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  <meta name="dcterms.date" content="2022-10-05" />
  <title>Member visit</title>
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<h1 class="title" style="text-align:center">Member <code class="sourceCode cpp">visit</code></h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2637R1</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2022-10-05</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>
      LEWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.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="#stdvisit"><span class="toc-section-number">2.1</span> <code class="sourceCode cpp">std<span class="op">::</span>visit</code><span></span></a></li>
<li><a href="#stdvisit_format_arg"><span class="toc-section-number">2.2</span> <code class="sourceCode cpp">std<span class="op">::</span>visit_format_arg</code><span></span></a></li>
<li><a href="#implementation"><span class="toc-section-number">2.3</span> Implementation<span></span></a></li>
</ul></li>
<li><a href="#wording"><span class="toc-section-number">3</span> Wording<span></span></a>
<ul>
<li><a href="#feature-test-macro"><span class="toc-section-number">3.1</span> Feature-test macro<span></span></a></li>
</ul></li>
<li><a href="#bibliography"><span class="toc-section-number">4</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="P2637R0">[<a href="#ref-P2637R0" role="doc-biblioref">P2637R0</a>]</span>, dropped <code class="sourceCode cpp">apply</code>, added member <code class="sourceCode cpp">visit<span class="op">&lt;</span>R<span class="op">&gt;</span></code> to <code class="sourceCode cpp">basic_format_arg</code>, and added support for types privately inheriting from <code class="sourceCode cpp">std<span class="op">::</span>variant</code> for member <code class="sourceCode cpp">visit</code> and <code class="sourceCode cpp">visit<span class="op">&lt;</span>R<span class="op">&gt;</span></code></p>
<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>The standard library currently has two free function templates for variant visitation: <code class="sourceCode cpp">std<span class="op">::</span>visit</code> and <code class="sourceCode cpp">std<span class="op">::</span>visit_format_arg</code>. The goal of this paper is to add member function versions of each of them, simply for ergonomic reasons. This paper adds no new functionality that did not exist before.</p>
<h2 data-number="2.1" id="stdvisit"><span class="header-section-number">2.1</span> <code class="sourceCode cpp">std<span class="op">::</span>visit</code><a href="#stdvisit" class="self-link"></a></h2>
<p><code class="sourceCode cpp">std<span class="op">::</span>visit</code> is a variadic function template, which is the correct design since binary (and more) visitation is a useful and important piece of functionality. However, the common case is simply unary visitation. Even in that case, however, a non-member function was a superior implementation choice for forwarding const-ness and value category.<a href="#fn1" class="footnote-ref" id="fnref1" role="doc-noteref"><sup>1</sup></a></p>
<p>But this decision logic changes in C++23 with the introduction of deducing <code class="sourceCode cpp"><span class="kw">this</span></code> <span class="citation" data-cites="P0847R7">[<a href="#ref-P0847R7" role="doc-biblioref">P0847R7</a>]</span>. Now, it is possible to implement unary <code class="sourceCode cpp">visit</code> as a member function without any loss of functionality. We simply gain better syntax:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Existing</strong>
</div></th>
<th><div style="text-align:center">
<strong>Proposed</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a>std<span class="op">::</span>visit<span class="op">(</span>overload<span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a>  <span class="op">[](</span><span class="dt">int</span> i<span class="op">){</span> std<span class="op">::</span>print<span class="op">(</span><span class="st">&quot;i={}</span><span class="sc">\n</span><span class="st">&quot;</span>, i<span class="op">)</span>; <span class="op">}</span>,</span>
