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<title>Checking if a union alternative is active</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Checking if a union alternative is active</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2641R1</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-10-08</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, LEWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
Daveed Vandevoorde<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#introduction"><span class="toc-section-number">1</span> Introduction<span></span></a></li>
<li><a href="#proposal"><span class="toc-section-number">2</span> Proposal<span></span></a>
<ul>
<li><a href="#interesting-cases"><span class="toc-section-number">2.1</span> Interesting Cases<span></span></a>
<ul>
<li><a href="#nested-anonymous-unions"><span class="toc-section-number">2.1.1</span> Nested Anonymous Unions<span></span></a></li>
<li><a href="#subobject"><span class="toc-section-number">2.1.2</span> Subobject<span></span></a></li>
<li><a href="#other-rules"><span class="toc-section-number">2.1.3</span> Other rules<span></span></a></li>
</ul></li>
<li><a href="#alternative-naming"><span class="toc-section-number">2.2</span> Alternative Naming<span></span></a></li>
<li><a href="#implementation-experience"><span class="toc-section-number">2.3</span> Implementation Experience<span></span></a></li>
</ul></li>
<li><a href="#wording"><span class="toc-section-number">3</span> Wording<span></span></a></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="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>Let’s say you want to implement an <code class="sourceCode cpp">Optional<span class="op"><</span><span class="dt">bool</span><span class="op">></span></code> such that <code class="sourceCode cpp"><span class="kw">sizeof</span><span class="op">(</span>Optional<span class="op"><</span><span class="dt">bool</span><span class="op">>)</span> <span class="op">==</span> <span class="dv">1</span></code>. This is conceptually doable since <code class="sourceCode cpp"><span class="dt">bool</span></code> only has 2 states, but takes up a whole byte anyway, which leaves 254 bit patterns to use to express other criteria (i.e. that the <code class="sourceCode cpp">Optional</code> is disengaged).</p>
<p>There are two ways to do this, neither of which have undefined behavior:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Union</strong>
</div></th>
<th><div style="text-align:center">
<strong>Reinterpret</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><span class="kw">struct</span> OptBool <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a> <span class="kw">union</span> <span class="op">{</span> <span class="dt">bool</span> b; <span class="dt">char</span> c; <span class="op">}</span>;</span>
<span id="cb1-3"><a href="#cb1-3"></a></span>
<span id="cb1-4"><a href="#cb1-4"></a> OptBool<span class="op">()</span> <span class="op">:</span> c<span class="op">(</span><span class="dv">2</span><span class="op">)</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb1-5"><a href="#cb1-5"></a> OptBool<span class="op">(</span><span class="dt">bool</span> b<span class="op">)</span> <span class="op">:</span> b<span class="op">(</span>b<span class="op">)</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb1-6"><a href="#cb1-6"></a></span>
<span id="cb1-7"><a href="#cb1-7"></a> <span class="kw">auto</span> has_value<span class="op">()</span> <span class="kw">const</span> <span class="op">-></span> <span class="dt">bool</span> <span class="op">{</span></span>
<span id="cb1-8"><a href="#cb1-8"></a> <span class="cf">return</span> c <span class="op">!=</span> <span class="dv">2</span>;</span>
<span id="cb1-9"><a href="#cb1-9"></a> <span class="op">}</span></span>
<span id="cb1-10"><a href="#cb1-10"></a></span>
<span id="cb1-11"><a href="#cb1-11"></a> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">*()</span> <span class="op">-></span> <span class="dt">bool</span><span class="op">&</span> <span class="op">{</span></span>
<span id="cb1-12"><a href="#cb1-12"></a> <span class="cf">return</span> b;</span>
<span id="cb1-13"><a href="#cb1-13"></a> <span class="op">}</span></span>
