</span>
<span id="cb14-6"><a href="#cb14-6"></a>      <span class="op">[](</span>std<span class="op">::</span>string_view arg<span class="op">){</span></span>
<span id="cb14-7"><a href="#cb14-7"></a>        <span class="cf">return</span> arg<span class="op">.</span>starts_with<span class="op">(</span><span class="st">&quot;--&quot;</span><span class="op">)</span> <span class="op">&amp;&amp;</span> arg<span class="op">.</span>substr<span class="op">(</span><span class="dv">2</span><span class="op">)</span> <span class="op">==</span> name_of<span class="op">(</span>dm<span class="op">)</span>;</span>
<span id="cb14-8"><a href="#cb14-8"></a>      <span class="op">})</span>;</span>
<span id="cb14-9"><a href="#cb14-9"></a></span>
<span id="cb14-10"><a href="#cb14-10"></a>    <span class="cf">if</span> <span class="op">(</span>it <span class="op">==</span> args<span class="op">.</span>end<span class="op">())</span> <span class="op">{</span></span>
<span id="cb14-11"><a href="#cb14-11"></a>      <span class="co">// no option provided, use default</span></span>
<span id="cb14-12"><a href="#cb14-12"></a>      <span class="cf">continue</span>;</span>
<span id="cb14-13"><a href="#cb14-13"></a>    <span class="op">}</span> <span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>it <span class="op">+</span> <span class="dv">1</span> <span class="op">==</span> args<span class="op">.</span>end<span class="op">())</span> <span class="op">{</span></span>
<span id="cb14-14"><a href="#cb14-14"></a>      std<span class="op">::</span>print<span class="op">(</span>stderr, <span class="st">&quot;Option {} is missing a value</span><span class="sc">\n</span><span class="st">&quot;</span>, <span class="op">*</span>it<span class="op">)</span>;</span>
<span id="cb14-15"><a href="#cb14-15"></a>      std<span class="op">::</span>exit<span class="op">(</span>EXIT_FAILURE<span class="op">)</span>;</span>
<span id="cb14-16"><a href="#cb14-16"></a>    <span class="op">}</span></span>
<span id="cb14-17"><a href="#cb14-17"></a></span>
<span id="cb14-18"><a href="#cb14-18"></a>    <span class="kw">using</span> T <span class="op">=</span> <span class="kw">typename</span><span class="op">[:</span>type_of<span class="op">(</span>dm<span class="op">):]</span>;</span>
<span id="cb14-19"><a href="#cb14-19"></a>    <span class="kw">auto</span> iss <span class="op">=</span> std<span class="op">::</span>ispanstream<span class="op">(</span>it<span class="op">[</span><span class="dv">1</span><span class="op">])</span>;</span>
<span id="cb14-20"><a href="#cb14-20"></a>    <span class="cf">if</span> <span class="op">(</span>iss <span class="op">&gt;&gt;</span> opts<span class="op">.[:</span>dm<span class="op">:]</span>; <span class="op">!</span>iss<span class="op">)</span> <span class="op">{</span></span>
<span id="cb14-21"><a href="#cb14-21"></a>      std<span class="op">::</span>print<span class="op">(</span>stderr, <span class="st">&quot;Failed to parse option {} into a {}</span><span class="sc">\n</span><span class="st">&quot;</span>, <span class="op">*</span>it, display_name_of<span class="op">(^</span>T<span class="op">))</span>;</span>
<span id="cb14-22"><a href="#cb14-22"></a>      std<span class="op">::</span>exit<span class="op">(</span>EXIT_FAILURE<span class="op">)</span>;</span>
<span id="cb14-23"><a href="#cb14-23"></a>    <span class="op">}</span></span>
<span id="cb14-24"><a href="#cb14-24"></a>  <span class="op">}</span></span>
<span id="cb14-25"><a href="#cb14-25"></a>  <span class="cf">return</span> opts;</span>
<span id="cb14-26"><a href="#cb14-26"></a><span class="op">}</span></span>
<span id="cb14-27"><a href="#cb14-27"></a></span>
<span id="cb14-28"><a href="#cb14-28"></a><span class="kw">struct</span> MyOpts <span class="op">{</span></span>
<span id="cb14-29"><a href="#cb14-29"></a>  std<span class="op">::</span>string file_name <span class="op">=</span> <span class="st">&quot;input.txt&quot;</span>;  <span class="co">// Option &quot;--file_name &lt;string&gt;&quot;</span></span>
<span id="cb14-30"><a href="#cb14-30"></a>  <span class="dt">int</span>    count <span class="op">=</span> <span class="dv">1</span>;                     <span class="co">// Option &quot;--count &lt;int&gt;&quot;</span></span>
<span id="cb14-31"><a href="#cb14-31"></a><span class="op">}</span>;</span>
<span id="cb14-32"><a href="#cb14-32"></a></span>
<span id="cb14-33"><a href="#cb14-33"></a><span class="dt">int</span> main<span class="op">(</span><span class="dt">int</span> argc, <span class="dt">char</span> <span class="op">*</span>argv<span class="op">[])</span> <span class="op">{</span></span>
<span id="cb14-34"><a href="#cb14-34"></a>  MyOpts opts <span class="op">=</span> parse_options<span class="op">&lt;</span>MyOpts<span class="op">&gt;(</span>std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>string_view<span class="op">&gt;(</span>argv<span class="op">+</span><span class="dv">1</span>, argv<span class="op">+</span>argc<span class="op">))</span>;</span>
<span id="cb14-35"><a href="#cb14-35"></a>  <span class="co">// ...</span></span>
<span id="cb14-36"><a href="#cb14-36"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This example is based on a presentation by Matúš Chochlík.</p>
<p><a href="https://godbolt.org/z/G4dh3jq8a">On Compiler Explorer</a>.</p>
<h2 data-number="3.8" id="a-simple-tuple-type"><span class="header-section-number">3.8</span> A Simple Tuple Type<a href="#a-simple-tuple-type" class="self-link"></a></h2>
<blockquote>
<div class="sourceCode" id="cb15"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb15-1"><a href="#cb15-1"></a><span class="pp">#include </span><span class="im">&lt;meta&gt;</span></span>
<span id="cb15-2"><a href="#cb15-2"></a></span>
<span id="cb15-3"><a href="#cb15-3"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span> <span class="kw">struct</span> Tuple <span class="op">{</span></span>
<span id="cb15-4"><a href="#cb15-4"></a>  <span class="kw">struct</span> storage;</span>
<span id="cb15-5"><a href="#cb15-5"></a></span>
<span id="cb15-6"><a href="#cb15-6"></a>  <span class="kw">static_assert</span><span class="op">(</span>is_type<span class="op">(</span>define_class<span class="op">(^</span>storage, <span class="op">{</span>nsdm_description<span class="op">(^</span>Ts<span class="op">)...})))</span>;</span>
<span id="cb15-7"><a href="#cb15-7"></a>  storage data;</span>
<span id="cb15-8"><a href="#cb15-8"></a></span>
<span id="cb15-9"><a href="#cb15-9"></a>  Tuple<span class="op">():</span> data<span class="op">{}</span> <span class="op">{}</span></span>
<span id="cb15-10"><a href="#cb15-10"></a>  Tuple<span class="op">(</span>Ts <span class="kw">const</span><span class="op">&amp;</span> <span class="op">...</span>vs<span class="op">):</span> data<span class="op">{</span> vs<span class="op">...</span> <span class="op">}</span> <span class="op">{}</span></span>
<span id="cb15-11"><a href="#cb15-11"></a><span class="op">}</span>;</span>
<span id="cb15-12"><a href="#cb15-12"></a></span>
<span id="cb15-13"><a href="#cb15-13"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb15-14"><a href="#cb15-14"></a>  <span class="kw">struct</span> std<span class="op">::</span>tuple_size<span class="op">&lt;</span>Tuple<span class="op">&lt;</span>Ts<span class="op">...&gt;&gt;:</span> <span class="kw">public</span> integral_constant<span class="op">&lt;</span><span class="dt">size_t</span>, <span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)&gt;</span> <span class="op">{}</span>;</span>
<span id="cb15-15"><a href="#cb15-15"></a></span>
<span id="cb15-16"><a href="#cb15-16"></a><span class="kw">template</span><span class="op">&lt;</span>std<span class="op">::</span><span class="dt">size_t</span> I, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb15-17"><a href="#cb15-17"></a>  <span class="kw">struct</span> std<span class="op">::</span>tuple_element<span class="op">&lt;</span>I, Tuple<span class="op">&lt;</span>Ts<span class="op">...&gt;&gt;</span> <span class="op">{</span></span>
<span id="cb15-18"><a href="#cb15-18"></a>    <span class="kw">static</span> <span class="kw">constexpr</span> std<span class="op">::</span>array types <span class="op">=</span> <span class="op">{^</span>Ts<span class="op">...}</span>;</span>
<span id="cb15-19"><a href="#cb15-19"></a>    <span class="kw">using</span> type <span class="op">=</span> <span class="op">[:</span> types<span class="op">[</span>I<span class="op">]</span> <span class="op">:]</span>;</span>
<span id="cb15-20"><a href="#cb15-20"></a>  <span class="op">}</span>;</span>
<span id="cb15-21"><a href="#cb15-21"></a></span>
<span id="cb15-22"><a href="#cb15-22"></a><span class="kw">consteval</span> std<span class="op">::</span>meta<span class="op">::</span>info get_nth_nsdm<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>info r, std<span class="op">::</span><span class="dt">size_t</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb15-23"><a href="#cb15-23"></a>  <span class="cf">return</span> nonstatic_data_members_of<span class="op">(</span>r<span class="op">)[</span>n<span class="op">]</span>;</span>
<span id="cb15-24"><a href="#cb15-24"></a><span class="op">}</span></span>
<span id="cb15-25"><a href="#cb15-25"></a></span>
<span id="cb15-26"><a href="#cb15-26"></a><span class="kw">template</span><span class="op">&lt;</span>std<span class="op">::</span><span class="dt">size_t</span> I, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb15-27"><a href="#cb15-27"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> get<span class="op">(</span>Tuple<span class="op">&lt;</span>Ts<span class="op">...&gt;</span> <span class="op">&amp;</span>t<span class="op">)</span> <span class="kw">noexcept</span> <span class="op">-&gt;</span> std<span class="op">::</span>tuple_element_t<span class="op">&lt;</span>I, Tuple<span class="op">&lt;</span>Ts<span class="op">...&gt;&gt;&amp;</span> <span class="op">{</span></span>
<span id="cb15-28"><a href="#cb15-28"></a>    <span class="cf">return</span> t<span class="op">.</span>data<span class="op">.[:</span>get_nth_nsdm<span class="op">(^</span><span class="kw">decltype</span><span class="op">(</span>t<span class="op">.</span>data<span class="op">)</span>, I<span class="op">):]</span>;</span>
<span id="cb15-29"><a href="#cb15-29"></a>  <span class="op">}</span></span>
<span id="cb15-30"><a href="#cb15-30"></a><span class="co">// Similarly for other value categories...</span></span></code></pre></div>
</blockquote>
<p>This example uses a “magic” <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>define_class</code> template along with member reflection through the <code class="sourceCode cpp">nonstatic_data_members_of</code> metafunction to implement a <code class="sourceCode cpp">std<span class="op">::</span>tuple</code>-like type without the usual complex and costly template metaprogramming tricks that that involves when these facilities are not available. <code class="sourceCode cpp">define_class</code> takes a reflection for an incomplete class or union plus a vector of nonstatic data member descriptions, and completes the give class or union type to have the described members.</p>
<p><a href="https://godbolt.org/z/4P15rnbxh">On Compiler Explorer</a>.</p>
<h2 data-number="3.9" id="a-simple-variant-type"><span class="header-section-number">3.9</span> A Simple Variant Type<a href="#a-simple-variant-type" class="self-link"></a></h2>
<p>Similarly to how we can implement a tuple using <code class="sourceCode cpp">define_class</code> to create on the fly a type with one member for each <code class="sourceCode cpp">Ts<span class="op">...</span></code>, we can implement a variant that simply defines a <code class="sourceCode cpp"><span class="kw">union</span></code> instead of a <code class="sourceCode cpp"><span class="kw">struct</span></code>. One difference here is how the destructor of a <code class="sourceCode cpp"><span class="kw">union</span></code> is currently defined:</p>
<blockquote>
<div class="sourceCode" id="cb16"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb16-1"><a href="#cb16-1"></a><span class="kw">union</span> U1 <span class="op">{</span></span>
<span id="cb16-2"><a href="#cb16-2"></a>  <span class="dt">int</span> i;</span>
<span id="cb16-3"><a href="#cb16-3"></a>  <span class="dt">char</span> c;</span>
<span id="cb16-4"><a href="#cb16-4"></a><span class="op">}</span>;</span>
<span id="cb16-5"><a href="#cb16-5"></a></span>
<span id="cb16-6"><a href="#cb16-6"></a><span class="kw">union</span> U2 <span class="op">{</span></span>
<span id="cb16-7"><a href="#cb16-7"></a>  <span class="dt">int</span> i;</span>
<span id="cb16-8"><a href="#cb16-8"></a>  std<span class="op">::</span>string s;</span>
<span id="cb16-9"><a href="#cb16-9"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p><code class="sourceCode cpp">U1</code> has a trivial destructor, but <code class="sourceCode cpp">U2</code>’s destructor is defined as deleted (because <code class="sourceCode cpp">std<span class="op">::</span>string</code> has a non-trivial destructor). This is a problem because we need to define this thing… somehow. However, for the purposes of <code class="sourceCode cpp">define_class</code>, there really is only one reasonable option to choose here:</p>
<blockquote>
<div class="sourceCode" id="cb17"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb17-1"><a href="#cb17-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">class</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb17-2"><a href="#cb17-2"></a><span class="kw">union</span> U <span class="op">{</span></span>
<span id="cb17-3"><a href="#cb17-3"></a>  <span class="co">// all of our members</span></span>
<span id="cb17-4"><a href="#cb17-4"></a>  Ts<span class="op">...</span> members;</span>
<span id="cb17-5"><a href="#cb17-5"></a></span>
<span id="cb17-6"><a href="#cb17-6"></a>  <span class="co">// a defaulted destructor if all of the types are trivially destructible</span></span>
<span id="cb17-7"><a href="#cb17-7"></a>  <span class="kw">constexpr</span> <span class="op">~</span>U<span class="op">()</span> <span class="kw">requires</span> <span class="op">(</span>std<span class="op">::</span>is_trivially_destructible_v<span class="op">&lt;</span>Ts<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> <span class="op">...)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb17-8"><a href="#cb17-8"></a></span>
<span id="cb17-9"><a href="#cb17-9"></a>  <span class="co">// ... otherwise a destructor that does nothing</span></span>
<span id="cb17-10"><a href="#cb17-10"></a>  <span class="kw">constexpr</span> <span class="op">~</span>U<span class="op">()</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb17-11"><a href="#cb17-11"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p>If we make <a href="#nsdm_description-define_class"><code class="sourceCode cpp">define_class</code></a> for a <code class="sourceCode cpp"><span class="kw">union</span></code> have this behavior, then we can implement a <code class="sourceCode cpp">variant</code> in a much more straightforward way than in current implementations. This is not a complete implementation of <code class="sourceCode cpp">std<span class="op">::</span>variant</code> (and cheats using libstdc++ internals, and also uses Boost.Mp11’s <code class="sourceCode cpp">mp_with_index</code>) but should demonstrate the idea:</p>
<blockquote>
<div class="sourceCode" id="cb18"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb18-1"><a href="#cb18-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb18-2"><a href="#cb18-2"></a><span class="kw">class</span> Variant <span class="op">{</span></span>
<span id="cb18-3"><a href="#cb18-3"></a>    <span class="kw">union</span> Storage;</span>
<span id="cb18-4"><a href="#cb18-4"></a>    <span class="kw">struct</span> Empty <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb18-5"><a href="#cb18-5"></a></span>
<span id="cb18-6"><a href="#cb18-6"></a>    <span class="kw">static_assert</span><span class="op">(</span>is_type<span class="op">(</span>define_class<span class="op">(^</span>Storage, <span class="op">{</span></span>
<span id="cb18-7"><a href="#cb18-7"></a>        nsdm_description<span class="op">(^</span>Empty, <span class="op">{.</span>name<span class="op">=</span><span class="st">&quot;empty&quot;</span><span class="op">})</span>,</span>
<span id="cb18-8"><a href="#cb18-8"></a>        nsdm_description<span class="op">(^</span>Ts<span class="op">)...</span></span>
<span id="cb18-9"><a href="#cb18-9"></a>    <span class="op">})))</span>;</span>
<span id="cb18-10"><a href="#cb18-10"></a></span>
<span id="cb18-11"><a href="#cb18-11"></a>    <span class="kw">static</span> <span class="kw">constexpr</span> std<span class="op">::</span>array<span class="op">&lt;</span>std<span class="op">::</span>meta<span class="op">::</span>info, <span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)&gt;</span> types <span class="op">=</span> <span class="op">{^</span>Ts<span class="op">...}</span>;</span>
<span id="cb18-12"><a href="#cb18-12"></a></span>
<span id="cb18-13"><a href="#cb18-13"></a>    <span class="kw">static</span> <span class="kw">consteval</span> std<span class="op">::</span>meta<span class="op">::</span>info get_nth_nsdm<span class="op">(</span>std<span class="op">::</span><span class="dt">size_t</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb18-14"><a href="#cb18-14"></a>        <span class="cf">return</span> nonstatic_data_members_of<span class="op">(^</span>Storage<span class="op">)[</span>n<span class="op">+</span><span class="dv">1</span><span class="op">]</span>;</span>
<span id="cb18-15"><a href="#cb18-15"></a>    <span class="op">}</span></span>
<span id="cb18-16"><a href="#cb18-16"></a></span>
<span id="cb18-17"><a href="#cb18-17"></a>    Storage storage_;</span>
<span id="cb18-18"><a href="#cb18-18"></a>    <span class="dt">int</span> index_ <span class="op">=</span> <span class="op">-</span><span class="dv">1</span>;</span>
<span id="cb18-19"><a href="#cb18-19"></a></span>
<span id="cb18-20"><a href="#cb18-20"></a>    <span class="co">// cheat: use libstdc++&#39;s implementation</span></span>
<span id="cb18-21"><a href="#cb18-21"></a>    <span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb18-22"><a href="#cb18-22"></a>    <span class="kw">static</span> <span class="kw">constexpr</span> <span class="dt">size_t</span> accepted_index <span class="op">=</span> std<span class="op">::</span>__detail<span class="op">::</span>__variant<span class="op">::</span>__accepted_index<span class="op">&lt;</span>T, std<span class="op">::</span>variant<span class="op">&lt;</span>Ts<span class="op">...&gt;&gt;</span>;</span>
<span id="cb18-23"><a href="#cb18-23"></a></span>
<span id="cb18-24"><a href="#cb18-24"></a>    <span class="kw">template</span> <span class="op">&lt;</span><span class="kw">class</span> F<span class="op">&gt;</span></span>
<span id="cb18-25"><a href="#cb18-25"></a>    <span class="kw">constexpr</span> <span class="kw">auto</span> with_index<span class="op">(</span>F<span class="op">&amp;&amp;</span> f<span class="op">)</span> <span class="kw">const</span> <span class="op">-&gt;</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb18-26"><a href="#cb18-26"></a>        <span class="cf">return</span> mp_with_index<span class="op">&lt;</span><span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)&gt;(</span>index_, <span class="op">(</span>F<span class="op">&amp;&amp;)</span>f<span class="op">)</span>;</span>
<span id="cb18-27"><a href="#cb18-27"></a>    <span class="op">}</span></span>
<span id="cb18-28"><a href="#cb18-28"></a></span>