<span id="cb1-3"><a href="#cb1-3"></a>  <span class="op">[](</span>std<span class="op">::</span>string s<span class="op">){</span> std<span class="op">::</span>print<span class="op">(</span><span class="st">&quot;s={:?}</span><span class="sc">\n</span><span class="st">&quot;</span>, s<span class="op">)</span>; <span class="op">}</span></span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="op">}</span>, value<span class="op">)</span>;</span></code></pre></div></td>
<td><div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a>value<span class="op">.</span>visit<span class="op">(</span>overload<span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a>  <span class="op">[](</span><span class="dt">int</span> i<span class="op">){</span> std<span class="op">::</span>print<span class="op">(</span><span class="st">&quot;i={}</span><span class="sc">\n</span><span class="st">&quot;</span>, i<span class="op">)</span>; <span class="op">}</span>,</span>
<span id="cb2-3"><a href="#cb2-3"></a>  <span class="op">[](</span>std<span class="op">::</span>string s<span class="op">){</span> std<span class="op">::</span>print<span class="op">(</span><span class="st">&quot;s={:?}</span><span class="sc">\n</span><span class="st">&quot;</span>, s<span class="op">)</span>; <span class="op">}</span></span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="op">})</span>;</span></code></pre></div></td>
</tr>
</tbody>
</table>
<h2 data-number="2.2" id="stdvisit_format_arg"><span class="header-section-number">2.2</span> <code class="sourceCode cpp">std<span class="op">::</span>visit_format_arg</code><a href="#stdvisit_format_arg" class="self-link"></a></h2>
<p>One of the components of the format library is <code class="sourceCode cpp">basic_format_arg<span class="op">&lt;</span>Context<span class="op">&gt;</span></code> (see <span>22.14.8.1 <a href="https://wg21.link/format.arg">[format.arg]</a></span>), which is basically a <code class="sourceCode cpp">std<span class="op">::</span>variant</code>. As such, it also needs to be visited in order to be used. To that end, the library provides:</p>
<blockquote>
<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">class</span> Visitor, <span class="kw">class</span> Context<span class="op">&gt;</span></span>
<span id="cb3-2"><a href="#cb3-2"></a>  <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> visit_format_arg<span class="op">(</span>Visitor<span class="op">&amp;&amp;</span> vis, basic_format_arg<span class="op">&lt;</span>Context<span class="op">&gt;</span> arg<span class="op">)</span>;</span></code></pre></div>
</blockquote>
<p>But here, the only reason <code class="sourceCode cpp">std<span class="op">::</span>visit_format_arg</code> is a non-member function was to mirror the interface for <code class="sourceCode cpp">std<span class="op">::</span>visit</code>. There is neither multiple visitation nor forwarding of value category or const-ness here. It could always have been a member function without any loss of functionality. With deducing <code class="sourceCode cpp"><span class="kw">this</span></code>, it can even be by-value member function.</p>
<p>This example is from the standard itself:</p>
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<th><div style="text-align:center">
<strong>Existing</strong>
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<th><div style="text-align:center">
<strong>Proposed</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><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">auto</span> format<span class="op">(</span>S s, format_context<span class="op">&amp;</span> ctx<span class="op">)</span> <span class="op">{</span></span>
<span id="cb4-2"><a href="#cb4-2"></a>  <span class="dt">int</span> width <span class="op">=</span> visit_format_arg<span class="op">([](</span><span class="kw">auto</span> value<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">int</span> <span class="op">{</span></span>
