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<style> @media print{ .hljs{overflow: visible; word-wrap: break-word !important;} }</style></head><body><div class="markdown-body">
<p><strong>P0598</strong><br/>
To: SG7 (Reflection) &amp; EWG<br/>
From: Daveed Vandevoorde (<code>daveed@edg.com</code>)<br/>
Date: 2017-02-02</p>

<h1 id="toc_0">Reflect Through Values Instead of Types</h1>

<p>P0194R2 proposes adding compile-time reflection facilities to the language.  I&rsquo;m looking forward to the capabilities themselves, but I&rsquo;m concerned about using types to perform computations when we&rsquo;ve come such a long way in providing efficient compile-time computation facilities through <code>constexpr</code>. Let&rsquo;s design with modern C++ in mind and leave the template metaprogramming shackles of the past behind.</p>

<h3 id="toc_1">The Cost of &ldquo;Types as Values&rdquo;</h3>

<p>The main downside of using types as values is that types and template instances are persistent: Every intermediate type and template instance we use in a meta-computation persists until the end of the compilation process, even though these things are only briefly needed to produce the final &ldquo;value&rdquo;. That is almost the definition of &ldquo;not scalable&rdquo;.</p>

<p>FAQ 10.3 in P0385R1 argues that the meta types can be very &ldquo;lightweight&rdquo; and need not be &ldquo;regular&rdquo; types.  That doesn&rsquo;t address the issue of persistence (which is not just a about the meta-information types, but all the templates instantiated from them), and it potentially introduces new problems: We now get a new kind of beast in our type system, which may be &ldquo;heavyweight&rdquo; when it comes to specification and implementation. </p>

<h3 id="toc_2">An Alternative Approach</h3>

<p>Instead of trafficking in types, let&rsquo;s deal with ordinary values as we typically do in imperative programming.  We can add a single &ldquo;magical&rdquo; literal class type to encapsulate a hook to compiler internals (e.g., <code>std::metainfo</code>).  Along with that type, a few operators can transition back and forth between the meta and non-meta worlds.  E.g.:</p>

<pre><code>std::metainfo tp = reflexpr(X);
typename(tp) x;  // Same as &quot;X x;&quot;
</code></pre>

<p>Instead of the <code>typename(...)</code> operator, we could use the suggested <code>unreflexpr(...)</code> operator.  I find the former clearer however and it avoids a parsing problem in some contexts.  Similarly, I&rsquo;d suggest a <code>template(...)</code> operator for materializing templates, and maybe notations to materialize various names and operators (e.g., <code>x.(info)</code>, <code>N::(info)</code>, <code>(:info)(x, y)</code>, etc.).</p>

<p>We can add a literal dynamic container type (e.g., <code>std::constexpr_vector&lt;T&gt;</code>; it&rsquo;s a literal class type by fiat) to ease dealing with collections of metadata (and, incidentally, such a facility would be more broadly useful; see P0597).  Beyond that, many of the facilities proposed in P00194R2 can just be translated to use ordinary function interfaces (instead of template instantiation interfaces).  E.g.:</p>

<pre><code>struct foo { ... };
typedef reflexpr(foo) meta_foo;
meta::get_public_data_members_m&lt;meta_foo&gt; meta_data_mems;
</code></pre>

<p>becomes</p>

<pre><code>struct foo { ... };
constexpr auto meta_foo = reflexpr(foo);
auto meta_data_mems = get_public_data_members(meta_foo);
</code></pre>

<p>The type of <code>meta_data_mems</code> in this example could be <code>std::constexpr_vector&lt;std::metainfo&gt;</code>.</p>

<p>This approach addresses the scalability of reflection itself. It doesn&rsquo;t address the scalability of synthesis, since there is no <code>constexpr</code> mechanism to do that at this time (i.e., that still requires recursive template instantiations).  There is, however, nothing to stop us from adding such capabilities: <code>idreflexpr</code> is an example of this, but a more general code injection system could be provided too (N1471, &ldquo;Reflexive Metaprogramming in C++&rdquo; has some ideas that could be adapted to the <code>constexpr</code> world).</p>

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