<span id="cb18-29"><a href="#cb18-29"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb18-30"><a href="#cb18-30"></a>    <span class="kw">constexpr</span> Variant<span class="op">()</span> <span class="kw">requires</span> std<span class="op">::</span>is_default_constructible_v<span class="op">&lt;[:</span> types<span class="op">[</span><span class="dv">0</span><span class="op">]</span> <span class="op">:]&gt;</span></span>
<span id="cb18-31"><a href="#cb18-31"></a>        <span class="co">// should this work: storage_{. [: get_nth_nsdm(0) :]{} }</span></span>
<span id="cb18-32"><a href="#cb18-32"></a>        <span class="op">:</span> storage_<span class="op">{.</span>empty<span class="op">={}}</span></span>
<span id="cb18-33"><a href="#cb18-33"></a>        , index_<span class="op">(</span><span class="dv">0</span><span class="op">)</span></span>
<span id="cb18-34"><a href="#cb18-34"></a>    <span class="op">{</span></span>
<span id="cb18-35"><a href="#cb18-35"></a>        std<span class="op">::</span>construct_at<span class="op">(&amp;</span>storage_<span class="op">.[:</span> get_nth_nsdm<span class="op">(</span><span class="dv">0</span><span class="op">)</span> <span class="op">:])</span>;</span>
<span id="cb18-36"><a href="#cb18-36"></a>    <span class="op">}</span></span>
<span id="cb18-37"><a href="#cb18-37"></a></span>
<span id="cb18-38"><a href="#cb18-38"></a>    <span class="kw">constexpr</span> <span class="op">~</span>Variant<span class="op">()</span> <span class="kw">requires</span> <span class="op">(</span>std<span class="op">::</span>is_trivially_destructible_v<span class="op">&lt;</span>Ts<span class="op">&gt;</span> <span class="kw">and</span> <span class="op">...)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb18-39"><a href="#cb18-39"></a>    <span class="kw">constexpr</span> <span class="op">~</span>Variant<span class="op">()</span> <span class="op">{</span></span>
<span id="cb18-40"><a href="#cb18-40"></a>        <span class="cf">if</span> <span class="op">(</span>index_ <span class="op">!=</span> <span class="op">-</span><span class="dv">1</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb18-41"><a href="#cb18-41"></a>            with_index<span class="op">([&amp;](</span><span class="kw">auto</span> I<span class="op">){</span></span>
<span id="cb18-42"><a href="#cb18-42"></a>                std<span class="op">::</span>destroy_at<span class="op">(&amp;</span>storage_<span class="op">.[:</span> get_nth_nsdm<span class="op">(</span>I<span class="op">)</span> <span class="op">:])</span>;</span>
<span id="cb18-43"><a href="#cb18-43"></a>            <span class="op">})</span>;</span>
<span id="cb18-44"><a href="#cb18-44"></a>        <span class="op">}</span></span>
<span id="cb18-45"><a href="#cb18-45"></a>    <span class="op">}</span></span>
<span id="cb18-46"><a href="#cb18-46"></a></span>
<span id="cb18-47"><a href="#cb18-47"></a>    <span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T, <span class="dt">size_t</span> I <span class="op">=</span> accepted_index<span class="op">&lt;</span>T<span class="op">&amp;&amp;&gt;&gt;</span></span>
<span id="cb18-48"><a href="#cb18-48"></a>        <span class="kw">requires</span> <span class="op">(!</span>std<span class="op">::</span>is_base_of_v<span class="op">&lt;</span>Variant, std<span class="op">::</span>decay_t<span class="op">&lt;</span>T<span class="op">&gt;&gt;)</span></span>
<span id="cb18-49"><a href="#cb18-49"></a>    <span class="kw">constexpr</span> Variant<span class="op">(</span>T<span class="op">&amp;&amp;</span> t<span class="op">)</span></span>
<span id="cb18-50"><a href="#cb18-50"></a>        <span class="op">:</span> storage_<span class="op">{.</span>empty<span class="op">={}}</span></span>
<span id="cb18-51"><a href="#cb18-51"></a>        , index_<span class="op">(-</span><span class="dv">1</span><span class="op">)</span></span>
<span id="cb18-52"><a href="#cb18-52"></a>    <span class="op">{</span></span>
<span id="cb18-53"><a href="#cb18-53"></a>        std<span class="op">::</span>construct_at<span class="op">(&amp;</span>storage_<span class="op">.[:</span> get_nth_nsdm<span class="op">(</span>I<span class="op">)</span> <span class="op">:]</span>, <span class="op">(</span>T<span class="op">&amp;&amp;)</span>t<span class="op">)</span>;</span>
<span id="cb18-54"><a href="#cb18-54"></a>        index_ <span class="op">=</span> <span class="op">(</span><span class="dt">int</span><span class="op">)</span>I;</span>
<span id="cb18-55"><a href="#cb18-55"></a>    <span class="op">}</span></span>
<span id="cb18-56"><a href="#cb18-56"></a></span>
<span id="cb18-57"><a href="#cb18-57"></a>    <span class="co">// you can&#39;t actually express this constraint nicely until P2963</span></span>
<span id="cb18-58"><a href="#cb18-58"></a>    <span class="kw">constexpr</span> Variant<span class="op">(</span>Variant <span class="kw">const</span><span class="op">&amp;)</span> <span class="kw">requires</span> <span class="op">(</span>std<span class="op">::</span>is_trivially_copyable_v<span class="op">&lt;</span>Ts<span class="op">&gt;</span> <span class="kw">and</span> <span class="op">...)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb18-59"><a href="#cb18-59"></a>    <span class="kw">constexpr</span> Variant<span class="op">(</span>Variant <span class="kw">const</span><span class="op">&amp;</span> rhs<span class="op">)</span></span>
<span id="cb18-60"><a href="#cb18-60"></a>            <span class="kw">requires</span> <span class="op">((</span>std<span class="op">::</span>is_copy_constructible_v<span class="op">&lt;</span>Ts<span class="op">&gt;</span> <span class="kw">and</span> <span class="op">...)</span></span>
<span id="cb18-61"><a href="#cb18-61"></a>                <span class="kw">and</span> <span class="kw">not</span> <span class="op">(</span>std<span class="op">::</span>is_trivially_copyable_v<span class="op">&lt;</span>Ts<span class="op">&gt;</span> <span class="kw">and</span> <span class="op">...))</span></span>
<span id="cb18-62"><a href="#cb18-62"></a>        <span class="op">:</span> storage_<span class="op">{.</span>empty<span class="op">={}}</span></span>
<span id="cb18-63"><a href="#cb18-63"></a>        , index_<span class="op">(-</span><span class="dv">1</span><span class="op">)</span></span>
<span id="cb18-64"><a href="#cb18-64"></a>    <span class="op">{</span></span>
<span id="cb18-65"><a href="#cb18-65"></a>        rhs<span class="op">.</span>with_index<span class="op">([&amp;](</span><span class="kw">auto</span> I<span class="op">){</span></span>
<span id="cb18-66"><a href="#cb18-66"></a>            <span class="kw">constexpr</span> <span class="kw">auto</span> nsdm <span class="op">=</span> get_nth_nsdm<span class="op">(</span>I<span class="op">)</span>;</span>
<span id="cb18-67"><a href="#cb18-67"></a>            std<span class="op">::</span>construct_at<span class="op">(&amp;</span>storage_<span class="op">.[:</span> nsdm <span class="op">:]</span>, rhs<span class="op">.</span>storage_<span class="op">.[:</span> nsdm <span class="op">:])</span>;</span>
<span id="cb18-68"><a href="#cb18-68"></a>            index_ <span class="op">=</span> I;</span>
<span id="cb18-69"><a href="#cb18-69"></a>        <span class="op">})</span>;</span>
<span id="cb18-70"><a href="#cb18-70"></a>    <span class="op">}</span></span>
<span id="cb18-71"><a href="#cb18-71"></a></span>
<span id="cb18-72"><a href="#cb18-72"></a>    <span class="kw">constexpr</span> <span class="kw">auto</span> index<span class="op">()</span> <span class="kw">const</span> <span class="op">-&gt;</span> <span class="dt">int</span> <span class="op">{</span> <span class="cf">return</span> index_; <span class="op">}</span></span>
<span id="cb18-73"><a href="#cb18-73"></a></span>
<span id="cb18-74"><a href="#cb18-74"></a>    <span class="kw">template</span> <span class="op">&lt;</span><span class="kw">class</span> F<span class="op">&gt;</span></span>
<span id="cb18-75"><a href="#cb18-75"></a>    <span class="kw">constexpr</span> <span class="kw">auto</span> visit<span class="op">(</span>F<span class="op">&amp;&amp;</span> f<span class="op">)</span> <span class="kw">const</span> <span class="op">-&gt;</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb18-76"><a href="#cb18-76"></a>        <span class="cf">if</span> <span class="op">(</span>index_ <span class="op">==</span> <span class="op">-</span><span class="dv">1</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb18-77"><a href="#cb18-77"></a>            <span class="cf">throw</span> std<span class="op">::</span>bad_variant_access<span class="op">()</span>;</span>
<span id="cb18-78"><a href="#cb18-78"></a>        <span class="op">}</span></span>
<span id="cb18-79"><a href="#cb18-79"></a></span>
<span id="cb18-80"><a href="#cb18-80"></a>        <span class="cf">return</span> mp_with_index<span class="op">&lt;</span><span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)&gt;(</span>index_, <span class="op">[&amp;](</span><span class="kw">auto</span> I<span class="op">)</span> <span class="op">-&gt;</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb18-81"><a href="#cb18-81"></a>            <span class="cf">return</span> std<span class="op">::</span>invoke<span class="op">((</span>F<span class="op">&amp;&amp;)</span>f,  storage_<span class="op">.[:</span> get_nth_nsdm<span class="op">(</span>I<span class="op">)</span> <span class="op">:])</span>;</span>
<span id="cb18-82"><a href="#cb18-82"></a>        <span class="op">})</span>;</span>
<span id="cb18-83"><a href="#cb18-83"></a>    <span class="op">}</span></span>
<span id="cb18-84"><a href="#cb18-84"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p>Effectively, <code class="sourceCode cpp">Variant<span class="op">&lt;</span>T, U<span class="op">&gt;</span></code> synthesizes a union type <code class="sourceCode cpp">Storage</code> which looks like this:</p>
<blockquote>
<div class="sourceCode" id="cb19"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb19-1"><a href="#cb19-1"></a><span class="kw">union</span> Storage <span class="op">{</span></span>
<span id="cb19-2"><a href="#cb19-2"></a>    Empty empty;</span>
<span id="cb19-3"><a href="#cb19-3"></a>    T <em>unnamed<sub>0</sub></em>;</span>
<span id="cb19-4"><a href="#cb19-4"></a>    U <em>unnamed<sub>1</sub></em>;</span>
<span id="cb19-5"><a href="#cb19-5"></a></span>
<span id="cb19-6"><a href="#cb19-6"></a>    <span class="op">~</span>Storage<span class="op">()</span> <span class="kw">requires</span> std<span class="op">::</span>is_trivially_destructible_v<span class="op">&lt;</span>T<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> std<span class="op">::</span>is_trivially_destructible_v<span class="op">&lt;</span>U<span class="op">&gt;</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb19-7"><a href="#cb19-7"></a>    <span class="op">~</span>Storage<span class="op">()</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb19-8"><a href="#cb19-8"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>The question here is whether we should be should be able to directly initialize members of a defined union using a splicer, as in:</p>
<blockquote>
<div class="sourceCode" id="cb20"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb20-1"><a href="#cb20-1"></a><span class="op">:</span> storage<span class="op">{.[:</span> get_nth_nsdm<span class="op">(</span><span class="dv">0</span><span class="op">)</span> <span class="op">:]={}}</span></span></code></pre></div>
</blockquote>
<p>Arguably, the answer should be yes - this would be consistent with how other accesses work.</p>
<p><a href="https://godbolt.org/z/Efz5vsjaa">On Compiler Explorer</a>.</p>
<h2 data-number="3.10" id="struct-to-struct-of-arrays"><span class="header-section-number">3.10</span> Struct to Struct of Arrays<a href="#struct-to-struct-of-arrays" class="self-link"></a></h2>
<blockquote>
<div class="sourceCode" id="cb21"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb21-1"><a href="#cb21-1"></a><span class="pp">#include </span><span class="im">&lt;meta&gt;</span></span>
<span id="cb21-2"><a href="#cb21-2"></a><span class="pp">#include </span><span class="im">&lt;array&gt;</span></span>
<span id="cb21-3"><a href="#cb21-3"></a></span>
<span id="cb21-4"><a href="#cb21-4"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T, std<span class="op">::</span><span class="dt">size_t</span> N<span class="op">&gt;</span></span>
<span id="cb21-5"><a href="#cb21-5"></a><span class="kw">struct</span> struct_of_arrays_impl;</span>
<span id="cb21-6"><a href="#cb21-6"></a></span>
<span id="cb21-7"><a href="#cb21-7"></a><span class="kw">consteval</span> <span class="kw">auto</span> make_struct_of_arrays<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>info type,</span>
<span id="cb21-8"><a href="#cb21-8"></a>                                     std<span class="op">::</span>meta<span class="op">::</span>info N<span class="op">)</span> <span class="op">-&gt;</span> std<span class="op">::</span>meta<span class="op">::</span>info <span class="op">{</span></span>
<span id="cb21-9"><a href="#cb21-9"></a>  std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>meta<span class="op">::</span>info<span class="op">&gt;</span> old_members <span class="op">=</span> nonstatic_data_members_of<span class="op">(</span>type<span class="op">)</span>;</span>
<span id="cb21-10"><a href="#cb21-10"></a>  std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>meta<span class="op">::</span>info<span class="op">&gt;</span> new_members <span class="op">=</span> <span class="op">{}</span>;</span>
<span id="cb21-11"><a href="#cb21-11"></a>  <span class="cf">for</span> <span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>info member <span class="op">:</span> old_members<span class="op">)</span> <span class="op">{</span></span>
<span id="cb21-12"><a href="#cb21-12"></a>    <span class="kw">auto</span> array_type <span class="op">=</span> substitute<span class="op">(^</span>std<span class="op">::</span>array, <span class="op">{</span>type_of<span class="op">(</span>member<span class="op">)</span>, N <span class="op">})</span>;</span>
<span id="cb21-13"><a href="#cb21-13"></a>    <span class="kw">auto</span> mem_descr <span class="op">=</span> nsdm_description<span class="op">(</span>array_type, <span class="op">{.</span>name <span class="op">=</span> name_of<span class="op">(</span>member<span class="op">)})</span>;</span>
<span id="cb21-14"><a href="#cb21-14"></a>    new_members<span class="op">.</span>push_back<span class="op">(</span>mem_descr<span class="op">)</span>;</span>
<span id="cb21-15"><a href="#cb21-15"></a>  <span class="op">}</span></span>
<span id="cb21-16"><a href="#cb21-16"></a>  <span class="cf">return</span> std<span class="op">::</span>meta<span class="op">::</span>define_class<span class="op">(</span></span>
<span id="cb21-17"><a href="#cb21-17"></a>    substitute<span class="op">(^</span>struct_of_arrays_impl, <span class="op">{</span>type, N<span class="op">})</span>,</span>
<span id="cb21-18"><a href="#cb21-18"></a>    new_members<span class="op">)</span>;</span>
<span id="cb21-19"><a href="#cb21-19"></a><span class="op">}</span></span>
<span id="cb21-20"><a href="#cb21-20"></a></span>
<span id="cb21-21"><a href="#cb21-21"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T, <span class="dt">size_t</span> N<span class="op">&gt;</span></span>
<span id="cb21-22"><a href="#cb21-22"></a><span class="kw">using</span> struct_of_arrays <span class="op">=</span> <span class="op">[:</span> make_struct_of_arrays<span class="op">(^</span>T, <span class="op">^</span>N<span class="op">)</span> <span class="op">:]</span>;</span></code></pre></div>
</blockquote>
<p>Example:</p>
<blockquote>
<div class="sourceCode" id="cb22"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb22-1"><a href="#cb22-1"></a><span class="kw">struct</span> point <span class="op">{</span></span>
<span id="cb22-2"><a href="#cb22-2"></a>  <span class="dt">float</span> x;</span>
<span id="cb22-3"><a href="#cb22-3"></a>  <span class="dt">float</span> y;</span>
<span id="cb22-4"><a href="#cb22-4"></a>  <span class="dt">float</span> z;</span>
<span id="cb22-5"><a href="#cb22-5"></a><span class="op">}</span>;</span>
<span id="cb22-6"><a href="#cb22-6"></a></span>
<span id="cb22-7"><a href="#cb22-7"></a><span class="kw">using</span> points <span class="op">=</span> struct_of_arrays<span class="op">&lt;</span>point, <span class="dv">30</span><span class="op">&gt;</span>;</span>
<span id="cb22-8"><a href="#cb22-8"></a><span class="co">// equivalent to:</span></span>
<span id="cb22-9"><a href="#cb22-9"></a><span class="co">// struct points {</span></span>
<span id="cb22-10"><a href="#cb22-10"></a><span class="co">//   std::array&lt;float, 30&gt; x;</span></span>
<span id="cb22-11"><a href="#cb22-11"></a><span class="co">//   std::array&lt;float, 30&gt; y;</span></span>
<span id="cb22-12"><a href="#cb22-12"></a><span class="co">//   std::array&lt;float, 30&gt; z;</span></span>
<span id="cb22-13"><a href="#cb22-13"></a><span class="co">// };</span></span></code></pre></div>
</blockquote>
<p>Again, the combination of <code class="sourceCode cpp">nonstatic_data_members_of</code> and <code class="sourceCode cpp">define_class</code> is put to good use.</p>
<p><a href="https://godbolt.org/z/8rT77KxjP">On Compiler Explorer</a>.</p>
<h2 data-number="3.11" id="parsing-command-line-options-ii"><span class="header-section-number">3.11</span> Parsing Command-Line Options II<a href="#parsing-command-line-options-ii" class="self-link"></a></h2>
<p>Now that we’ve seen a couple examples of using <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>define_class</code> to create a type, we can create a more sophisticated command-line parser example.</p>
<p>This is the opening example for <a href="https://docs.rs/clap/latest/clap/">clap</a> (Rust’s <strong>C</strong>ommand <strong>L</strong>ine <strong>A</strong>rgument <strong>P</strong>arser):</p>
<blockquote>
<div class="sourceCode" id="cb23"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb23-1"><a href="#cb23-1"></a><span class="kw">struct</span> Args <span class="op">:</span> Clap <span class="op">{</span></span>
<span id="cb23-2"><a href="#cb23-2"></a>  Option<span class="op">&lt;</span>std<span class="op">::</span>string, <span class="op">{.</span>use_short<span class="op">=</span><span class="kw">true</span>, <span class="op">.</span>use_long<span class="op">=</span><span class="kw">true</span><span class="op">}&gt;</span> name;</span>
<span id="cb23-3"><a href="#cb23-3"></a>  Option<span class="op">&lt;</span><span class="dt">int</span>, <span class="op">{.</span>use_short<span class="op">=</span><span class="kw">true</span>, <span class="op">.</span>use_long<span class="op">=</span><span class="kw">true</span><span class="op">}&gt;</span> count <span class="op">=</span> <span class="dv">1</span>;</span>
<span id="cb23-4"><a href="#cb23-4"></a><span class="op">}</span>;</span>
<span id="cb23-5"><a href="#cb23-5"></a></span>
<span id="cb23-6"><a href="#cb23-6"></a><span class="dt">int</span> main<span class="op">(</span><span class="dt">int</span> argc, <span class="dt">char</span><span class="op">**</span> argv<span class="op">)</span> <span class="op">{</span></span>
<span id="cb23-7"><a href="#cb23-7"></a>  <span class="kw">auto</span> opts <span class="op">=</span> Args<span class="op">{}.</span>parse<span class="op">(</span>argc, argv<span class="op">)</span>;</span>
<span id="cb23-8"><a href="#cb23-8"></a></span>