<span id="cb4-3"><a href="#cb4-3"></a>    <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(!</span>is_integral_v<span class="op">&lt;</span><span class="kw">decltype</span><span class="op">(</span>value<span class="op">)&gt;)</span></span>
<span id="cb4-4"><a href="#cb4-4"></a>      <span class="cf">throw</span> format_error<span class="op">(</span><span class="st">&quot;width is not integral&quot;</span><span class="op">)</span>;</span>
<span id="cb4-5"><a href="#cb4-5"></a>    <span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>value <span class="op">&lt;</span> <span class="dv">0</span> <span class="op">||</span> value <span class="op">&gt;</span> numeric_limits<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;::</span>max<span class="op">())</span></span>
<span id="cb4-6"><a href="#cb4-6"></a>      <span class="cf">throw</span> format_error<span class="op">(</span><span class="st">&quot;invalid width&quot;</span><span class="op">)</span>;</span>
<span id="cb4-7"><a href="#cb4-7"></a>    <span class="cf">else</span></span>
<span id="cb4-8"><a href="#cb4-8"></a>      <span class="cf">return</span> value;</span>
<span id="cb4-9"><a href="#cb4-9"></a>    <span class="op">}</span>, ctx<span class="op">.</span>arg<span class="op">(</span>width_arg_id<span class="op">))</span>;</span>
<span id="cb4-10"><a href="#cb4-10"></a>  <span class="cf">return</span> format_to<span class="op">(</span>ctx<span class="op">.</span>out<span class="op">()</span>, <span class="st">&quot;{0:x&lt;{1}}&quot;</span>, s<span class="op">.</span>value, width<span class="op">)</span>;</span>
<span id="cb4-11"><a href="#cb4-11"></a><span class="op">}</span></span></code></pre></div></td>
<td><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">auto</span> format<span class="op">(</span>S s, format_context<span class="op">&amp;</span> ctx<span class="op">)</span> <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a>  <span class="dt">int</span> width <span class="op">=</span> ctx<span class="op">.</span>arg<span class="op">(</span>width_arg_id<span class="op">).</span>visit<span class="op">([](</span><span class="kw">auto</span> value<span class="op">)</span> <span class="op">-&gt;</span> <span class="dt">int</span> <span class="op">{</span></span>
<span id="cb5-3"><a href="#cb5-3"></a>    <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(!</span>is_integral_v<span class="op">&lt;</span><span class="kw">decltype</span><span class="op">(</span>value<span class="op">)&gt;)</span></span>
<span id="cb5-4"><a href="#cb5-4"></a>      <span class="cf">throw</span> format_error<span class="op">(</span><span class="st">&quot;width is not integral&quot;</span><span class="op">)</span>;</span>
<span id="cb5-5"><a href="#cb5-5"></a>    <span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>value <span class="op">&lt;</span> <span class="dv">0</span> <span class="op">||</span> value <span class="op">&gt;</span> numeric_limits<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;::</span>max<span class="op">())</span></span>
<span id="cb5-6"><a href="#cb5-6"></a>      <span class="cf">throw</span> format_error<span class="op">(</span><span class="st">&quot;invalid width&quot;</span><span class="op">)</span>;</span>
<span id="cb5-7"><a href="#cb5-7"></a>    <span class="cf">else</span></span>
<span id="cb5-8"><a href="#cb5-8"></a>      <span class="cf">return</span> value;</span>
<span id="cb5-9"><a href="#cb5-9"></a>    <span class="op">})</span>;</span>
<span id="cb5-10"><a href="#cb5-10"></a>  <span class="cf">return</span> format_to<span class="op">(</span>ctx<span class="op">.</span>out<span class="op">()</span>, <span class="st">&quot;{0:x&lt;{1}}&quot;</span>, s<span class="op">.</span>value, width<span class="op">)</span>;</span>
<span id="cb5-11"><a href="#cb5-11"></a><span class="op">}</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>The proposed name here is just <code class="sourceCode cpp">visit</code> (rather than <code class="sourceCode cpp">visit_format_arg</code>), since as a member function we don’t need the longer name for differentiation.</p>
<h2 data-number="2.3" id="implementation"><span class="header-section-number">2.3</span> Implementation<a href="#implementation" class="self-link"></a></h2>
<p>In each case, the implementation is simple: simply redirect to the corresponding non-member function. Member <code class="sourceCode cpp">visit</code>, for instance:</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">template</span> <span class="op">&lt;</span><span class="kw">class</span><span class="op">...</span> Types<span class="op">&gt;</span></span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="kw">class</span> variant <span class="op">{</span></span>
<span id="cb6-3"><a href="#cb6-3"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb6-4"><a href="#cb6-4"></a>  <span class="kw">template</span> <span class="op">&lt;</span><span class="dt">int</span><span class="op">=</span><span class="dv">0</span>, <span class="kw">class</span> Self, <span class="kw">class</span> Visitor<span class="op">&gt;</span></span>
<span id="cb6-5"><a href="#cb6-5"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> visit<span class="op">(</span><span class="kw">this</span> Self<span class="op">&amp;&amp;</span> self, Visitor<span class="op">&amp;&amp;</span> vis<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="cb6-6"><a href="#cb6-6"></a>    <span class="cf">return</span> std<span class="op">::</span>visit<span class="op">(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>Visitor<span class="op">&gt;(</span>vis<span class="op">)</span>, <span class="op">(</span>copy_cvref_t<span class="op">&lt;</span>Self, variant<span class="op">&gt;&amp;&amp;)</span>self<span class="op">)</span>;</span>
<span id="cb6-7"><a href="#cb6-7"></a>  <span class="op">}</span></span>
<span id="cb6-8"><a href="#cb6-8"></a></span>
<span id="cb6-9"><a href="#cb6-9"></a>  <span class="kw">template</span> <span class="op">&lt;</span><span class="kw">class</span> R, <span class="kw">class</span> Self, <span class="kw">class</span> Visitor<span class="op">&gt;</span></span>
<span id="cb6-10"><a href="#cb6-10"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> visit<span class="op">(</span><span class="kw">this</span> Self<span class="op">&amp;&amp;</span> self, Visitor<span class="op">&amp;&amp;</span> vis<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="cb6-11"><a href="#cb6-11"></a>    <span class="cf">return</span> std<span class="op">::</span>visit<span class="op">&lt;</span>R<span class="op">&gt;(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>Visitor<span class="op">&gt;(</span>vis<span class="op">)</span>, <span class="op">(</span>copy_cvref_t<span class="op">&lt;</span>Self, variant<span class="op">&gt;&amp;&amp;)</span>self<span class="op">)</span>;</span>
<span id="cb6-12"><a href="#cb6-12"></a>  <span class="op">}</span></span>
<span id="cb6-13"><a href="#cb6-13"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p><code class="sourceCode cpp">copy_cvref_t<span class="op">&lt;</span>A, B<span class="op">&gt;</span></code> is a metafunction that simply pastes the const/ref qualifiers from <code class="sourceCode cpp">A</code> onto <code class="sourceCode cpp">B</code>. It will be added by <span class="citation" data-cites="P1450R3">[<a href="#ref-P1450R3" role="doc-biblioref">P1450R3</a>]</span>.</p>
<p>The C-style cast here is deliberate because <code class="sourceCode cpp">variant</code> might be a private base of <code class="sourceCode cpp">Self</code>. This is a case that <code class="sourceCode cpp">std<span class="op">::</span>visit</code> does not support, but LEWG preferred if member <code class="sourceCode cpp">visit</code> did.</p>