<span id="cb23-9"><a href="#cb23-9"></a>  <span class="cf">for</span> <span class="op">(</span><span class="dt">int</span> i <span class="op">=</span> <span class="dv">0</span>; i <span class="op">&lt;</span> opts<span class="op">.</span>count; <span class="op">++</span>i<span class="op">)</span> <span class="op">{</span>  <span class="co">// opts.count has type int</span></span>
<span id="cb23-10"><a href="#cb23-10"></a>    std<span class="op">::</span>print<span class="op">(</span><span class="st">&quot;Hello {}!&quot;</span>, opts<span class="op">.</span>name<span class="op">)</span>;   <span class="co">// opts.name has type std::string</span></span>
<span id="cb23-11"><a href="#cb23-11"></a>  <span class="op">}</span></span>
<span id="cb23-12"><a href="#cb23-12"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Which we can implement like this:</p>
<blockquote>
<div class="sourceCode" id="cb24"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb24-1"><a href="#cb24-1"></a><span class="kw">struct</span> Flags <span class="op">{</span></span>
<span id="cb24-2"><a href="#cb24-2"></a>  <span class="dt">bool</span> use_short;</span>
<span id="cb24-3"><a href="#cb24-3"></a>  <span class="dt">bool</span> use_long;</span>
<span id="cb24-4"><a href="#cb24-4"></a><span class="op">}</span>;</span>
<span id="cb24-5"><a href="#cb24-5"></a></span>
<span id="cb24-6"><a href="#cb24-6"></a><span class="co">// type that has a member optional&lt;T&gt; with some suitable constructors and members</span></span>
<span id="cb24-7"><a href="#cb24-7"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T, Flags flags<span class="op">&gt;</span></span>
<span id="cb24-8"><a href="#cb24-8"></a><span class="kw">struct</span> Option;</span>
<span id="cb24-9"><a href="#cb24-9"></a></span>
<span id="cb24-10"><a href="#cb24-10"></a><span class="co">// convert a type (all of whose non-static data members are specializations of Option)</span></span>
<span id="cb24-11"><a href="#cb24-11"></a><span class="co">// to a type that is just the appropriate members.</span></span>
<span id="cb24-12"><a href="#cb24-12"></a><span class="co">// For example, if type is a reflection of the Args presented above, then this</span></span>
<span id="cb24-13"><a href="#cb24-13"></a><span class="co">// function would evaluate to a reflection of the type</span></span>
<span id="cb24-14"><a href="#cb24-14"></a><span class="co">// struct {</span></span>
<span id="cb24-15"><a href="#cb24-15"></a><span class="co">//   std::string name;</span></span>
<span id="cb24-16"><a href="#cb24-16"></a><span class="co">//   int count;</span></span>
<span id="cb24-17"><a href="#cb24-17"></a><span class="co">// }</span></span>
<span id="cb24-18"><a href="#cb24-18"></a><span class="kw">consteval</span> <span class="kw">auto</span> spec_to_opts<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>info opts,</span>
<span id="cb24-19"><a href="#cb24-19"></a>                            std<span class="op">::</span>meta<span class="op">::</span>info spec<span class="op">)</span> <span class="op">-&gt;</span> std<span class="op">::</span>meta<span class="op">::</span>info <span class="op">{</span></span>
<span id="cb24-20"><a href="#cb24-20"></a>  std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>meta<span class="op">::</span>info<span class="op">&gt;</span> new_members;</span>
<span id="cb24-21"><a href="#cb24-21"></a>  <span class="cf">for</span> <span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>info member <span class="op">:</span> nonstatic_data_members_of<span class="op">(</span>spec<span class="op">))</span> <span class="op">{</span></span>
<span id="cb24-22"><a href="#cb24-22"></a>    <span class="kw">auto</span> new_type <span class="op">=</span> template_arguments_of<span class="op">(</span>type_of<span class="op">(</span>member<span class="op">))[</span><span class="dv">0</span><span class="op">]</span>;</span>
<span id="cb24-23"><a href="#cb24-23"></a>    new_members<span class="op">.</span>push_back<span class="op">(</span>nsdm_description<span class="op">(</span>new_type, <span class="op">{.</span>name<span class="op">=</span>name_of<span class="op">(</span>member<span class="op">)}))</span>;</span>
<span id="cb24-24"><a href="#cb24-24"></a>  <span class="op">}</span></span>
<span id="cb24-25"><a href="#cb24-25"></a>  <span class="cf">return</span> define_class<span class="op">(</span>opts, new_members<span class="op">)</span>;</span>
<span id="cb24-26"><a href="#cb24-26"></a><span class="op">}</span></span>
<span id="cb24-27"><a href="#cb24-27"></a></span>
<span id="cb24-28"><a href="#cb24-28"></a><span class="kw">struct</span> Clap <span class="op">{</span></span>
<span id="cb24-29"><a href="#cb24-29"></a>  <span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> Spec<span class="op">&gt;</span></span>
<span id="cb24-30"><a href="#cb24-30"></a>  <span class="kw">auto</span> parse<span class="op">(</span><span class="kw">this</span> Spec <span class="kw">const</span><span class="op">&amp;</span> spec, <span class="dt">int</span> argc, <span class="dt">char</span><span class="op">**</span> argv<span class="op">)</span> <span class="op">{</span></span>
<span id="cb24-31"><a href="#cb24-31"></a>    std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>string_view<span class="op">&gt;</span> cmdline<span class="op">(</span>argv<span class="op">+</span><span class="dv">1</span>, argv<span class="op">+</span>argc<span class="op">)</span></span>
<span id="cb24-32"><a href="#cb24-32"></a></span>
<span id="cb24-33"><a href="#cb24-33"></a>    <span class="co">// check if cmdline contains --help, etc.</span></span>
<span id="cb24-34"><a href="#cb24-34"></a></span>
<span id="cb24-35"><a href="#cb24-35"></a>    <span class="kw">struct</span> Opts;</span>
<span id="cb24-36"><a href="#cb24-36"></a>    <span class="kw">static_assert</span><span class="op">(</span>is_type<span class="op">(</span>spec_to_opts<span class="op">(^</span>Opts, <span class="op">^</span>Spec<span class="op">)))</span>;</span>
<span id="cb24-37"><a href="#cb24-37"></a>    Opts opts;</span>
<span id="cb24-38"><a href="#cb24-38"></a></span>
<span id="cb24-39"><a href="#cb24-39"></a>    <span class="kw">template</span> <span class="cf">for</span> <span class="op">(</span><span class="kw">constexpr</span> <span class="kw">auto</span> <span class="op">[</span>sm, om<span class="op">]</span> <span class="op">:</span> std<span class="op">::</span>views<span class="op">::</span>zip<span class="op">(</span>nonstatic_data_members_of<span class="op">(^</span>Spec<span class="op">)</span>,</span>
<span id="cb24-40"><a href="#cb24-40"></a>                                                            nonstatic_data_members_of<span class="op">(^</span>Opts<span class="op">)))</span> <span class="op">{</span></span>
<span id="cb24-41"><a href="#cb24-41"></a>      <span class="kw">auto</span> <span class="kw">const</span><span class="op">&amp;</span> cur <span class="op">=</span> spec<span class="op">.[:</span>sm<span class="op">:]</span>;</span>
<span id="cb24-42"><a href="#cb24-42"></a>      <span class="kw">constexpr</span> <span class="kw">auto</span> type <span class="op">=</span> type_of<span class="op">(</span>om<span class="op">)</span>;</span>
<span id="cb24-43"><a href="#cb24-43"></a></span>
<span id="cb24-44"><a href="#cb24-44"></a>      <span class="co">// find the argument associated with this option</span></span>
<span id="cb24-45"><a href="#cb24-45"></a>      <span class="kw">auto</span> it <span class="op">=</span> std<span class="op">::</span>ranges<span class="op">::</span>find_if<span class="op">(</span>cmdline,</span>
<span id="cb24-46"><a href="#cb24-46"></a>        <span class="op">[&amp;](</span>std<span class="op">::</span>string_view arg<span class="op">){</span></span>
<span id="cb24-47"><a href="#cb24-47"></a>          <span class="cf">return</span> <span class="op">(</span>cur<span class="op">.</span>use_short <span class="op">&amp;&amp;</span> arg<span class="op">.</span>size<span class="op">()</span> <span class="op">==</span> <span class="dv">2</span> <span class="op">&amp;&amp;</span> arg<span class="op">[</span><span class="dv">0</span><span class="op">]</span> <span class="op">==</span> <span class="ch">&#39;-&#39;</span> <span class="op">&amp;&amp;</span> arg<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op">==</span> name_of<span class="op">(</span>sm<span class="op">)[</span><span class="dv">0</span><span class="op">])</span></span>
<span id="cb24-48"><a href="#cb24-48"></a>              <span class="op">||</span> <span class="op">(</span>cur<span class="op">.</span>use_long <span class="op">&amp;&amp;</span> arg<span class="op">.</span>starts_with<span class="op">(</span><span class="st">&quot;--&quot;</span><span class="op">)</span> <span class="op">&amp;&amp;</span> arg<span class="op">.</span>substr<span class="op">(</span><span class="dv">2</span><span class="op">)</span> <span class="op">==</span> name_of<span class="op">(</span>sm<span class="op">))</span>;</span>
<span id="cb24-49"><a href="#cb24-49"></a>        <span class="op">})</span>;</span>
<span id="cb24-50"><a href="#cb24-50"></a></span>
<span id="cb24-51"><a href="#cb24-51"></a>      <span class="co">// no such argument</span></span>
<span id="cb24-52"><a href="#cb24-52"></a>      <span class="cf">if</span> <span class="op">(</span>it <span class="op">==</span> cmdline<span class="op">.</span>end<span class="op">())</span> <span class="op">{</span></span>
<span id="cb24-53"><a href="#cb24-53"></a>        <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>has_template_arguments<span class="op">(</span>type<span class="op">)</span> <span class="kw">and</span> template_of<span class="op">(</span>type<span class="op">)</span> <span class="op">==</span> <span class="op">^</span>std<span class="op">::</span>optional<span class="op">)</span> <span class="op">{</span></span>
<span id="cb24-54"><a href="#cb24-54"></a>          <span class="co">// the type is optional, so the argument is too</span></span>
<span id="cb24-55"><a href="#cb24-55"></a>          <span class="cf">continue</span>;</span>
<span id="cb24-56"><a href="#cb24-56"></a>        <span class="op">}</span> <span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>cur<span class="op">.</span>initializer<span class="op">)</span> <span class="op">{</span></span>
<span id="cb24-57"><a href="#cb24-57"></a>          <span class="co">// the type isn&#39;t optional, but an initializer is provided, use that</span></span>
<span id="cb24-58"><a href="#cb24-58"></a>          opts<span class="op">.[:</span>om<span class="op">:]</span> <span class="op">=</span> <span class="op">*</span>cur<span class="op">.</span>initializer;</span>
<span id="cb24-59"><a href="#cb24-59"></a>          <span class="cf">continue</span>;</span>
<span id="cb24-60"><a href="#cb24-60"></a>        <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb24-61"><a href="#cb24-61"></a>          std<span class="op">::</span>print<span class="op">(</span>stderr, <span class="st">&quot;Missing required option {}</span><span class="sc">\n</span><span class="st">&quot;</span>, name_of<span class="op">(</span>sm<span class="op">))</span>;</span>
<span id="cb24-62"><a href="#cb24-62"></a>          std<span class="op">::</span>exit<span class="op">(</span>EXIT_FAILURE<span class="op">)</span>;</span>
<span id="cb24-63"><a href="#cb24-63"></a>        <span class="op">}</span></span>
<span id="cb24-64"><a href="#cb24-64"></a>      <span class="op">}</span> <span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>it <span class="op">+</span> <span class="dv">1</span> <span class="op">==</span> cmdline<span class="op">.</span>end<span class="op">())</span> <span class="op">{</span></span>
<span id="cb24-65"><a href="#cb24-65"></a>        std<span class="op">::</span>print<span class="op">(</span>stderr, <span class="st">&quot;Option {} for {} is missing a value</span><span class="sc">\n</span><span class="st">&quot;</span>, <span class="op">*</span>it, name_of<span class="op">(</span>sm<span class="op">))</span>;</span>
<span id="cb24-66"><a href="#cb24-66"></a>        std<span class="op">::</span>exit<span class="op">(</span>EXIT_FAILURE<span class="op">)</span>;</span>
<span id="cb24-67"><a href="#cb24-67"></a>      <span class="op">}</span></span>
<span id="cb24-68"><a href="#cb24-68"></a></span>
<span id="cb24-69"><a href="#cb24-69"></a>      <span class="co">// found our argument, try to parse it</span></span>
<span id="cb24-70"><a href="#cb24-70"></a>      <span class="kw">auto</span> iss <span class="op">=</span> ispanstream<span class="op">(</span>it<span class="op">[</span><span class="dv">1</span><span class="op">])</span>;</span>
<span id="cb24-71"><a href="#cb24-71"></a>      <span class="cf">if</span> <span class="op">(</span>iss <span class="op">&gt;&gt;</span> opts<span class="op">.[:</span>om<span class="op">:]</span>; <span class="op">!</span>iss<span class="op">)</span> <span class="op">{</span></span>
<span id="cb24-72"><a href="#cb24-72"></a>        std<span class="op">::</span>print<span class="op">(</span>stderr, <span class="st">&quot;Failed to parse {:?} into option {} of type {}</span><span class="sc">\n</span><span class="st">&quot;</span>,</span>
<span id="cb24-73"><a href="#cb24-73"></a>          it<span class="op">[</span><span class="dv">1</span><span class="op">]</span>, name_of<span class="op">(</span>sm<span class="op">)</span>, display_name_of<span class="op">(</span>type<span class="op">))</span>;</span>
<span id="cb24-74"><a href="#cb24-74"></a>        std<span class="op">::</span>exit<span class="op">(</span>EXIT_FAILURE<span class="op">)</span>;</span>
<span id="cb24-75"><a href="#cb24-75"></a>      <span class="op">}</span></span>
<span id="cb24-76"><a href="#cb24-76"></a>    <span class="op">}</span></span>
<span id="cb24-77"><a href="#cb24-77"></a>    <span class="cf">return</span> opts;</span>
<span id="cb24-78"><a href="#cb24-78"></a>  <span class="op">}</span></span>
<span id="cb24-79"><a href="#cb24-79"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p><a href="https://godbolt.org/z/1esbcq4jq">On Compiler Explorer</a>.</p>
<h2 data-number="3.12" id="a-universal-formatter"><span class="header-section-number">3.12</span> A Universal Formatter<a href="#a-universal-formatter" class="self-link"></a></h2>
<p>This example is taken from Boost.Describe:</p>
<blockquote>
<div class="sourceCode" id="cb25"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb25-1"><a href="#cb25-1"></a><span class="kw">struct</span> universal_formatter <span class="op">{</span></span>
<span id="cb25-2"><a href="#cb25-2"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> parse<span class="op">(</span><span class="kw">auto</span><span class="op">&amp;</span> ctx<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> ctx<span class="op">.</span>begin<span class="op">()</span>; <span class="op">}</span></span>
<span id="cb25-3"><a href="#cb25-3"></a></span>
<span id="cb25-4"><a href="#cb25-4"></a>  <span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb25-5"><a href="#cb25-5"></a>  <span class="kw">auto</span> format<span class="op">(</span>T <span class="kw">const</span><span class="op">&amp;</span> t, <span class="kw">auto</span><span class="op">&amp;</span> ctx<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb25-6"><a href="#cb25-6"></a>    <span class="kw">auto</span> out <span class="op">=</span> std<span class="op">::</span>format_to<span class="op">(</span>ctx<span class="op">.</span>out<span class="op">()</span>, <span class="st">&quot;{}{{&quot;</span>, name_of<span class="op">(^</span>T<span class="op">))</span>;</span>
<span id="cb25-7"><a href="#cb25-7"></a></span>
<span id="cb25-8"><a href="#cb25-8"></a>    <span class="kw">auto</span> delim <span class="op">=</span> <span class="op">[</span>first<span class="op">=</span><span class="kw">true</span><span class="op">]()</span> <span class="kw">mutable</span> <span class="op">{</span></span>
<span id="cb25-9"><a href="#cb25-9"></a>      <span class="cf">if</span> <span class="op">(!</span>first<span class="op">)</span> <span class="op">{</span></span>
<span id="cb25-10"><a href="#cb25-10"></a>        <span class="op">*</span>out<span class="op">++</span> <span class="op">=</span> <span class="ch">&#39;,&#39;</span>;</span>
<span id="cb25-11"><a href="#cb25-11"></a>        <span class="op">*</span>out<span class="op">++</span> <span class="op">=</span> <span class="ch">&#39; &#39;</span>;</span>
<span id="cb25-12"><a href="#cb25-12"></a>      <span class="op">}</span></span>
<span id="cb25-13"><a href="#cb25-13"></a>      first <span class="op">=</span> <span class="kw">false</span>;</span>
<span id="cb25-14"><a href="#cb25-14"></a>    <span class="op">}</span>;</span>
<span id="cb25-15"><a href="#cb25-15"></a></span>
<span id="cb25-16"><a href="#cb25-16"></a>    <span class="kw">template</span> <span class="cf">for</span> <span class="op">(</span><span class="kw">constexpr</span> <span class="kw">auto</span> base <span class="op">:</span> bases_of<span class="op">(^</span>T<span class="op">))</span> <span class="op">{</span></span>
<span id="cb25-17"><a href="#cb25-17"></a>      delim<span class="op">()</span>;</span>
<span id="cb25-18"><a href="#cb25-18"></a>      out <span class="op">=</span> std<span class="op">::</span>format_to<span class="op">(</span>out, <span class="st">&quot;{}&quot;</span>, <span class="kw">static_cast</span><span class="op">&lt;[:</span>base<span class="op">:]</span> <span class="kw">const</span><span class="op">&amp;&gt;(</span>t<span class="op">))</span>;</span>
<span id="cb25-19"><a href="#cb25-19"></a>    <span class="op">}</span></span>
<span id="cb25-20"><a href="#cb25-20"></a></span>
<span id="cb25-21"><a href="#cb25-21"></a>    <span class="kw">template</span> <span class="cf">for</span> <span class="op">(</span><span class="kw">constexpr</span> <span class="kw">auto</span> mem <span class="op">:</span> nonstatic_data_members_of<span class="op">(^</span>T<span class="op">))</span> <span class="op">{</span></span>
<span id="cb25-22"><a href="#cb25-22"></a>      delim<span class="op">()</span>;</span>
<span id="cb25-23"><a href="#cb25-23"></a>      out <span class="op">=</span> std<span class="op">::</span>format_to<span class="op">(</span>out, <span class="st">&quot;.{}={}&quot;</span>, name_of<span class="op">(</span>mem<span class="op">)</span>, t<span class="op">.[:</span>mem<span class="op">:])</span>;</span>
<span id="cb25-24"><a href="#cb25-24"></a>    <span class="op">}</span></span>
<span id="cb25-25"><a href="#cb25-25"></a></span>
<span id="cb25-26"><a href="#cb25-26"></a>    <span class="op">*</span>out<span class="op">++</span> <span class="op">=</span> <span class="ch">&#39;}&#39;</span>;</span>
<span id="cb25-27"><a href="#cb25-27"></a>    <span class="cf">return</span> out;</span>
<span id="cb25-28"><a href="#cb25-28"></a>  <span class="op">}</span></span>
<span id="cb25-29"><a href="#cb25-29"></a><span class="op">}</span>;</span>
<span id="cb25-30"><a href="#cb25-30"></a></span>
<span id="cb25-31"><a href="#cb25-31"></a><span class="kw">struct</span> X <span class="op">{</span> <span class="dt">int</span> m1 <span class="op">=</span> <span class="dv">1</span>; <span class="op">}</span>;</span>
<span id="cb25-32"><a href="#cb25-32"></a><span class="kw">struct</span> Y <span class="op">{</span> <span class="dt">int</span> m2 <span class="op">=</span> <span class="dv">2</span>; <span class="op">}</span>;</span>
<span id="cb25-33"><a href="#cb25-33"></a><span class="kw">class</span> Z <span class="op">:</span> <span class="kw">public</span> X, <span class="kw">private</span> Y <span class="op">{</span> <span class="dt">int</span> m3 <span class="op">=</span> <span class="dv">3</span>; <span class="dt">int</span> m4 <span class="op">=</span> <span class="dv">4</span>; <span class="op">}</span>;</span>