<p>There is also an extra leading <code class="sourceCode cpp"><span class="dt">int</span><span class="op">=</span><span class="dv">0</span></code> template parameter for the overload that just calls <code class="sourceCode cpp">std<span class="op">::</span>visit</code> (rather than <code class="sourceCode cpp">std<span class="op">::</span>visit<span class="op">&lt;</span>R<span class="op">&gt;</span></code>). This is because, unlike with the non-member functions, an ambiguity would otherwise arise if you attempted to do:</p>
<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">using</span> State <span class="op">=</span> std<span class="op">::</span>variant<span class="op">&lt;</span>A, B, C<span class="op">&gt;</span>;</span>
<span id="cb7-2"><a href="#cb7-2"></a></span>
<span id="cb7-3"><a href="#cb7-3"></a>State state <span class="op">=</span> <span class="co">/* ... */</span>;</span>
<span id="cb7-4"><a href="#cb7-4"></a>state <span class="op">=</span> std<span class="op">::</span>move<span class="op">(</span>state<span class="op">).</span>visit<span class="op">&lt;</span>State<span class="op">&gt;(</span>f<span class="op">)</span>;</span></code></pre></div>
<p>With non-member <code class="sourceCode cpp">visit</code>, there’s no real possible ambiguity because the function goes first. But here, unless we protect the non-return-type taking overload, <code class="sourceCode cpp">Self</code> could deduce as <code class="sourceCode cpp">State</code>, which would then be a perfectly valid overload. And this pattern isn’t rare either - so it’s important to support. The added <code class="sourceCode cpp"><span class="dt">int</span><span class="op">=</span><span class="dv">0</span></code> parameter ensures that only the first overload is viable for <code class="sourceCode cpp">v<span class="op">.</span>visit<span class="op">(</span>f<span class="op">)</span></code> and only the second is viable for <code class="sourceCode cpp">v<span class="op">.</span>visit<span class="op">&lt;</span>R<span class="op">&gt;(</span>f<span class="op">)</span></code>.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Add to <span>22.6.3.1 <a href="https://wg21.link/variant.variant.general">[variant.variant.general]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb8"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb8-1"><a href="#cb8-1"></a>namespace std {</span>
<span id="cb8-2"><a href="#cb8-2"></a>  template&lt;class... Types&gt;</span>
<span id="cb8-3"><a href="#cb8-3"></a>  class variant {</span>
<span id="cb8-4"><a href="#cb8-4"></a>  public:</span>
<span id="cb8-5"><a href="#cb8-5"></a>    // ...</span>
<span id="cb8-6"><a href="#cb8-6"></a></span>
<span id="cb8-7"><a href="#cb8-7"></a>    // [variant.status], value status</span>
<span id="cb8-8"><a href="#cb8-8"></a>    constexpr bool valueless_by_exception() const noexcept;</span>
<span id="cb8-9"><a href="#cb8-9"></a>    constexpr size_t index() const noexcept;</span>
<span id="cb8-10"><a href="#cb8-10"></a></span>
<span id="cb8-11"><a href="#cb8-11"></a>    // [variant.swap], swap</span>
<span id="cb8-12"><a href="#cb8-12"></a>    constexpr void swap(variant&amp;) noexcept(see below);</span>
<span id="cb8-13"><a href="#cb8-13"></a></span>
<span id="cb8-14"><a href="#cb8-14"></a><span class="va">+   // [variant.visit], visitation</span></span>
<span id="cb8-15"><a href="#cb8-15"></a><span class="va">+   template&lt;class Self, class Visitor&gt;</span></span>
<span id="cb8-16"><a href="#cb8-16"></a><span class="va">+     constexpr <em>see below</em> visit(this Self&amp;&amp;, Visitor&amp;&amp;);</span></span>
<span id="cb8-17"><a href="#cb8-17"></a><span class="va">+   template&lt;class R, class Self, class Visitor&gt;</span></span>
<span id="cb8-18"><a href="#cb8-18"></a><span class="va">+     constexpr R visit(this Self&amp;&amp;, Visitor&amp;&amp;);</span></span>
<span id="cb8-19"><a href="#cb8-19"></a>  };</span>
<span id="cb8-20"><a href="#cb8-20"></a>}</span></code></pre></div>
</div>
</blockquote>
<p>Add to <span>22.6.7 <a href="https://wg21.link/variant.visit">[variant.visit]</a></span>, after the definition of non-member <code class="sourceCode cpp">visit</code>:</p>
<blockquote>
<div class="addu">
<div class="sourceCode" id="cb9"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb9-1"><a href="#cb9-1"></a>template&lt;class Self, class Visitor&gt;</span>
<span id="cb9-2"><a href="#cb9-2"></a>  constexpr <em>see below</em> visit(this Self&amp;&amp; self, Visitor&amp;&amp; vis);</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">9</a></span> Let <code class="sourceCode cpp">V</code> be <code class="sourceCode cpp"><em>OVERRIDE_REF</em><span class="op">(</span>Self<span class="op">&amp;&amp;</span>, <em>COPY_CONST</em><span class="op">(</span>remove_reference_t<span class="op">&lt;</span>Self<span class="op">&gt;</span>, variant<span class="op">))</span></code> ([forward]).</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">10</a></span> <em>Constraints</em>: The call to <code class="sourceCode cpp">visit</code> does not use an explicit <code class="sourceCode cpp"><em>template-argument-list</em></code> that begins with a type <code class="sourceCode cpp"><em>template-argument</em></code>.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">11</a></span> <em>Effects</em>: Equivalent to <code class="sourceCode cpp"><span class="cf">return</span> std<span class="op">::</span>visit<span class="op">(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>Visitor<span class="op">&gt;(</span>vis<span class="op">)</span>, <span class="op">(</span>V<span class="op">)</span>self<span class="op">)</span>;</code></p>
<div class="sourceCode" id="cb10"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb10-1"><a href="#cb10-1"></a>template&lt;class R, class Self, class Visitor&gt;</span>
<span id="cb10-2"><a href="#cb10-2"></a>  constexpr R visit(this Self&amp;&amp; self, Visitor&amp;&amp; vis);</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">12</a></span> Let <code class="sourceCode cpp">V</code> be <code class="sourceCode cpp"><em>OVERRIDE_REF</em><span class="op">(</span>Self<span class="op">&amp;&amp;</span>, <em>COPY_CONST</em><span class="op">(</span>remove_reference_t<span class="op">&lt;</span>Self<span class="op">&gt;</span>, variant<span class="op">))</span></code> ([forward]).</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">13</a></span> <em>Effects</em>: Equivalent to <code class="sourceCode cpp"><span class="cf">return</span> std<span class="op">::</span>visit<span class="op">&lt;</span>R<span class="op">&gt;(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>Visitor<span class="op">&gt;(</span>vis<span class="op">)</span>, <span class="op">(</span>V<span class="op">)</span>self<span class="op">)</span>;</code></p>
</div>
</blockquote>
<p>Change the example in <span>22.14.6.6 <a href="https://wg21.link/format.context">[format.context]</a></span>/8:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb11"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb11-1"><a href="#cb11-1"></a>struct S { int value; };</span>
<span id="cb11-2"><a href="#cb11-2"></a></span>
<span id="cb11-3"><a href="#cb11-3"></a>template&lt;&gt; struct std::formatter&lt;S&gt; {</span>
<span id="cb11-4"><a href="#cb11-4"></a>  size_t width_arg_id = 0;</span>
<span id="cb11-5"><a href="#cb11-5"></a></span>
<span id="cb11-6"><a href="#cb11-6"></a>  // Parses a width argument id in the format { digit }.</span>
<span id="cb11-7"><a href="#cb11-7"></a>  constexpr auto parse(format_parse_context&amp; ctx) {</span>
<span id="cb11-8"><a href="#cb11-8"></a>    auto iter = ctx.begin();</span>
<span id="cb11-9"><a href="#cb11-9"></a>    auto get_char = [&amp;]() { return iter != ctx.end() ? *iter : 0; };</span>
<span id="cb11-10"><a href="#cb11-10"></a>    if (get_char() != &#39;{&#39;)</span>
<span id="cb11-11"><a href="#cb11-11"></a>      return iter;</span>
<span id="cb11-12"><a href="#cb11-12"></a>    ++iter;</span>
<span id="cb11-13"><a href="#cb11-13"></a>    char c = get_char();</span>
<span id="cb11-14"><a href="#cb11-14"></a>    if (!isdigit(c) || (++iter, get_char()) != &#39;}&#39;)</span>