<span id="cb25-34"><a href="#cb25-34"></a></span>
<span id="cb25-35"><a href="#cb25-35"></a><span class="kw">template</span> <span class="op">&lt;&gt;</span> <span class="kw">struct</span> std<span class="op">::</span>formatter<span class="op">&lt;</span>X<span class="op">&gt;</span> <span class="op">:</span> universal_formatter <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb25-36"><a href="#cb25-36"></a><span class="kw">template</span> <span class="op">&lt;&gt;</span> <span class="kw">struct</span> std<span class="op">::</span>formatter<span class="op">&lt;</span>Y<span class="op">&gt;</span> <span class="op">:</span> universal_formatter <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb25-37"><a href="#cb25-37"></a><span class="kw">template</span> <span class="op">&lt;&gt;</span> <span class="kw">struct</span> std<span class="op">::</span>formatter<span class="op">&lt;</span>Z<span class="op">&gt;</span> <span class="op">:</span> universal_formatter <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb25-38"><a href="#cb25-38"></a></span>
<span id="cb25-39"><a href="#cb25-39"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb25-40"><a href="#cb25-40"></a>    std<span class="op">::</span>println<span class="op">(</span><span class="st">&quot;{}&quot;</span>, Z<span class="op">())</span>; <span class="co">// Z{X{.m1 = 1}, Y{.m2 = 2}, .m3 = 3, .m4 = 4}</span></span>
<span id="cb25-41"><a href="#cb25-41"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This example is not implemented on compiler explorer at this time, but only because of issues compiling both <code class="sourceCode cpp">std<span class="op">::</span>format</code> and <code class="sourceCode cpp">fmt<span class="op">::</span>format<span class="op">.</span></code></p>
<h2 data-number="3.13" id="implementing-member-wise-hash_append"><span class="header-section-number">3.13</span> Implementing member-wise <code class="sourceCode cpp">hash_append</code><a href="#implementing-member-wise-hash_append" class="self-link"></a></h2>
<p>Based on the <span class="citation" data-cites="N3980">[<a href="#ref-N3980" role="doc-biblioref">N3980</a>]</span> API:</p>
<blockquote>
<div class="sourceCode" id="cb26"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb26-1"><a href="#cb26-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> H, <span class="kw">typename</span> T<span class="op">&gt;</span> <span class="kw">requires</span> std<span class="op">::</span>is_standard_layout_v<span class="op">&lt;</span>T<span class="op">&gt;</span></span>
<span id="cb26-2"><a href="#cb26-2"></a><span class="dt">void</span> hash_append<span class="op">(</span>H<span class="op">&amp;</span> algo, T <span class="kw">const</span><span class="op">&amp;</span> t<span class="op">)</span> <span class="op">{</span></span>
<span id="cb26-3"><a href="#cb26-3"></a>    <span class="kw">template</span> <span class="cf">for</span> <span class="op">(</span><span class="kw">constexpr</span> <span class="kw">auto</span> mem <span class="op">:</span> nonstatic_data_members_of<span class="op">(^</span>T<span class="op">))</span> <span class="op">{</span></span>
<span id="cb26-4"><a href="#cb26-4"></a>        hash_append<span class="op">(</span>algo, t<span class="op">.[:</span>mem<span class="op">:])</span>;</span>
<span id="cb26-5"><a href="#cb26-5"></a>    <span class="op">}</span></span>
<span id="cb26-6"><a href="#cb26-6"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<h2 data-number="3.14" id="converting-a-struct-to-a-tuple"><span class="header-section-number">3.14</span> Converting a Struct to a Tuple<a href="#converting-a-struct-to-a-tuple" class="self-link"></a></h2>
<p>This approach requires allowing packs in structured bindings <span class="citation" data-cites="P1061R5">[<a href="#ref-P1061R5" role="doc-biblioref">P1061R5</a>]</span>, but can also be written using <code class="sourceCode cpp">std<span class="op">::</span>make_index_sequence</code>:</p>
<blockquote>
<div class="sourceCode" id="cb27"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb27-1"><a href="#cb27-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb27-2"><a href="#cb27-2"></a><span class="kw">constexpr</span> <span class="kw">auto</span> struct_to_tuple<span class="op">(</span>T <span class="kw">const</span><span class="op">&amp;</span> t<span class="op">)</span> <span class="op">{</span></span>
<span id="cb27-3"><a href="#cb27-3"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> members <span class="op">=</span> nonstatic_data_members_of<span class="op">(^</span>T<span class="op">)</span>;</span>
<span id="cb27-4"><a href="#cb27-4"></a></span>
<span id="cb27-5"><a href="#cb27-5"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> indices <span class="op">=</span> <span class="op">[]{</span></span>
<span id="cb27-6"><a href="#cb27-6"></a>    std<span class="op">::</span>array<span class="op">&lt;</span><span class="dt">int</span>, members<span class="op">.</span>size<span class="op">()&gt;</span> indices;</span>
<span id="cb27-7"><a href="#cb27-7"></a>    std<span class="op">::</span>ranges<span class="op">::</span>iota<span class="op">(</span>indices, <span class="dv">0</span><span class="op">)</span>;</span>
<span id="cb27-8"><a href="#cb27-8"></a>    <span class="cf">return</span> indices;</span>
<span id="cb27-9"><a href="#cb27-9"></a>  <span class="op">}()</span>;</span>
<span id="cb27-10"><a href="#cb27-10"></a></span>
<span id="cb27-11"><a href="#cb27-11"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="op">[...</span>Is<span class="op">]</span> <span class="op">=</span> indices;</span>
<span id="cb27-12"><a href="#cb27-12"></a>  <span class="cf">return</span> std<span class="op">::</span>make_tuple<span class="op">(</span>t<span class="op">.[:</span> members<span class="op">[</span>Is<span class="op">]</span> <span class="op">:]...)</span>;</span>
<span id="cb27-13"><a href="#cb27-13"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>An alternative approach is:</p>
<blockquote>
<div class="sourceCode" id="cb28"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb28-1"><a href="#cb28-1"></a><span class="kw">consteval</span> <span class="kw">auto</span> struct_to_tuple_type<span class="op">(</span>info type<span class="op">)</span> <span class="op">-&gt;</span> info <span class="op">{</span></span>
<span id="cb28-2"><a href="#cb28-2"></a>  <span class="cf">return</span> substitute<span class="op">(^</span>std<span class="op">::</span>tuple,</span>
<span id="cb28-3"><a href="#cb28-3"></a>                    nonstatic_data_members_of<span class="op">(</span>type<span class="op">)</span></span>
<span id="cb28-4"><a href="#cb28-4"></a>                    <span class="op">|</span> std<span class="op">::</span>ranges<span class="op">::</span>transform<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>type_of<span class="op">)</span></span>
<span id="cb28-5"><a href="#cb28-5"></a>                    <span class="op">|</span> std<span class="op">::</span>ranges<span class="op">::</span>transform<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>remove_cvref<span class="op">)</span></span>
<span id="cb28-6"><a href="#cb28-6"></a>                    <span class="op">|</span> std<span class="op">::</span>ranges<span class="op">::</span>to<span class="op">&lt;</span>std<span class="op">::</span>vector<span class="op">&gt;())</span>;</span>
<span id="cb28-7"><a href="#cb28-7"></a><span class="op">}</span></span>
<span id="cb28-8"><a href="#cb28-8"></a></span>
<span id="cb28-9"><a href="#cb28-9"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> To, <span class="kw">typename</span> From, std<span class="op">::</span>meta<span class="op">::</span>info <span class="op">...</span> members<span class="op">&gt;</span></span>
<span id="cb28-10"><a href="#cb28-10"></a><span class="kw">constexpr</span> <span class="kw">auto</span> struct_to_tuple_helper<span class="op">(</span>From <span class="kw">const</span><span class="op">&amp;</span> from<span class="op">)</span> <span class="op">-&gt;</span> To <span class="op">{</span></span>
<span id="cb28-11"><a href="#cb28-11"></a>  <span class="cf">return</span> To<span class="op">(</span>from<span class="op">.[:</span>members<span class="op">:]...)</span>;</span>
<span id="cb28-12"><a href="#cb28-12"></a><span class="op">}</span></span>
<span id="cb28-13"><a href="#cb28-13"></a></span>
<span id="cb28-14"><a href="#cb28-14"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> From<span class="op">&gt;</span></span>
<span id="cb28-15"><a href="#cb28-15"></a><span class="kw">consteval</span> <span class="kw">auto</span> get_struct_to_tuple_helper<span class="op">()</span> <span class="op">{</span></span>
<span id="cb28-16"><a href="#cb28-16"></a>  <span class="kw">using</span> To <span class="op">=</span> <span class="op">[:</span> struct_to_tuple_type<span class="op">(^</span>From<span class="op">):</span> <span class="op">]</span>;</span>
<span id="cb28-17"><a href="#cb28-17"></a></span>
<span id="cb28-18"><a href="#cb28-18"></a>  std<span class="op">::</span>vector args <span class="op">=</span> <span class="op">{^</span>To, <span class="op">^</span>From<span class="op">}</span>;</span>
<span id="cb28-19"><a href="#cb28-19"></a>  <span class="cf">for</span> <span class="op">(</span><span class="kw">auto</span> mem <span class="op">:</span> nonstatic_data_members_of<span class="op">(^</span>From<span class="op">))</span> <span class="op">{</span></span>
<span id="cb28-20"><a href="#cb28-20"></a>    args<span class="op">.</span>push_back<span class="op">(</span>reflect_value<span class="op">(</span>mem<span class="op">))</span>;</span>
<span id="cb28-21"><a href="#cb28-21"></a>  <span class="op">}</span></span>
<span id="cb28-22"><a href="#cb28-22"></a></span>
<span id="cb28-23"><a href="#cb28-23"></a>  <span class="co">/*</span></span>
<span id="cb28-24"><a href="#cb28-24"></a><span class="co">  Alternatively, with Ranges:</span></span>
<span id="cb28-25"><a href="#cb28-25"></a><span class="co">  args.append_range(</span></span>
<span id="cb28-26"><a href="#cb28-26"></a><span class="co">    nonstatic_data_members_of(^From)</span></span>
<span id="cb28-27"><a href="#cb28-27"></a><span class="co">    | std::views::transform(std::meta::reflect_value)</span></span>
<span id="cb28-28"><a href="#cb28-28"></a><span class="co">    );</span></span>
<span id="cb28-29"><a href="#cb28-29"></a><span class="co">  */</span></span>
<span id="cb28-30"><a href="#cb28-30"></a></span>
<span id="cb28-31"><a href="#cb28-31"></a>  <span class="cf">return</span> value_of<span class="op">&lt;</span>To<span class="op">(*)(</span>From <span class="kw">const</span><span class="op">&amp;)&gt;(</span></span>
<span id="cb28-32"><a href="#cb28-32"></a>    substitute<span class="op">(^</span>struct_to_tuple_helper, args<span class="op">))</span>;</span>
<span id="cb28-33"><a href="#cb28-33"></a><span class="op">}</span></span>
<span id="cb28-34"><a href="#cb28-34"></a></span>
<span id="cb28-35"><a href="#cb28-35"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> From<span class="op">&gt;</span></span>
<span id="cb28-36"><a href="#cb28-36"></a><span class="kw">constexpr</span> <span class="kw">auto</span> struct_to_tuple<span class="op">(</span>From <span class="kw">const</span><span class="op">&amp;</span> from<span class="op">)</span> <span class="op">{</span></span>
<span id="cb28-37"><a href="#cb28-37"></a>  <span class="cf">return</span> get_struct_to_tuple_helper<span class="op">&lt;</span>From<span class="op">&gt;()(</span>from<span class="op">)</span>;</span>
<span id="cb28-38"><a href="#cb28-38"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Here, <code class="sourceCode cpp">struct_to_tuple_type</code> takes a reflection of a type like <code class="sourceCode cpp"><span class="kw">struct</span> <span class="op">{</span> T t; U <span class="kw">const</span><span class="op">&amp;</span> u; V v; <span class="op">}</span></code> and returns a reflection of the type <code class="sourceCode cpp">std<span class="op">::</span>tuple<span class="op">&lt;</span>T, U, V<span class="op">&gt;</span></code>. That gives us the return type. Then, <code class="sourceCode cpp">struct_to_tuple_helper</code> is a function template that does the actual conversion — which it can do by having all the reflections of the members as a non-type template parameter pack. This is a <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function and not a <code class="sourceCode cpp"><span class="kw">consteval</span></code> function because in the general case the conversion is a run-time operation. However, determining the instance of <code class="sourceCode cpp">struct_to_tuple_helper</code> that is needed is a compile-time operation and has to be performed with a <code class="sourceCode cpp"><span class="kw">consteval</span></code> function (because the function invokes <code class="sourceCode cpp">nonstatic_data_members_of</code>), hence the separate function template <code class="sourceCode cpp">get_struct_to_tuple_helper<span class="op">()</span></code>.</p>
<p>Everything is put together by using <code class="sourceCode cpp">substitute</code> to create the instantiation of <code class="sourceCode cpp">struct_to_tuple_helper</code> that we need, and a compile-time reference to that instance is obtained with <code class="sourceCode cpp">value_of</code>. Thus <code class="sourceCode cpp">f</code> is a function reference to the correct specialization of <code class="sourceCode cpp">struct_to_tuple_helper</code>, which we can simply invoke.</p>
<p><a href="https://godbolt.org/z/Moqf84nc1">On Compiler Explorer</a>, with a different implementation than either of the above.</p>
<h2 data-number="3.15" id="compile-time-ticket-counter"><span class="header-section-number">3.15</span> Compile-Time Ticket Counter<a href="#compile-time-ticket-counter" class="self-link"></a></h2>
<p>The features proposed here make it a little easier to update a ticket counter at compile time. This is not an ideal implementation (we’d prefer direct support for compile-time —– i.e., <code class="sourceCode cpp"><span class="kw">consteval</span></code> — variables), but it shows how compile-time mutable state surfaces in new ways.</p>
<blockquote>
<div class="sourceCode" id="cb29"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb29-1"><a href="#cb29-1"></a><span class="kw">class</span> TU_Ticket <span class="op">{</span></span>
<span id="cb29-2"><a href="#cb29-2"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="dt">int</span> N<span class="op">&gt;</span> <span class="kw">struct</span> Helper <span class="op">{</span></span>
<span id="cb29-3"><a href="#cb29-3"></a>    <span class="kw">static</span> <span class="kw">constexpr</span> <span class="dt">int</span> value <span class="op">=</span> N;</span>
<span id="cb29-4"><a href="#cb29-4"></a>  <span class="op">}</span>;</span>
<span id="cb29-5"><a href="#cb29-5"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb29-6"><a href="#cb29-6"></a>  <span class="kw">static</span> <span class="kw">consteval</span> <span class="dt">int</span> next<span class="op">()</span> <span class="op">{</span></span>
<span id="cb29-7"><a href="#cb29-7"></a>    <span class="co">// Search for the next incomplete Helper&lt;k&gt;.</span></span>
<span id="cb29-8"><a href="#cb29-8"></a>    std<span class="op">::</span>meta<span class="op">::</span>info r;</span>
<span id="cb29-9"><a href="#cb29-9"></a>    <span class="cf">for</span> <span class="op">(</span><span class="dt">int</span> k <span class="op">=</span> <span class="dv">0</span>;; <span class="op">++</span>k<span class="op">)</span> <span class="op">{</span></span>
<span id="cb29-10"><a href="#cb29-10"></a>      r <span class="op">=</span> substitute<span class="op">(^</span>Helper, <span class="op">{</span> std<span class="op">::</span>meta<span class="op">::</span>reflect_value<span class="op">(</span>k<span class="op">)</span> <span class="op">})</span>;</span>
<span id="cb29-11"><a href="#cb29-11"></a>      <span class="cf">if</span> <span class="op">(</span>is_incomplete_type<span class="op">(</span>r<span class="op">))</span> <span class="cf">break</span>;</span>
<span id="cb29-12"><a href="#cb29-12"></a>    <span class="op">}</span></span>
<span id="cb29-13"><a href="#cb29-13"></a>    <span class="co">// Return the value of its member.  Calling static_data_members_of</span></span>
<span id="cb29-14"><a href="#cb29-14"></a>    <span class="co">// triggers the instantiation (i.e., completion) of Helper&lt;k&gt;.</span></span>
<span id="cb29-15"><a href="#cb29-15"></a>    <span class="cf">return</span> value_of<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;(</span>static_data_members_of<span class="op">(</span>r<span class="op">)[</span><span class="dv">0</span><span class="op">])</span>;</span>
<span id="cb29-16"><a href="#cb29-16"></a>  <span class="op">}</span></span>
<span id="cb29-17"><a href="#cb29-17"></a><span class="op">}</span>;</span>
<span id="cb29-18"><a href="#cb29-18"></a></span>
<span id="cb29-19"><a href="#cb29-19"></a><span class="dt">int</span> x <span class="op">=</span> TU_Ticket<span class="op">::</span>next<span class="op">()</span>;  <span class="co">// x initialized to 0.</span></span>
<span id="cb29-20"><a href="#cb29-20"></a><span class="dt">int</span> y <span class="op">=</span> TU_Ticket<span class="op">::</span>next<span class="op">()</span>;  <span class="co">// y initialized to 1.</span></span>
<span id="cb29-21"><a href="#cb29-21"></a><span class="dt">int</span> z <span class="op">=</span> TU_Ticket<span class="op">::</span>next<span class="op">()</span>;  <span class="co">// z initialized to 2.</span></span></code></pre></div>
</blockquote>
<p>Note that this relies on the fact that a call to <code class="sourceCode cpp">substitute</code> returns a specialization of a template, but doesn’t trigger the instantiation of that specialization. Thus, the only instantiations of <code class="sourceCode cpp">TU_Ticket<span class="op">::</span>Helper</code> occur because of the call to <code class="sourceCode cpp">nonstatic_data_members_of</code> (which is a singleton representing the lone <code class="sourceCode cpp">value</code> member).</p>
<p><a href="https://godbolt.org/z/1vEjW4sTr">On Compiler Explorer</a>.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="proposed-features"><span class="header-section-number">4</span> Proposed Features<a href="#proposed-features" class="self-link"></a></h1>
<h2 data-number="4.1" id="the-reflection-operator"><span class="header-section-number">4.1</span> The Reflection Operator (<code class="sourceCode cpp"><span class="op">^</span></code>)<a href="#the-reflection-operator" class="self-link"></a></h2>
<p>The reflection operator produces a reflection value from a grammatical construct (its operand):</p>
<blockquote>
<div class="line-block"><em>unary-expression</em>:<br />
      …<br />
      <code class="sourceCode cpp"><span class="op">^</span></code> <code class="sourceCode cpp"><span class="op">::</span></code><br />
      <code class="sourceCode cpp"><span class="op">^</span></code> <em>namespace-name</em><br />
      <code class="sourceCode cpp"><span class="op">^</span></code> <em>type-id</em><br />
      <code class="sourceCode cpp"><span class="op">^</span></code> <em>cast-expression</em></div>
</blockquote>
<p>Note that <em>cast-expression</em> includes <em>id-expression</em>, which in turn can designate templates, member names, etc.</p>