<span id="cb11-15"><a href="#cb11-15"></a>      throw format_error(&quot;invalid format&quot;);</span>
<span id="cb11-16"><a href="#cb11-16"></a>    width_arg_id = c - &#39;0&#39;;</span>
<span id="cb11-17"><a href="#cb11-17"></a>    ctx.check_arg_id(width_arg_id);</span>
<span id="cb11-18"><a href="#cb11-18"></a>    return ++iter;</span>
<span id="cb11-19"><a href="#cb11-19"></a>  }</span>
<span id="cb11-20"><a href="#cb11-20"></a></span>
<span id="cb11-21"><a href="#cb11-21"></a>  // Formats an S with width given by the argument width_­arg_­id.</span>
<span id="cb11-22"><a href="#cb11-22"></a>  auto format(S s, format_context&amp; ctx) {</span>
<span id="cb11-23"><a href="#cb11-23"></a><span class="st">-   int width = visit_format_arg([](auto value) -&gt; int {</span></span>
<span id="cb11-24"><a href="#cb11-24"></a><span class="va">+   int width = ctx.arg(width_arg_id).visit([](auto value) -&gt; int {</span></span>
<span id="cb11-25"><a href="#cb11-25"></a>      if constexpr (!is_integral_v&lt;decltype(value)&gt;)</span>
<span id="cb11-26"><a href="#cb11-26"></a>        throw format_error(&quot;width is not integral&quot;);</span>
<span id="cb11-27"><a href="#cb11-27"></a>      else if (value &lt; 0 || value &gt; numeric_limits&lt;int&gt;::max())</span>
<span id="cb11-28"><a href="#cb11-28"></a>        throw format_error(&quot;invalid width&quot;);</span>
<span id="cb11-29"><a href="#cb11-29"></a>      else</span>
<span id="cb11-30"><a href="#cb11-30"></a>        return value;</span>
<span id="cb11-31"><a href="#cb11-31"></a><span class="st">-     }, ctx.arg(width_arg_id));</span></span>
<span id="cb11-32"><a href="#cb11-32"></a><span class="va">+     });</span></span>
<span id="cb11-33"><a href="#cb11-33"></a>    return format_to(ctx.out(), &quot;{0:x&lt;{1}}&quot;, s.value, width);</span>
<span id="cb11-34"><a href="#cb11-34"></a>  }</span>
<span id="cb11-35"><a href="#cb11-35"></a>};</span>
<span id="cb11-36"><a href="#cb11-36"></a></span>
<span id="cb11-37"><a href="#cb11-37"></a>std::string s = std::format(&quot;{0:{1}}&quot;, S{42}, 10);  // value of s is &quot;xxxxxxxx42&quot;</span></code></pre></div>
</div>
</blockquote>
<p>Add to <span>22.14.8.1 <a href="https://wg21.link/format.arg">[format.arg]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb12"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb12-1"><a href="#cb12-1"></a>namespace std {</span>
<span id="cb12-2"><a href="#cb12-2"></a>  template&lt;class Context&gt;</span>
<span id="cb12-3"><a href="#cb12-3"></a>  class basic_format_arg {</span>
<span id="cb12-4"><a href="#cb12-4"></a>    // ...</span>
<span id="cb12-5"><a href="#cb12-5"></a>  public:</span>
<span id="cb12-6"><a href="#cb12-6"></a>    basic_format_arg() noexcept;</span>
<span id="cb12-7"><a href="#cb12-7"></a></span>
<span id="cb12-8"><a href="#cb12-8"></a>    explicit operator bool() const noexcept;</span>
<span id="cb12-9"><a href="#cb12-9"></a></span>
<span id="cb12-10"><a href="#cb12-10"></a><span class="va">+   template&lt;class Visitor&gt;</span></span>
<span id="cb12-11"><a href="#cb12-11"></a><span class="va">+     decltype(auto) visit(this basic_format_arg arg, Visitor&amp;&amp; vis);</span></span>
<span id="cb12-12"><a href="#cb12-12"></a><span class="va">+   template&lt;class R, class Visitor&gt;</span></span>
<span id="cb12-13"><a href="#cb12-13"></a><span class="va">+     R visit(this basic_format_arg arg, Visitor&amp;&amp; vis);</span></span>
<span id="cb12-14"><a href="#cb12-14"></a></span>
<span id="cb12-15"><a href="#cb12-15"></a>  };</span>
<span id="cb12-16"><a href="#cb12-16"></a>}</span></code></pre></div>
</div>
</blockquote>
<p>And:</p>
<blockquote>
<div class="sourceCode" id="cb13"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb13-1"><a href="#cb13-1"></a>explicit operator bool() const noexcept;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">15</a></span> <em>Returns</em>: <code class="sourceCode cpp"><span class="op">!</span>holds_­alternative<span class="op">&lt;</span>monostate<span class="op">&gt;(</span>value<span class="op">)</span></code>.</p>
<div class="addu">
<div class="sourceCode" id="cb14"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb14-1"><a href="#cb14-1"></a>template&lt;class Visitor&gt;</span>
<span id="cb14-2"><a href="#cb14-2"></a>  decltype(auto) visit(this basic_format_arg arg, Visitor&amp;&amp; vis);</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">16</a></span> <em>Effects</em>: Equivalent to <code class="sourceCode cpp"><span class="cf">return</span> arg<span class="op">.</span>value<span class="op">.</span>visit<span class="op">(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>Visitor<span class="op">&gt;(</span>vis<span class="op">))</span>;</code></p>
<div class="sourceCode" id="cb15"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb15-1"><a href="#cb15-1"></a>template&lt;class R, class Visitor&gt;</span>
<span id="cb15-2"><a href="#cb15-2"></a>  R visit(this basic_format_arg arg, Visitor&amp;&amp; vis);</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">17</a></span> <em>Effects</em>: Equivalent to <code class="sourceCode cpp"><span class="cf">return</span> arg<span class="op">.</span>value<span class="op">.</span>visit<span class="op">&lt;</span>R<span class="op">&gt;(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>Visitor<span class="op">&gt;(</span>vis<span class="op">))</span>;</code></p>
</div>
</blockquote>
<h2 data-number="3.1" id="feature-test-macro"><span class="header-section-number">3.1</span> Feature-test macro<a href="#feature-test-macro" class="self-link"></a></h2>
<p>There isn’t much reason to provide one, since would anybody write this?</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">auto</span> result <span class="op">=</span></span>
<span id="cb16-2"><a href="#cb16-2"></a>  <span class="pp">#ifdef __cpp_lib_member_visit </span><span class="co">// or whatever</span></span>
<span id="cb16-3"><a href="#cb16-3"></a>    var<span class="op">.</span>visit<span class="op">(</span>f<span class="op">)</span>;</span>
<span id="cb16-4"><a href="#cb16-4"></a>  <span class="pp">#else</span></span>
<span id="cb16-5"><a href="#cb16-5"></a>    std<span class="op">::</span>visit<span class="op">(</span>f, var<span class="op">)</span>;</span>
<span id="cb16-6"><a href="#cb16-6"></a>  <span class="pp">#endif</span></span></code></pre></div>
</blockquote>
<p>If you have to write the old code, the new code doesn’t give you any benefit. Moreover, a lot of visits are more complicated than just <code class="sourceCode cpp">f</code> - at the very least the’re a lambda, but potentially lots of lambdas.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">4</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-P0847R7">
<p>[P0847R7] Barry Revzin, Gašper Ažman, Sy Brand, Ben Deane. 2021-07-14. Deducing this. <br />
<a href="https://wg21.link/p0847r7">https://wg21.link/p0847r7</a></p>
</div>
<div id="ref-P1450R3">
<p>[P1450R3] Vincent Reverdy. 2020-06-15. Enriching type modification traits. <br />
<a href="https://wg21.link/p1450r3">https://wg21.link/p1450r3</a></p>
</div>
<div id="ref-P2637R0">
<p>[P2637R0] Barry Revzin. 2022-09-17. Member <code class="sourceCode cpp">visit</code> and <code class="sourceCode cpp">apply</code>. <br />
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2637r0.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2637r0.html</a></p>
</div>
</div>
<section class="footnotes" role="doc-endnotes">
<hr />
<ol>
<li id="fn1" role="doc-endnote"><p>A single non-member function template is still superior to four member function overloads due to proper handling of certain edge cases. See the section on <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0847r7.html#sfinae-friendly-callables">SFINAE-friendly</a> for more information.<a href="#fnref1" class="footnote-back" role="doc-backlink">↩︎</a></p></li>
</ol>
</section>
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