<p>The current proposal requires that the <em>cast-expression</em> be:</p>
<ul>
<li>a <em>primary-expression</em> referring to a function or member function, or</li>
<li>a <em>primary-expression</em> referring to a variable, static data member, or structured binding, or</li>
<li>a <em>primary-expression</em> referring to a nonstatic data member, or</li>
<li>a <em>primary-expression</em> referring to a template, or</li>
<li>a constant-expression.</li>
</ul>
<p>In a SFINAE context, a failure to substitute the operand of a reflection operator construct causes that construct to not evaluate to constant.</p>
<h2 data-number="4.2" id="splicers"><span class="header-section-number">4.2</span> Splicers (<code class="sourceCode cpp"><span class="op">[:</span></code>…<code class="sourceCode cpp"><span class="op">:]</span></code>)<a href="#splicers" class="self-link"></a></h2>
<p>A reflection can be “spliced” into source code using one of several <em>splicer</em> forms:</p>
<ul>
<li><code class="sourceCode cpp"><span class="op">[:</span> r <span class="op">:]</span></code> produces an <em>expression</em> evaluating to the entity or constant value represented by <code class="sourceCode cpp">r</code>.</li>
<li><code class="sourceCode cpp"><span class="kw">typename</span><span class="op">[:</span> r <span class="op">:]</span></code> produces a <em>simple-type-specifier</em> corresponding to the type represented by <code class="sourceCode cpp">r</code>.</li>
<li><code class="sourceCode cpp"><span class="kw">template</span><span class="op">[:</span> r <span class="op">:]</span></code> produces a <em>template-name</em> corresponding to the template represented by <code class="sourceCode cpp">r</code>.</li>
<li><code class="sourceCode cpp"><span class="kw">namespace</span><span class="op">[:</span> r <span class="op">:]</span></code> produces a <em>namespace-name</em> corresponding to the namespace represented by <code class="sourceCode cpp">r</code>.</li>
<li><code class="sourceCode cpp"><span class="op">[:</span>r<span class="op">:]::</span></code> produces a <em>nested-name-specifier</em> corresponding to the namespace, enumeration type, or class type represented by <code class="sourceCode cpp">r</code>.</li>
</ul>
<p>The operand of a splicer is implicitly converted to a <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code> prvalue (i.e., if the operand expression has a class type that with a conversion function to convert to <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code>, splicing can still work.)</p>
<p>Attempting to splice a reflection value that does not meet the requirement of the splice is ill-formed. For example:</p>
<blockquote>
<div class="sourceCode" id="cb30"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb30-1"><a href="#cb30-1"></a><span class="kw">typename</span><span class="op">[:</span> <span class="op">^::</span> <span class="op">:]</span> x <span class="op">=</span> <span class="dv">0</span>;  <span class="co">// Error.</span></span></code></pre></div>
</blockquote>
<p>(This proposal does not at this time propose range-based splicers as described in P1240. We still believe that those are desirable. However, they are more complex to implement and they involve syntactic choices that benefit from being considered along with other proposals that introduce pack-like constructs in non-template contexts. Meanwhile, we found that many very useful techniques are enabled with just the basic splicers presented here.)</p>
<h3 data-number="4.2.1" id="range-splicers"><span class="header-section-number">4.2.1</span> Range Splicers<a href="#range-splicers" class="self-link"></a></h3>
<p>The splicers described above all take a single object of type <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code> (described in more detail below). However, there are many cases where we don’t have a single reflection, we have a range of reflections - and we want to splice them all in one go. For that, we need a different form of splicer: a range splicer.</p>
<p>Construct the <a href="#converting-a-struct-to-a-tuple">struct-to-tuple</a> example from above. It was demonstrates using a single splice, but it would be simpler if we had a range splice:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>With Single Splice</strong>
</div></th>
<th><div style="text-align:center">
<strong>With Range Splice</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb31"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb31-1"><a href="#cb31-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb31-2"><a href="#cb31-2"></a><span class="kw">constexpr</span> <span class="kw">auto</span> struct_to_tuple<span class="op">(</span>T <span class="kw">const</span><span class="op">&amp;</span> t<span class="op">)</span> <span class="op">{</span></span>
<span id="cb31-3"><a href="#cb31-3"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> members <span class="op">=</span> nonstatic_data_members_of<span class="op">(^</span>T<span class="op">)</span>;</span>
<span id="cb31-4"><a href="#cb31-4"></a></span>
<span id="cb31-5"><a href="#cb31-5"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> indices <span class="op">=</span> <span class="op">[]{</span></span>
<span id="cb31-6"><a href="#cb31-6"></a>    std<span class="op">::</span>array<span class="op">&lt;</span><span class="dt">int</span>, members<span class="op">.</span>size<span class="op">()&gt;</span> indices;</span>
<span id="cb31-7"><a href="#cb31-7"></a>    std<span class="op">::</span>ranges<span class="op">::</span>iota<span class="op">(</span>indices, <span class="dv">0</span><span class="op">)</span>;</span>
<span id="cb31-8"><a href="#cb31-8"></a>    <span class="cf">return</span> indices;</span>
<span id="cb31-9"><a href="#cb31-9"></a>  <span class="op">}()</span>;</span>
<span id="cb31-10"><a href="#cb31-10"></a></span>
<span id="cb31-11"><a href="#cb31-11"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="op">[...</span>Is<span class="op">]</span> <span class="op">=</span> indices;</span>
<span id="cb31-12"><a href="#cb31-12"></a>  <span class="cf">return</span> std<span class="op">::</span>make_tuple<span class="op">(</span>t<span class="op">.[:</span> members<span class="op">[</span>Is<span class="op">]</span> <span class="op">:]...)</span>;</span>
<span id="cb31-13"><a href="#cb31-13"></a><span class="op">}</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb32"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb32-1"><a href="#cb32-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb32-2"><a href="#cb32-2"></a><span class="kw">constexpr</span> <span class="kw">auto</span> struct_to_tuple<span class="op">(</span>T <span class="kw">const</span><span class="op">&amp;</span> t<span class="op">)</span> <span class="op">{</span></span>
<span id="cb32-3"><a href="#cb32-3"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> members <span class="op">=</span> nonstatic_data_members_of<span class="op">(^</span>T<span class="op">)</span>;</span>
<span id="cb32-4"><a href="#cb32-4"></a>  <span class="cf">return</span> std<span class="op">::</span>make_tuple<span class="op">(</span>t<span class="op">.[:</span> <span class="op">...</span>members <span class="op">:]...)</span>;</span>
<span id="cb32-5"><a href="#cb32-5"></a><span class="op">}</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>A range splice, <code class="sourceCode cpp"><span class="op">[:</span> <span class="op">...</span> r <span class="op">:]</span></code>, would accept as its argument a constant range of <code class="sourceCode cpp">meta<span class="op">::</span>info</code>, <code class="sourceCode cpp">r</code>, and would behave as an unexpanded pack of splices. So the above expression</p>
<blockquote>
<div class="sourceCode" id="cb33"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb33-1"><a href="#cb33-1"></a>make_tuple<span class="op">(</span>t<span class="op">.[:</span> <span class="op">...</span> members <span class="op">:]...)</span></span></code></pre></div>
</blockquote>
<p>would evaluate as</p>
<blockquote>
<div class="sourceCode" id="cb34"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb34-1"><a href="#cb34-1"></a>make_tuple<span class="op">(</span>t<span class="op">.[:</span>members<span class="op">[</span><span class="dv">0</span><span class="op">]:]</span>, t<span class="op">.[:</span>members<span class="op">[</span><span class="dv">1</span><span class="op">]:]</span>, <span class="op">...</span>, t<span class="op">.[:</span>members<span class="op">[</span><em>N-1</em><span class="op">]:])</span></span></code></pre></div>
</blockquote>
<p>This is a very useful facility indeed!</p>
<p>However, range splicing of dependent arguments is at least an order of magnitude harder to implement than ordinary splicing. We think that not including range splicing gives us a better chance of having reflection in C++26. Especially since, as this paper’s examples demonstrate, a lot can be done without them.</p>
<p>Another way to work around a lack of range splicing would be to implement <code class="sourceCode cpp">with_size<span class="op">&lt;</span>N<span class="op">&gt;(</span>f<span class="op">)</span></code>, which would behave like <code class="sourceCode cpp">f<span class="op">(</span>integral_constant<span class="op">&lt;</span><span class="dt">size_t</span>, <span class="dv">0</span><span class="op">&gt;{}</span>, integral_constant<span class="op">&lt;</span><span class="dt">size_t</span>, <span class="dv">0</span><span class="op">&gt;{}</span>, <span class="op">...</span>, integral_constant<span class="op">&lt;</span><span class="dt">size_t</span>, N<span class="op">-</span><span class="dv">1</span><span class="op">&gt;{})</span></code>. Which is enough for a tolerable implementation:</p>
<blockquote>
<div class="sourceCode" id="cb35"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb35-1"><a href="#cb35-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb35-2"><a href="#cb35-2"></a><span class="kw">constexpr</span> <span class="kw">auto</span> struct_to_tuple<span class="op">(</span>T <span class="kw">const</span><span class="op">&amp;</span> t<span class="op">)</span> <span class="op">{</span></span>
<span id="cb35-3"><a href="#cb35-3"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> members <span class="op">=</span> nonstatic_data_members_of<span class="op">(^</span>T<span class="op">)</span>;</span>
<span id="cb35-4"><a href="#cb35-4"></a>  <span class="cf">return</span> with_size<span class="op">&lt;</span>members<span class="op">.</span>size<span class="op">()&gt;([&amp;](</span><span class="kw">auto</span><span class="op">...</span> Is<span class="op">){</span></span>
<span id="cb35-5"><a href="#cb35-5"></a>    <span class="cf">return</span> std<span class="op">::</span>make_tuple<span class="op">(</span>t<span class="op">.[:</span> members<span class="op">[</span>Is<span class="op">]</span> <span class="op">:]...)</span>;</span>
<span id="cb35-6"><a href="#cb35-6"></a>  <span class="op">})</span>;</span>
<span id="cb35-7"><a href="#cb35-7"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<h2 data-number="4.3" id="stdmetainfo"><span class="header-section-number">4.3</span> <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code><a href="#stdmetainfo" class="self-link"></a></h2>
<p>The type <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code> can be defined as follows:</p>
<blockquote>
<div class="sourceCode" id="cb36"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb36-1"><a href="#cb36-1"></a><span class="kw">namespace</span> std <span class="op">{</span></span>
<span id="cb36-2"><a href="#cb36-2"></a>  <span class="kw">namespace</span> meta <span class="op">{</span></span>
<span id="cb36-3"><a href="#cb36-3"></a>    <span class="kw">using</span> info <span class="op">=</span> <span class="kw">decltype</span><span class="op">(^</span><span class="dt">int</span><span class="op">)</span>;</span>
<span id="cb36-4"><a href="#cb36-4"></a>  <span class="op">}</span></span>
<span id="cb36-5"><a href="#cb36-5"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>In our initial proposal a value of type <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code> can represent:</p>
<ul>
<li>any (C++) type and type-alias</li>
<li>any function or member function</li>
<li>any variable, static data member, or structured binding</li>
<li>any non-static data member</li>
<li>any constant value</li>
<li>any template</li>
<li>any namespace</li>
</ul>
<p>Notably absent at this time are general non-constant expressions (that aren’t <em>expression-id</em>s referring to functions, variables or structured bindings). For example:</p>
<blockquote>
<div class="sourceCode" id="cb37"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb37-1"><a href="#cb37-1"></a><span class="dt">int</span> x <span class="op">=</span> <span class="dv">0</span>;</span>
<span id="cb37-2"><a href="#cb37-2"></a><span class="dt">void</span> g<span class="op">()</span> <span class="op">{</span></span>
<span id="cb37-3"><a href="#cb37-3"></a>  <span class="op">[:^</span>x<span class="op">:]</span> <span class="op">=</span> <span class="dv">42</span>;     <span class="co">// Okay.  Same as: x = 42;</span></span>
<span id="cb37-4"><a href="#cb37-4"></a>  x <span class="op">=</span> <span class="op">[:^(</span><span class="dv">2</span><span class="op">*</span>x<span class="op">):]</span>;  <span class="co">// Error: &quot;2*x&quot; is a general non-constant expression.</span></span>
<span id="cb37-5"><a href="#cb37-5"></a>  <span class="kw">constexpr</span> <span class="dt">int</span> N <span class="op">=</span> <span class="dv">42</span>;</span>
<span id="cb37-6"><a href="#cb37-6"></a>  x <span class="op">=</span> <span class="op">[:^(</span><span class="dv">2</span><span class="op">*</span>N<span class="op">):]</span>;  <span class="co">// Okay: &quot;2*N&quot; is a constant-expression.</span></span>
<span id="cb37-7"><a href="#cb37-7"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Note that for <code class="sourceCode cpp"><span class="op">^(</span><span class="dv">2</span><span class="op">*</span>N<span class="op">)</span></code> an implementation only has to capture the constant value of <code class="sourceCode cpp"><span class="dv">2</span><span class="op">*</span>N</code> and not various other properties of the underlying expression (such as any temporaries it involves, etc.).</p>
<p>The type <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code> is a <em>scalar</em> type. Nontype template arguments of type <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code> are permitted. The entity being reflected can affect the linkage of a template instance involving a reflection. For example:</p>
<blockquote>
<div class="sourceCode" id="cb38"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb38-1"><a href="#cb38-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">auto</span> R<span class="op">&gt;</span> <span class="kw">struct</span> S <span class="op">{}</span>;</span>
<span id="cb38-2"><a href="#cb38-2"></a></span>
<span id="cb38-3"><a href="#cb38-3"></a><span class="kw">extern</span> <span class="dt">int</span> x;</span>
<span id="cb38-4"><a href="#cb38-4"></a><span class="kw">static</span> <span class="dt">int</span> y;</span>
<span id="cb38-5"><a href="#cb38-5"></a></span>
<span id="cb38-6"><a href="#cb38-6"></a>S<span class="op">&lt;^</span>x<span class="op">&gt;</span> sx;  <span class="co">// S&lt;^x&gt; has external name linkage.</span></span>
<span id="cb38-7"><a href="#cb38-7"></a>S<span class="op">&lt;^</span>y<span class="op">&gt;</span> sy;  <span class="co">// S&lt;^y&gt; has internal name linkage.</span></span></code></pre></div>
</blockquote>
<p>Namespace <code class="sourceCode cpp">std<span class="op">::</span>meta</code> is associated with type <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code>: That allows the core meta functions to be invoked without explicit qualification. For example:</p>
<blockquote>
<div class="sourceCode" id="cb39"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb39-1"><a href="#cb39-1"></a><span class="pp">#include </span><span class="im">&lt;meta&gt;</span></span>
<span id="cb39-2"><a href="#cb39-2"></a><span class="kw">struct</span> S <span class="op">{}</span>;</span>
<span id="cb39-3"><a href="#cb39-3"></a>std<span class="op">::</span>string name2 <span class="op">=</span> std<span class="op">::</span>meta<span class="op">::</span>name_of<span class="op">(^</span>S<span class="op">)</span>;  <span class="co">// Okay.</span></span>
<span id="cb39-4"><a href="#cb39-4"></a>std<span class="op">::</span>string name1 <span class="op">=</span> name_of<span class="op">(^</span>S<span class="op">)</span>;             <span class="co">// Also okay.</span></span></code></pre></div>
</blockquote>
<h2 data-number="4.4" id="metafunctions"><span class="header-section-number">4.4</span> Metafunctions<a href="#metafunctions" class="self-link"></a></h2>
<p>We propose a number of metafunctions declared in namespace <code class="sourceCode cpp">std<span class="op">::</span>meta</code> to operator on reflection values. Adding metafunctions to an implementation is expected to be relatively “easy” compared to implementing the core language features described previously. However, despite offering a normal consteval C++ function interface, each on of these relies on “compiler magic” to a significant extent.</p>
<h3 data-number="4.4.1" id="error-handling-in-reflection"><span class="header-section-number">4.4.1</span> Error-Handling in Reflection<a href="#error-handling-in-reflection" class="self-link"></a></h3>
<p>One important question we have to answer is: How do we handle errors in reflection metafunctions? For example, what does <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>template_of<span class="op">(^</span><span class="dt">int</span><span class="op">)</span></code> do? <code class="sourceCode cpp"><span class="op">^</span><span class="dt">int</span></code> is a reflection of a type, but that type is not a specialization of a template, so there is no valid reflected template for us to return.</p>
<p>There are a few options available to us today:</p>
<ol type="1">
<li>This fails to be a constant expression (unspecified mechanism).</li>
<li>This returns an invalid reflection (similar to <code class="sourceCode cpp">NaN</code> for floating point) which carries source location info and some useful message. (This was the approach suggested in P1240.)</li>
<li>This returns <code class="sourceCode cpp">std<span class="op">::</span>expected<span class="op">&lt;</span>std<span class="op">::</span>meta<span class="op">::</span>info, E<span class="op">&gt;</span></code> for some reflection-specific error type <code class="sourceCode cpp">E</code> which carries source location info and some useful message (this could be just <code class="sourceCode cpp">info</code> but probably should not be).</li>
<li>This throws an exception of type <code class="sourceCode cpp">E</code> (which requires allowing exceptions to work during <code class="sourceCode cpp"><span class="kw">constexpr</span></code> evaluation, such that an uncaught exception would fail to be a constant exception).</li>
</ol>
<p>The immediate downside of (2), yielding a <code class="sourceCode cpp">NaN</code>-like reflection for <code class="sourceCode cpp">template_of<span class="op">(^</span><span class="dt">int</span><span class="op">)</span></code> is what we do for those functions that need to return a range. That is, what does <code class="sourceCode cpp">template_arguments_of<span class="op">(^</span><span class="dt">int</span><span class="op">)</span></code> return?</p>
<ol type="1">
<li>This fails to be a constant expression (unspecified mechanism).</li>
<li>This returns a <code class="sourceCode cpp">std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>meta<span class="op">::</span>info<span class="op">&gt;</span></code> containing one invalid reflection.</li>
<li>This returns a <code class="sourceCode cpp">std<span class="op">::</span>expected<span class="op">&lt;</span>std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>meta<span class="op">::</span>info<span class="op">&gt;</span>, E<span class="op">&gt;</span></code>.</li>
<li>This throws an exception of type <code class="sourceCode cpp">E</code>.</li>
</ol>
<p>Having range-based functions return a single invalid reflection would make for awkward error handling code. Using <code class="sourceCode cpp">std<span class="op">::</span>expected</code> or exceptions for error handling allow for a consistent, more straightforward interface.</p>
<p>This becomes another situation where we need to decide an error handling mechanism between exceptions and not exceptions, although importantly in this context a lot of usual concerns about exceptions do not apply:</p>
<ul>
<li>there is no runtime (so concerns about runtime performance, object file size, etc. do not exist), and</li>
<li>there is no runtime (so concerns about code evolving to add a new uncaught exception type do not apply)</li>
</ul>
<p>There is one interesting example to consider to decide between <code class="sourceCode cpp">std<span class="op">::</span>expected</code> and exceptions here:</p>
<blockquote>
<div class="sourceCode" id="cb40"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb40-1"><a href="#cb40-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb40-2"><a href="#cb40-2"></a>  <span class="kw">requires</span> <span class="op">(</span>template_of<span class="op">(^</span>T<span class="op">)</span> <span class="op">==</span> <span class="op">^</span>std<span class="op">::</span>optional<span class="op">)</span></span>
<span id="cb40-3"><a href="#cb40-3"></a><span class="dt">void</span> foo<span class="op">()</span>;</span></code></pre></div>
</blockquote>
<p>If <code class="sourceCode cpp">template_of</code> returns an <code class="sourceCode cpp">excepted<span class="op">&lt;</span>info, E<span class="op">&gt;</span></code>, then <code class="sourceCode cpp">foo<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span></code> is a substitution failure — <code class="sourceCode cpp">expected<span class="op">&lt;</span>T, E<span class="op">&gt;</span></code> is equality-comparable to <code class="sourceCode cpp">T</code>, that comparison would evaluate to <code class="sourceCode cpp"><span class="kw">false</span></code> but still be a constant expression.</p>
<p>If <code class="sourceCode cpp">template_of</code> returns <code class="sourceCode cpp">info</code> but throws an exception, then <code class="sourceCode cpp">foo<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span></code> would cause that exception to be uncaught, which would make the comparison not a constant expression. This actually makes the constraint ill-formed - not a substitution failure. In order to have <code class="sourceCode cpp">foo<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span></code> be a substitution failure, either the constraint would have to first check that <code class="sourceCode cpp">T</code> is a template or we would have to change the language rule that requires constraints to be constant expressions (we would of course still keep the requirement that the constraint is a <code class="sourceCode cpp"><span class="dt">bool</span></code>).</p>
<p>The other thing to consider are compiler modes that disable exception support (like <code class="sourceCode cpp"><span class="op">-</span>fno<span class="op">-</span>exceptions</code> in GCC and Clang). Today, implementations reject using <code class="sourceCode cpp"><span class="cf">try</span></code>, <code class="sourceCode cpp"><span class="cf">catch</span></code>, or <code class="sourceCode cpp"><span class="cf">throw</span></code> at all when such modes are enabled. With support for <code class="sourceCode cpp"><span class="kw">constexpr</span></code> exceptions, implementations would have to come up with a strategy for how to support compile-time exceptions — probably by only allowing them in <code class="sourceCode cpp"><span class="kw">consteval</span></code> functions (including <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function templates that were propagated to <code class="sourceCode cpp"><span class="kw">consteval</span></code>).</p>
<p>Despite these concerns (and the requirement of a whole new language feature), we believe that exceptions will be the more user-friendly choice for error handling here, simply because exceptions are more ergonomic to use than <code class="sourceCode cpp">std<span class="op">::</span>expected</code> (even if we adopt language features that make this type easier to use - like pattern matching and a control flow operator).</p>
<h3 data-number="4.4.2" id="synopsis"><span class="header-section-number">4.4.2</span> Synopsis<a href="#synopsis" class="self-link"></a></h3>
<p>Here is a synopsis for the proposed library API. The functions will be explained below.</p>
<blockquote>
<div class="sourceCode" id="cb41"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb41-1"><a href="#cb41-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb41-2"><a href="#cb41-2"></a>  <span class="co">// <a href="#name_of-display_name_of-source_location_of">name and location</a></span></span>
<span id="cb41-3"><a href="#cb41-3"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> name_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> string_view;</span>
<span id="cb41-4"><a href="#cb41-4"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> display_name_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> string_view;</span>
<span id="cb41-5"><a href="#cb41-5"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> source_location_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> source_location;</span>
<span id="cb41-6"><a href="#cb41-6"></a></span>
<span id="cb41-7"><a href="#cb41-7"></a>  <span class="co">// <a href="#type_of-parent_of-dealias">type queries</a></span></span>
<span id="cb41-8"><a href="#cb41-8"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> type_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb41-9"><a href="#cb41-9"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> parent_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb41-10"><a href="#cb41-10"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> dealias<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb41-11"><a href="#cb41-11"></a></span>
<span id="cb41-12"><a href="#cb41-12"></a>  <span class="co">// <a href="#template_of-template_arguments_of">template queries</a></span></span>
<span id="cb41-13"><a href="#cb41-13"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> template_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb41-14"><a href="#cb41-14"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> template_arguments_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb41-15"><a href="#cb41-15"></a></span>
<span id="cb41-16"><a href="#cb41-16"></a>  <span class="co">// <a href="#members_of-static_data_members_of-nonstatic_data_members_of-bases_of-enumerators_of-subobjects_of">member queries</a></span></span>
<span id="cb41-17"><a href="#cb41-17"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> <span class="op">...</span>Fs<span class="op">&gt;</span></span>
<span id="cb41-18"><a href="#cb41-18"></a>    <span class="kw">consteval</span> <span class="kw">auto</span> members_of<span class="op">(</span>info class_type, Fs <span class="op">...</span>filters<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb41-19"><a href="#cb41-19"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> <span class="op">...</span>Fs<span class="op">&gt;</span></span>
<span id="cb41-20"><a href="#cb41-20"></a>    <span class="kw">consteval</span> <span class="kw">auto</span> bases_of<span class="op">(</span>info class_type, Fs <span class="op">...</span>filters<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb41-21"><a href="#cb41-21"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> static_data_members_of<span class="op">(</span>info class_type<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb41-22"><a href="#cb41-22"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> nonstatic_data_members_of<span class="op">(</span>info class_type<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb41-23"><a href="#cb41-23"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> subobjects_of<span class="op">(</span>info class_type<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb41-24"><a href="#cb41-24"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> enumerators_of<span class="op">(</span>info enum_type<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb41-25"><a href="#cb41-25"></a></span>
<span id="cb41-26"><a href="#cb41-26"></a>  <span class="co">// <a href="#substitute">substitute</a></span></span>
<span id="cb41-27"><a href="#cb41-27"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> substitute<span class="op">(</span>info templ, span<span class="op">&lt;</span>info <span class="kw">const</span><span class="op">&gt;</span> args<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb41-28"><a href="#cb41-28"></a></span>
<span id="cb41-29"><a href="#cb41-29"></a>   <span class="co">// <a href="#value_oft">value_of<T></a></span></span>
<span id="cb41-30"><a href="#cb41-30"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb41-31"><a href="#cb41-31"></a>    <span class="kw">consteval</span> <span class="kw">auto</span> value_of<span class="op">(</span>info<span class="op">)</span> <span class="op">-&gt;</span> T;</span>
<span id="cb41-32"><a href="#cb41-32"></a></span>
<span id="cb41-33"><a href="#cb41-33"></a>  <span class="co">// <a href="#test_type-test_types">test_type</a></span></span>
<span id="cb41-34"><a href="#cb41-34"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> test_type<span class="op">(</span>info templ, info type<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-35"><a href="#cb41-35"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> test_types<span class="op">(</span>info templ, span<span class="op">&lt;</span>info <span class="kw">const</span><span class="op">&gt;</span> types<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-36"><a href="#cb41-36"></a></span>
<span id="cb41-37"><a href="#cb41-37"></a>  <span class="co">// <a href="#other-singular-reflection-predicates">other type predicates</a></span></span>
<span id="cb41-38"><a href="#cb41-38"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_public<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-39"><a href="#cb41-39"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_protected<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-40"><a href="#cb41-40"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_private<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-41"><a href="#cb41-41"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_accessible<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-42"><a href="#cb41-42"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_virtual<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-43"><a href="#cb41-43"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_deleted<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-44"><a href="#cb41-44"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_defaulted<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-45"><a href="#cb41-45"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_explicit<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-46"><a href="#cb41-46"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_override<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-47"><a href="#cb41-47"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_pure_virtual<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-48"><a href="#cb41-48"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_bit_field<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-49"><a href="#cb41-49"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> has_static_storage_duration<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-50"><a href="#cb41-50"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_nsdm<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-51"><a href="#cb41-51"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_base<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-52"><a href="#cb41-52"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_namespace<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-53"><a href="#cb41-53"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_function<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-54"><a href="#cb41-54"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_static<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-55"><a href="#cb41-55"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_variable<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-56"><a href="#cb41-56"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_type<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-57"><a href="#cb41-57"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_alias<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-58"><a href="#cb41-58"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_incomplete_type<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-59"><a href="#cb41-59"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-60"><a href="#cb41-60"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_function_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-61"><a href="#cb41-61"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_variable_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-62"><a href="#cb41-62"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_class_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-63"><a href="#cb41-63"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_alias_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-64"><a href="#cb41-64"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> has_template_arguments<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-65"><a href="#cb41-65"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_constructor<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-66"><a href="#cb41-66"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_destructor<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-67"><a href="#cb41-67"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_special_member<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb41-68"><a href="#cb41-68"></a></span>
<span id="cb41-69"><a href="#cb41-69"></a>  <span class="co">// <a href="#reflect_value">reflect_value</a></span></span>
<span id="cb41-70"><a href="#cb41-70"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb41-71"><a href="#cb41-71"></a>    <span class="kw">consteval</span> <span class="kw">auto</span> reflect_value<span class="op">(</span>T value<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb41-72"><a href="#cb41-72"></a></span>
<span id="cb41-73"><a href="#cb41-73"></a>  <span class="co">// <a href="#nsdm_description-define_class">define_class</a></span></span>
<span id="cb41-74"><a href="#cb41-74"></a>  <span class="kw">struct</span> nsdm_options_t;</span>
<span id="cb41-75"><a href="#cb41-75"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> nsdm_description<span class="op">(</span>info class_type, nsdm_options_t options <span class="op">=</span> <span class="op">{})</span> <span class="op">-&gt;</span> info;</span>
<span id="cb41-76"><a href="#cb41-76"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> define_class<span class="op">(</span>info class_type, span<span class="op">&lt;</span>info <span class="kw">const</span><span class="op">&gt;)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb41-77"><a href="#cb41-77"></a></span>
<span id="cb41-78"><a href="#cb41-78"></a>  <span class="co">// <a href="#data-layout-reflection">data layout</a></span></span>
<span id="cb41-79"><a href="#cb41-79"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> offset_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb41-80"><a href="#cb41-80"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> size_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb41-81"><a href="#cb41-81"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> bit_offset_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb41-82"><a href="#cb41-82"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> bit_size_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb41-83"><a href="#cb41-83"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> alignment_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb41-84"><a href="#cb41-84"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<h3 data-number="4.4.3" id="name_of-display_name_of-source_location_of"><span class="header-section-number">4.4.3</span> <code class="sourceCode cpp">name_of</code>, <code class="sourceCode cpp">display_name_of</code>, <code class="sourceCode cpp">source_location_of</code><a href="#name_of-display_name_of-source_location_of" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb42"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb42-1"><a href="#cb42-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb42-2"><a href="#cb42-2"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> name_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> string_view;</span>
<span id="cb42-3"><a href="#cb42-3"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> display_name_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> string_view;</span>
<span id="cb42-4"><a href="#cb42-4"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> source_location_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> source_location;</span>
<span id="cb42-5"><a href="#cb42-5"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Given a reflection <code class="sourceCode cpp">r</code> that designates a declared entity <code class="sourceCode cpp">X</code>, <code class="sourceCode cpp">name_of<span class="op">(</span>r<span class="op">)</span></code> returns a <code class="sourceCode cpp">string_view</code> holding the unqualified name of <code class="sourceCode cpp">X</code>. For all other reflections, an empty <code class="sourceCode cpp">string_view</code> is produced. For template instances, the name does not include the template argument list. The contents of the <code class="sourceCode cpp">string_view</code> consist of characters of the basic source character set only (an implementation can map other characters using universal character names).</p>
<p>Given a reflection <code class="sourceCode cpp">r</code>, <code class="sourceCode cpp">display_name_of<span class="op">(</span>r<span class="op">)</span></code> returns a unspecified non-empty <code class="sourceCode cpp">string_view</code>. Implementations are encouraged to produce text that is helpful in identifying the reflected construct.</p>
<p>Given a reflection <code class="sourceCode cpp">r</code>, <code class="sourceCode cpp">source_location_of<span class="op">(</span>r<span class="op">)</span></code> returns an unspecified <code class="sourceCode cpp">source_location</code>. Implementations are encouraged to produce the correct source location of the item designated by the reflection.</p>
<h3 data-number="4.4.4" id="type_of-parent_of-dealias"><span class="header-section-number">4.4.4</span> <code class="sourceCode cpp">type_of</code>, <code class="sourceCode cpp">parent_of</code>, <code class="sourceCode cpp">dealias</code><a href="#type_of-parent_of-dealias" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb43"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb43-1"><a href="#cb43-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb43-2"><a href="#cb43-2"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> type_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb43-3"><a href="#cb43-3"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> parent_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb43-4"><a href="#cb43-4"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> dealias<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb43-5"><a href="#cb43-5"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>If <code class="sourceCode cpp">r</code> is a reflection designating a typed entity, <code class="sourceCode cpp">type_of<span class="op">(</span>r<span class="op">)</span></code> is a reflection designating its type. If <code class="sourceCode cpp">r</code> is already a type, <code class="sourceCode cpp">type_of<span class="op">(</span>r<span class="op">)</span></code> is not a constant expression. This can be used to implement the C <code class="sourceCode cpp"><span class="ex">typeof</span></code> feature (which works on both types and expressions and strips qualifiers):</p>
<blockquote>
<div class="sourceCode" id="cb44"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb44-1"><a href="#cb44-1"></a><span class="kw">consteval</span> <span class="kw">auto</span> do_typeof<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>info r<span class="op">)</span> <span class="op">-&gt;</span> std<span class="op">::</span>meta<span class="op">::</span>info <span class="op">{</span></span>
<span id="cb44-2"><a href="#cb44-2"></a>  <span class="cf">return</span> remove_cvref<span class="op">(</span>is_type<span class="op">(</span>r<span class="op">)</span> <span class="op">?</span> r <span class="op">:</span> type_of<span class="op">(</span>r<span class="op">))</span>;</span>
<span id="cb44-3"><a href="#cb44-3"></a><span class="op">}</span></span>
<span id="cb44-4"><a href="#cb44-4"></a></span>
<span id="cb44-5"><a href="#cb44-5"></a><span class="pp">#define typeof</span><span class="op">(</span>e<span class="op">)</span><span class="pp"> </span><span class="op">[:</span><span class="pp"> </span>do_typeof<span class="op">(^</span>e<span class="op">)</span><span class="pp"> </span><span class="op">:]</span></span></code></pre></div>
</blockquote>
<p>If <code class="sourceCode cpp">r</code> designates a member of a class or namespace, <code class="sourceCode cpp">parent_of<span class="op">(</span>r<span class="op">)</span></code> is a reflection designating its immediately enclosing class or namespace.</p>
<p>If <code class="sourceCode cpp">r</code> designates an alias, <code class="sourceCode cpp">dealias<span class="op">(</span>r<span class="op">)</span></code> designates the underlying entity. Otherwise, <code class="sourceCode cpp">dealias<span class="op">(</span>r<span class="op">)</span></code> produces <code class="sourceCode cpp">r</code>. <code class="sourceCode cpp">dealias</code> is recursive - it strips all aliases:</p>
<blockquote>
<div class="sourceCode" id="cb45"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb45-1"><a href="#cb45-1"></a><span class="kw">using</span> X <span class="op">=</span> <span class="dt">int</span>;</span>
<span id="cb45-2"><a href="#cb45-2"></a><span class="kw">using</span> Y <span class="op">=</span> X;</span>
<span id="cb45-3"><a href="#cb45-3"></a><span class="kw">static_assert</span><span class="op">(</span>dealias<span class="op">(^</span><span class="dt">int</span><span class="op">)</span> <span class="op">==</span> <span class="op">^</span><span class="dt">int</span><span class="op">)</span>;</span>
<span id="cb45-4"><a href="#cb45-4"></a><span class="kw">static_assert</span><span class="op">(</span>dealias<span class="op">(^</span>X<span class="op">)</span> <span class="op">==</span> <span class="op">^</span><span class="dt">int</span><span class="op">)</span>;</span>
<span id="cb45-5"><a href="#cb45-5"></a><span class="kw">static_assert</span><span class="op">(</span>dealias<span class="op">(^</span>Y<span class="op">)</span> <span class="op">==</span> <span class="op">^</span><span class="dt">int</span><span class="op">)</span>;</span></code></pre></div>
</blockquote>
<h3 data-number="4.4.5" id="template_of-template_arguments_of"><span class="header-section-number">4.4.5</span> <code class="sourceCode cpp">template_of</code>, <code class="sourceCode cpp">template_arguments_of</code><a href="#template_of-template_arguments_of" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb46"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb46-1"><a href="#cb46-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb46-2"><a href="#cb46-2"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> template_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb46-3"><a href="#cb46-3"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> template_arguments_of<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb46-4"><a href="#cb46-4"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>If <code class="sourceCode cpp">r</code> is a reflection designated a type that is a specialization of some template, then <code class="sourceCode cpp">template_of<span class="op">(</span>r<span class="op">)</span></code> is a reflection of that template and <code class="sourceCode cpp">template_arguments_of<span class="op">(</span>r<span class="op">)</span></code> is a vector of the reflections of the template arguments. In other words, the preconditions on both is that <code class="sourceCode cpp">has_template_arguments<span class="op">(</span>r<span class="op">)</span></code> is <code class="sourceCode cpp"><span class="kw">true</span></code>.</p>
<p>For example:</p>
<blockquote>
<div class="sourceCode" id="cb47"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb47-1"><a href="#cb47-1"></a>std<span class="op">::</span>vector<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span> v <span class="op">=</span> <span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span>;</span>
<span id="cb47-2"><a href="#cb47-2"></a><span class="kw">static_assert</span><span class="op">(</span>template_of<span class="op">(</span>type_of<span class="op">(^</span>v<span class="op">))</span> <span class="op">==</span> <span class="op">^</span>std<span class="op">::</span>vector<span class="op">)</span>;</span>
<span id="cb47-3"><a href="#cb47-3"></a><span class="kw">static_assert</span><span class="op">(</span>template_arguments_of<span class="op">(</span>type_of<span class="op">(^</span>v<span class="op">))[</span><span class="dv">0</span><span class="op">]</span> <span class="op">==</span> <span class="op">^</span><span class="dt">int</span><span class="op">)</span>;</span></code></pre></div>
</blockquote>
<h3 data-number="4.4.6" id="members_of-static_data_members_of-nonstatic_data_members_of-bases_of-enumerators_of-subobjects_of"><span class="header-section-number">4.4.6</span> <code class="sourceCode cpp">members_of</code>, <code class="sourceCode cpp">static_data_members_of</code>, <code class="sourceCode cpp">nonstatic_data_members_of</code>, <code class="sourceCode cpp">bases_of</code>, <code class="sourceCode cpp">enumerators_of</code>, <code class="sourceCode cpp">subobjects_of</code><a href="#members_of-static_data_members_of-nonstatic_data_members_of-bases_of-enumerators_of-subobjects_of" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb48"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb48-1"><a href="#cb48-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb48-2"><a href="#cb48-2"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> <span class="op">...</span>Fs<span class="op">&gt;</span></span>
<span id="cb48-3"><a href="#cb48-3"></a>    <span class="kw">consteval</span> <span class="kw">auto</span> members_of<span class="op">(</span>info class_type, Fs <span class="op">...</span>filters<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb48-4"><a href="#cb48-4"></a></span>
<span id="cb48-5"><a href="#cb48-5"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> <span class="op">...</span>Fs<span class="op">&gt;</span></span>
<span id="cb48-6"><a href="#cb48-6"></a>    <span class="kw">consteval</span> <span class="kw">auto</span> bases_of<span class="op">(</span>info class_type, Fs <span class="op">...</span>filters<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb48-7"><a href="#cb48-7"></a></span>
<span id="cb48-8"><a href="#cb48-8"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> static_data_members_of<span class="op">(</span>info class_type<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span> <span class="op">{</span></span>
<span id="cb48-9"><a href="#cb48-9"></a>    <span class="cf">return</span> members_of<span class="op">(</span>class_type, is_variable<span class="op">)</span>;</span>
<span id="cb48-10"><a href="#cb48-10"></a>  <span class="op">}</span></span>
<span id="cb48-11"><a href="#cb48-11"></a></span>
<span id="cb48-12"><a href="#cb48-12"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> nonstatic_data_members_of<span class="op">(</span>info class_type<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span> <span class="op">{</span></span>
<span id="cb48-13"><a href="#cb48-13"></a>    <span class="cf">return</span> members_of<span class="op">(</span>class_type, is_nsdm<span class="op">)</span>;</span>
<span id="cb48-14"><a href="#cb48-14"></a>  <span class="op">}</span></span>
<span id="cb48-15"><a href="#cb48-15"></a></span>
<span id="cb48-16"><a href="#cb48-16"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> subobjects_of<span class="op">(</span>info class_type<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span> <span class="op">{</span></span>
<span id="cb48-17"><a href="#cb48-17"></a>    <span class="kw">auto</span> subobjects <span class="op">=</span> bases_of<span class="op">(</span>class_type<span class="op">)</span>;</span>
<span id="cb48-18"><a href="#cb48-18"></a>    subobjects<span class="op">.</span>append_range<span class="op">(</span>nonstatic_data_members_of<span class="op">(</span>class_type<span class="op">))</span>;</span>
<span id="cb48-19"><a href="#cb48-19"></a>    <span class="cf">return</span> subobjects;</span>
<span id="cb48-20"><a href="#cb48-20"></a>  <span class="op">}</span></span>
<span id="cb48-21"><a href="#cb48-21"></a></span>
<span id="cb48-22"><a href="#cb48-22"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> enumerators_of<span class="op">(</span>info enum_type<span class="op">)</span> <span class="op">-&gt;</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span>;</span>
<span id="cb48-23"><a href="#cb48-23"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>The template <code class="sourceCode cpp">members_of</code> returns a vector of reflections representing the direct members of the class type represented by its first argument. Any nonstatic data members appear in declaration order within that vector. Anonymous unions appear as a nonstatic data member of corresponding union type. If any <code class="sourceCode cpp">Filters<span class="op">...</span></code> argument is specified, a member is dropped from the result if any filter applied to that members reflection returns <code class="sourceCode cpp"><span class="kw">false</span></code>. E.g., <code class="sourceCode cpp">members_of<span class="op">(^</span>C, std<span class="op">::</span>meta<span class="op">::</span>is_type<span class="op">)</span></code> will only return types nested in the definition of <code class="sourceCode cpp">C</code> and <code class="sourceCode cpp">members_of<span class="op">(^</span>C, std<span class="op">::</span>meta<span class="op">::</span>is_type, std<span class="op">::</span>meta<span class="op">::</span>is_variable<span class="op">)</span></code> will return an empty vector since a member cannot be both a type and a variable.</p>
<p>The template <code class="sourceCode cpp">bases_of</code> returns the direct base classes of the class type represented by its first argument, in declaration order.</p>
<p><code class="sourceCode cpp">enumerators_of</code> returns the enumerator constants of the indicated enumeration type in declaration order.</p>
<h3 data-number="4.4.7" id="substitute"><span class="header-section-number">4.4.7</span> <code class="sourceCode cpp">substitute</code><a href="#substitute" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb49"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb49-1"><a href="#cb49-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb49-2"><a href="#cb49-2"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> substitute<span class="op">(</span>info templ, span<span class="op">&lt;</span>info <span class="kw">const</span><span class="op">&gt;</span> args<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb49-3"><a href="#cb49-3"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Given a reflection for a template and reflections for template arguments that match that template, <code class="sourceCode cpp">substitute</code> returns a reflection for the entity obtained by substituting the given arguments in the template. If the template is a concept template, the result is a reflection of a constant of type <code class="sourceCode cpp"><span class="dt">bool</span></code>.</p>
<p>For example:</p>
<blockquote>
<div class="sourceCode" id="cb50"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb50-1"><a href="#cb50-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> r <span class="op">=</span> substitute<span class="op">(^</span>std<span class="op">::</span>vector, std<span class="op">::</span>vector<span class="op">{^</span><span class="dt">int</span><span class="op">})</span>;</span>
<span id="cb50-2"><a href="#cb50-2"></a><span class="kw">using</span> T <span class="op">=</span> <span class="op">[:</span>r<span class="op">:]</span>; <span class="co">// Ok, T is std::vector&lt;int&gt;</span></span></code></pre></div>
</blockquote>
<p>This process might kick off instantiations outside the immediate context, which can lead to the program being ill-formed.</p>
<p>Note that the template is only substituted, not instantiated. For example:</p>
<blockquote>
<div class="sourceCode" id="cb51"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb51-1"><a href="#cb51-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span> <span class="kw">struct</span> S <span class="op">{</span> <span class="kw">typename</span> T<span class="op">::</span>X x; <span class="op">}</span>;</span>
<span id="cb51-2"><a href="#cb51-2"></a></span>
<span id="cb51-3"><a href="#cb51-3"></a><span class="kw">constexpr</span> <span class="kw">auto</span> r <span class="op">=</span> substitute<span class="op">(^</span>S, std<span class="op">::</span>vector<span class="op">{^</span><span class="dt">int</span><span class="op">})</span>;  <span class="co">// Okay.</span></span>
<span id="cb51-4"><a href="#cb51-4"></a><span class="kw">typename</span><span class="op">[:</span>r<span class="op">:]</span> si;  <span class="co">// Error: T::X is invalid for T = int.</span></span></code></pre></div>
</blockquote>
<h3 data-number="4.4.8" id="value_oft"><span class="header-section-number">4.4.8</span> <code class="sourceCode cpp">value_of<span class="op">&lt;</span>T<span class="op">&gt;</span></code><a href="#value_oft" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb52"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb52-1"><a href="#cb52-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb52-2"><a href="#cb52-2"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span> <span class="kw">consteval</span> <span class="kw">auto</span> value_of<span class="op">(</span>info<span class="op">)</span> <span class="op">-&gt;</span> T;</span>
<span id="cb52-3"><a href="#cb52-3"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>If <code class="sourceCode cpp">r</code> is a reflection for a constant-expression or a constant-valued entity of type <code class="sourceCode cpp">T</code>, <code class="sourceCode cpp">value_of<span class="op">&lt;</span>T<span class="op">&gt;(</span>r<span class="op">)</span></code> evaluates to that constant value.</p>
<p>If <code class="sourceCode cpp">r</code> is a reflection for a variable of non-reference type <code class="sourceCode cpp">T</code>, <code class="sourceCode cpp">value_of<span class="op">&lt;</span>T<span class="op">&amp;&gt;(</span>r<span class="op">)</span></code> and <code class="sourceCode cpp">value_of<span class="op">&lt;</span>T <span class="kw">const</span><span class="op">&amp;&gt;(</span>r<span class="op">)</span></code> are lvalues referring to that variable. If the variable is usable in constant expressions [expr.const], <code class="sourceCode cpp">value_of<span class="op">&lt;</span>T<span class="op">&gt;(</span>r<span class="op">)</span></code> evaluates to its value.</p>
<p>If <code class="sourceCode cpp">r</code> is a reflection for a variable of reference type <code class="sourceCode cpp">T</code> usable in constant-expressions, <code class="sourceCode cpp">value_of<span class="op">&lt;</span>T<span class="op">&gt;(</span>r<span class="op">)</span></code> evaluates to that reference.</p>
<p>If <code class="sourceCode cpp">r</code> is a reflection of an enumerator constant of type <code class="sourceCode cpp">E</code>, <code class="sourceCode cpp">value_of<span class="op">&lt;</span>E<span class="op">&gt;(</span>r<span class="op">)</span></code> evaluates to the value of that enumerator.</p>
<p>If <code class="sourceCode cpp">r</code> is a reflection of a non-bit-field non-reference non-static member of type <code class="sourceCode cpp">M</code> in a class <code class="sourceCode cpp">C</code>, <code class="sourceCode cpp">value_of<span class="op">&lt;</span>M C<span class="op">::*&gt;(</span>r<span class="op">)</span></code> is the pointer-to-member value for that nonstatic member.</p>
<p>For other reflection values <code class="sourceCode cpp">r</code>, <code class="sourceCode cpp">value_of<span class="op">&lt;</span>T<span class="op">&gt;(</span>r<span class="op">)</span></code> is ill-formed.</p>
<p>The function template <code class="sourceCode cpp">value_of</code> may feel similar to splicers, but unlike splicers it does not require its operand to be a constant-expression itself. Also unlike splicers, it requires knowledge of the type associated with the entity reflected by its operand.</p>
<h3 data-number="4.4.9" id="test_type-test_types"><span class="header-section-number">4.4.9</span> <code class="sourceCode cpp">test_type</code>, <code class="sourceCode cpp">test_types</code><a href="#test_type-test_types" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb53"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb53-1"><a href="#cb53-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb53-2"><a href="#cb53-2"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> test_type<span class="op">(</span>info templ, info type<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span> <span class="op">{</span></span>
<span id="cb53-3"><a href="#cb53-3"></a>    <span class="cf">return</span> test_types<span class="op">(</span>templ, <span class="op">{</span>type<span class="op">})</span>;</span>
<span id="cb53-4"><a href="#cb53-4"></a>  <span class="op">}</span></span>
<span id="cb53-5"><a href="#cb53-5"></a></span>
<span id="cb53-6"><a href="#cb53-6"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> test_types<span class="op">(</span>info templ, span<span class="op">&lt;</span>info <span class="kw">const</span><span class="op">&gt;</span> types<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span> <span class="op">{</span></span>
<span id="cb53-7"><a href="#cb53-7"></a>    <span class="cf">return</span> value_of<span class="op">&lt;</span><span class="dt">bool</span><span class="op">&gt;(</span>substitute<span class="op">(</span>templ, types<span class="op">))</span>;</span>
<span id="cb53-8"><a href="#cb53-8"></a>  <span class="op">}</span></span>
<span id="cb53-9"><a href="#cb53-9"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This utility translates existing metaprogramming predicates (expressed as constexpr variable templates or concept templates) to the reflection domain. For example:</p>
<blockquote>
<div class="sourceCode" id="cb54"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb54-1"><a href="#cb54-1"></a><span class="kw">struct</span> S <span class="op">{}</span>;</span>
<span id="cb54-2"><a href="#cb54-2"></a><span class="kw">static_assert</span><span class="op">(</span>test_type<span class="op">(^</span>std<span class="op">::</span>is_class_v, <span class="op">^</span>S<span class="op">))</span>;</span></code></pre></div>
</blockquote>
<p>An implementation is permitted to recognize standard predicate templates and implement <code class="sourceCode cpp">test_type</code> without actually instantiating the predicate template. In fact, that is recommended practice.</p>
<h3 data-number="4.4.10" id="other-singular-reflection-predicates"><span class="header-section-number">4.4.10</span> Other Singular Reflection Predicates<a href="#other-singular-reflection-predicates" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb55"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb55-1"><a href="#cb55-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb55-2"><a href="#cb55-2"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_public<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-3"><a href="#cb55-3"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_protected<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-4"><a href="#cb55-4"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_private<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-5"><a href="#cb55-5"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_accessible<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-6"><a href="#cb55-6"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_virtual<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-7"><a href="#cb55-7"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_deleted<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-8"><a href="#cb55-8"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_defaulted<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-9"><a href="#cb55-9"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_explicit<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-10"><a href="#cb55-10"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_override<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-11"><a href="#cb55-11"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_pure_virtual<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-12"><a href="#cb55-12"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_bit_field<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-13"><a href="#cb55-13"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> has_static_storage_duration<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-14"><a href="#cb55-14"></a></span>
<span id="cb55-15"><a href="#cb55-15"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_nsdm<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-16"><a href="#cb55-16"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_base<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-17"><a href="#cb55-17"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_namespace<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-18"><a href="#cb55-18"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_function<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-19"><a href="#cb55-19"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_static<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-20"><a href="#cb55-20"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_variable<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-21"><a href="#cb55-21"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_type<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-22"><a href="#cb55-22"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_alias<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-23"><a href="#cb55-23"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_incomplete_type<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-24"><a href="#cb55-24"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-25"><a href="#cb55-25"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_function_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-26"><a href="#cb55-26"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_variable_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-27"><a href="#cb55-27"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_class_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-28"><a href="#cb55-28"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_alias_template<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-29"><a href="#cb55-29"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> has_template_arguments<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-30"><a href="#cb55-30"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_constructor<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-31"><a href="#cb55-31"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_destructor<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-32"><a href="#cb55-32"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> is_special_member<span class="op">(</span>info r<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">bool</span>;</span>
<span id="cb55-33"><a href="#cb55-33"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<h3 data-number="4.4.11" id="reflect_value"><span class="header-section-number">4.4.11</span> <code class="sourceCode cpp">reflect_value</code><a href="#reflect_value" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb56"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb56-1"><a href="#cb56-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb56-2"><a href="#cb56-2"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span> <span class="kw">consteval</span> <span class="kw">auto</span> reflect_value<span class="op">(</span>T value<span class="op">)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb56-3"><a href="#cb56-3"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This metafunction produces a reflection representing the constant value of the operand.</p>
<h3 data-number="4.4.12" id="nsdm_description-define_class"><span class="header-section-number">4.4.12</span> <code class="sourceCode cpp">nsdm_description</code>, <code class="sourceCode cpp">define_class</code><a href="#nsdm_description-define_class" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb57"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb57-1"><a href="#cb57-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb57-2"><a href="#cb57-2"></a>  <span class="kw">struct</span> nsdm_options_t <span class="op">{</span></span>
<span id="cb57-3"><a href="#cb57-3"></a>    optional<span class="op">&lt;</span>string_view<span class="op">&gt;</span> name;</span>
<span id="cb57-4"><a href="#cb57-4"></a>    optional<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span> alignment;</span>
<span id="cb57-5"><a href="#cb57-5"></a>    optional<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span> width;</span>
<span id="cb57-6"><a href="#cb57-6"></a>  <span class="op">}</span>;</span>
<span id="cb57-7"><a href="#cb57-7"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> nsdm_description<span class="op">(</span>info type, nsdm_options options <span class="op">=</span> <span class="op">{})</span> <span class="op">-&gt;</span> info;</span>
<span id="cb57-8"><a href="#cb57-8"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> define_class<span class="op">(</span>info class_type, span<span class="op">&lt;</span>info <span class="kw">const</span><span class="op">&gt;)</span> <span class="op">-&gt;</span> info;</span>
<span id="cb57-9"><a href="#cb57-9"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p><code class="sourceCode cpp">nsdm_description</code> returns a reflection of a description of a non-static data member of given type. Optional alignment, bit-field-width, and name can be provided as well. If no <code class="sourceCode cpp">name</code> is provided, the name of the non-static data member is unspecified.</p>
<p><code class="sourceCode cpp">define_class</code> takes the reflection of an incomplete class/struct/union type and a range of reflections of non-static data member descriptions and it completes the given class type with nonstatic data members as described (in the given order). The given reflection is returned. For now, only non-static data member reflections are supported (via <code class="sourceCode cpp">nsdm_description</code>) but the API takes in a range of <code class="sourceCode cpp">info</code> anticipating expanding this in the near future.</p>
<p>For example:</p>
<blockquote>
<div class="sourceCode" id="cb58"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb58-1"><a href="#cb58-1"></a><span class="kw">union</span> U;</span>
<span id="cb58-2"><a href="#cb58-2"></a><span class="kw">static_assert</span><span class="op">(</span>is_type<span class="op">(</span>define_class<span class="op">(^</span>U, <span class="op">{</span></span>
<span id="cb58-3"><a href="#cb58-3"></a>  nsdm_description<span class="op">(^</span><span class="dt">int</span><span class="op">)</span>,</span>
<span id="cb58-4"><a href="#cb58-4"></a>  nsdm_description<span class="op">(^</span><span class="dt">char</span><span class="op">)</span>,</span>
<span id="cb58-5"><a href="#cb58-5"></a>  nsdm_description<span class="op">(^</span><span class="dt">double</span><span class="op">)</span>,</span>
<span id="cb58-6"><a href="#cb58-6"></a><span class="op">})))</span>;</span>
<span id="cb58-7"><a href="#cb58-7"></a></span>
<span id="cb58-8"><a href="#cb58-8"></a><span class="co">// U is now defined to the equivalent of</span></span>
<span id="cb58-9"><a href="#cb58-9"></a><span class="co">// union U {</span></span>
<span id="cb58-10"><a href="#cb58-10"></a><span class="co">//   int <em>_0</em>;</span></span>
<span id="cb58-11"><a href="#cb58-11"></a><span class="co">//   char <em>_1</em>;</span></span>
<span id="cb58-12"><a href="#cb58-12"></a><span class="co">//   double <em>_2</em>;</span></span>
<span id="cb58-13"><a href="#cb58-13"></a><span class="co">// };</span></span>
<span id="cb58-14"><a href="#cb58-14"></a></span>
<span id="cb58-15"><a href="#cb58-15"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span> <span class="kw">struct</span> S;</span>
<span id="cb58-16"><a href="#cb58-16"></a><span class="kw">constexpr</span> <span class="kw">auto</span> U <span class="op">=</span> define_class<span class="op">(^</span>S<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span>, <span class="op">{</span></span>
<span id="cb58-17"><a href="#cb58-17"></a>  nsdm_description<span class="op">(^</span><span class="dt">int</span>, <span class="op">{.</span>name<span class="op">=</span><span class="st">&quot;i&quot;</span>, <span class="op">.</span>align<span class="op">=</span><span class="dv">64</span><span class="op">})</span>,</span>
<span id="cb58-18"><a href="#cb58-18"></a>  nsdm_description<span class="op">(^</span><span class="dt">int</span>, <span class="op">{.</span>name<span class="op">=</span><span class="st">&quot;j&quot;</span>, <span class="op">.</span>align<span class="op">=</span><span class="dv">64</span><span class="op">})</span>,</span>
<span id="cb58-19"><a href="#cb58-19"></a><span class="op">})</span>;</span>
<span id="cb58-20"><a href="#cb58-20"></a></span>
<span id="cb58-21"><a href="#cb58-21"></a><span class="co">// S&lt;int&gt; is now defined to the equivalent of</span></span>
<span id="cb58-22"><a href="#cb58-22"></a><span class="co">// template&lt;&gt; struct S&lt;int&gt; {</span></span>
<span id="cb58-23"><a href="#cb58-23"></a><span class="co">//   alignas(64) int i;</span></span>
<span id="cb58-24"><a href="#cb58-24"></a><span class="co">//   alignas(64) int j;</span></span>
<span id="cb58-25"><a href="#cb58-25"></a><span class="co">// };</span></span></code></pre></div>
</blockquote>
<p>When defining a <code class="sourceCode cpp"><span class="kw">union</span></code>, if one of the alternatives has a non-trivial destructor, the defined union will <em>still</em> have a destructor provided - that simply does nothing. This allows implementing <a href="#a-simple-variant-type">variant</a> without having to further extend support in <code class="sourceCode cpp">define_class</code> for member functions.</p>
<h3 data-number="4.4.13" id="data-layout-reflection"><span class="header-section-number">4.4.13</span> Data Layout Reflection<a href="#data-layout-reflection" class="self-link"></a></h3>
<blockquote>
<div class="sourceCode" id="cb59"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb59-1"><a href="#cb59-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb59-2"><a href="#cb59-2"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> offset_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb59-3"><a href="#cb59-3"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> size_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb59-4"><a href="#cb59-4"></a></span>
<span id="cb59-5"><a href="#cb59-5"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> bit_offset_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb59-6"><a href="#cb59-6"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> bit_size_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb59-7"><a href="#cb59-7"></a></span>
<span id="cb59-8"><a href="#cb59-8"></a>  <span class="kw">consteval</span> <span class="kw">auto</span> alignment_of<span class="op">(</span>info entity<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">size_t</span>;</span>
<span id="cb59-9"><a href="#cb59-9"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<h3 data-number="4.4.14" id="other-type-traits"><span class="header-section-number">4.4.14</span> Other Type Traits<a href="#other-type-traits" class="self-link"></a></h3>
<p>There is a question of whether all the type traits should be provided in <code class="sourceCode cpp">std<span class="op">::</span>meta</code>. For instance, a few examples in this paper use <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>remove_cvref<span class="op">(</span>t<span class="op">)</span></code> as if that exists. Technically, the functionality isn’t strictly necessary - since it can be provided indirectly:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Direct</strong>
</div></th>
<th><div style="text-align:center">
<strong>Indirect</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb60"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb60-1"><a href="#cb60-1"></a>std<span class="op">::</span>meta<span class="op">::</span>remove_cvref<span class="op">(</span>type<span class="op">)</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb61"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb61-1"><a href="#cb61-1"></a>std<span class="op">::</span>meta<span class="op">::</span>substitute<span class="op">(^</span>std<span class="op">::</span>remove_cvref_t, <span class="op">{</span>type<span class="op">})</span></span></code></pre></div></td>
</tr>
<tr class="even">
<td><div class="sourceCode" id="cb62"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb62-1"><a href="#cb62-1"></a>std<span class="op">::</span>meta<span class="op">::</span>is_const<span class="op">(</span>type<span class="op">)</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb63"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb63-1"><a href="#cb63-1"></a>std<span class="op">::</span>meta<span class="op">::</span>value_of<span class="op">&lt;</span><span class="dt">bool</span><span class="op">&gt;(</span>std<span class="op">::</span>meta<span class="op">::</span>substitute<span class="op">(^</span>std<span class="op">::</span>is_const_v, <span class="op">{</span>type<span class="op">}))</span></span>
<span id="cb63-2"><a href="#cb63-2"></a>std<span class="op">::</span>meta<span class="op">::</span>test_type<span class="op">(^</span>std<span class="op">::</span>is_const_v, type<span class="op">)</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>Having <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>meow</code> for every trait <code class="sourceCode cpp">std<span class="op">::</span>meow</code> is more straightforward and will likely be faster to compile, though means we will have a much larger library API. There are quite a few traits in <span>21 <a href="https://wg21.link/meta">[meta]</a></span> - but it should be easy enough to specify all of them.</p>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">5</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-N3980">
<p>[N3980] H. Hinnant, V. Falco, J. Byteway. 2014-05-24. Types don’t know #. <br />
<a href="https://wg21.link/n3980">https://wg21.link/n3980</a></p>
</div>
<div id="ref-P0784R7">
<p>[P0784R7] Daveed Vandevoorde, Peter Dimov,Louis Dionne, Nina Ranns, Richard Smith, Daveed Vandevoorde. 2019-07-22. More constexpr containers. <br />
<a href="https://wg21.link/p0784r7">https://wg21.link/p0784r7</a></p>
</div>
<div id="ref-P1061R5">
<p>[P1061R5] Barry Revzin, Jonathan Wakely. 2023-05-18. Structured Bindings can introduce a Pack. <br />
<a href="https://wg21.link/p1061r5">https://wg21.link/p1061r5</a></p>
</div>
<div id="ref-P1240R2">
<p>[P1240R2] Daveed Vandevoorde, Wyatt Childers, Andrew Sutton, Faisal Vali. 2022-01-14. Scalable Reflection. <br />
<a href="https://wg21.link/p1240r2">https://wg21.link/p1240r2</a></p>
</div>
<div id="ref-P1306R1">
<p>[P1306R1] Andrew Sutton, Sam Goodrick, Daveed Vandevoorde. 2019-01-21. Expansion statements. <br />
<a href="https://wg21.link/p1306r1">https://wg21.link/p1306r1</a></p>
</div>
<div id="ref-P1974R0">
<p>[P1974R0] Jeff Snyder, Louis Dionne, Daveed Vandevoorde. 2020-05-15. Non-transient constexpr allocation using propconst. <br />
<a href="https://wg21.link/p1974r0">https://wg21.link/p1974r0</a></p>
</div>
<div id="ref-P2237R0">
<p>[P2237R0] Andrew Sutton. 2020-10-15. Metaprogramming. <br />
<a href="https://wg21.link/p2237r0">https://wg21.link/p2237r0</a></p>
</div>
<div id="ref-P2670R1">
<p>[P2670R1] Barry Revzin. 2023-02-03. Non-transient constexpr allocation. <br />
<a href="https://wg21.link/p2670r1">https://wg21.link/p2670r1</a></p>
</div>
<div id="ref-P2996R0">
<p>[P2996R0] Barry Revzin, Wyatt Childers, Peter Dimov, Andrew Sutton, Faisal Vali, Daveed Vandevoorde. 2023-10-15. Reflection for C++26. <br />
<a href="https://wg21.link/p2996r0">https://wg21.link/p2996r0</a></p>
</div>
</div>
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