<span id="cb1-14"><a href="#cb1-14"></a><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><span class="kw">struct</span> OptBool <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a> <span class="dt">char</span> c;</span>
<span id="cb2-3"><a href="#cb2-3"></a></span>
<span id="cb2-4"><a href="#cb2-4"></a> OptBool<span class="op">()</span> <span class="op">:</span> c<span class="op">(</span><span class="dv">2</span><span class="op">)</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb2-5"><a href="#cb2-5"></a> OptBool<span class="op">(</span><span class="dt">bool</span> b<span class="op">)</span> <span class="op">{</span> <span class="kw">new</span> <span class="op">(&</span>c<span class="op">)</span> <span class="dt">bool</span><span class="op">(</span>b<span class="op">)</span>; <span class="op">}</span></span>
<span id="cb2-6"><a href="#cb2-6"></a></span>
<span id="cb2-7"><a href="#cb2-7"></a> <span class="kw">auto</span> has_value<span class="op">()</span> <span class="kw">const</span> <span class="op">-></span> <span class="dt">bool</span> <span class="op">{</span></span>
<span id="cb2-8"><a href="#cb2-8"></a> <span class="cf">return</span> c <span class="op">!=</span> <span class="dv">2</span>;</span>
<span id="cb2-9"><a href="#cb2-9"></a> <span class="op">}</span></span>
<span id="cb2-10"><a href="#cb2-10"></a></span>
<span id="cb2-11"><a href="#cb2-11"></a> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">*()</span> <span class="op">-></span> <span class="dt">bool</span><span class="op">&</span> <span class="op">{</span></span>
<span id="cb2-12"><a href="#cb2-12"></a> <span class="cf">return</span> <span class="op">(</span><span class="dt">bool</span><span class="op">&)</span>c;</span>
<span id="cb2-13"><a href="#cb2-13"></a> <span class="op">}</span></span>
<span id="cb2-14"><a href="#cb2-14"></a><span class="op">}</span>;</span></code></pre></div></td>
</tr>
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</table>
<p>Both of these are fine: <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">*</span></code> works because we are returning a reference to a very-much-live <code class="sourceCode cpp"><span class="dt">bool</span></code> in both cases, and <code class="sourceCode cpp">has_value<span class="op">()</span></code> works because we are allowed read through a <code class="sourceCode cpp"><span class="dt">char</span></code> (explicitly, per <a href="https://timsong-cpp.github.io/cppwp/n4868/basic.lval#11.3">[basic.lval]/11.3</a><a href="#fn1" class="footnote-ref" id="fnref1" role="doc-noteref"><sup>1</sup></a>).</p>
<p>However, try to make this work in <code class="sourceCode cpp"><span class="kw">constexpr</span></code> and we immediately run into problems.</p>
<p>The union solution has a problem with <code class="sourceCode cpp">has_value<span class="op">()</span></code> because we’re not allowed to read the non-active member of a union (<a href="https://timsong-cpp.github.io/cppwp/n4868/expr.const#5.9">[expr.const]/5.9</a>). <code class="sourceCode cpp">c</code> isn’t the active member of that union, so we can’t read from it, even though it’s <code class="sourceCode cpp"><span class="dt">char</span></code>.</p>
<p>The reinterpet solution has two problems:</p>
<ul>
<li>the placement new into <code class="sourceCode cpp"><span class="op">&</span>c</code> doesn’t work. Placement-new is explicitly disallowed by <a href="https://timsong-cpp.github.io/cppwp/n4868/expr.const#5.17">[expr.const]/5.17</a>. While we have <code class="sourceCode cpp">std<span class="op">::</span>construct_at<span class="op">()</span></code> now to work around the lack of placement-new, that API is typed and would require a <code class="sourceCode cpp"><span class="dt">bool</span><span class="op">*</span></code>, which we don’t have.</li>
<li><code class="sourceCode cpp"><span class="kw">operator</span><span class="op">*()</span></code> involves a <code class="sourceCode cpp"><span class="kw">reinterpret_cast</span></code>, which is explicitly disallowed by <a href="https://timsong-cpp.github.io/cppwp/n4868/expr.const#5.15">[expr.const]/5.15</a>.</li>
</ul>
<p>This is unfortunate - we have a well-defined runtime solution that we cannot use during constant evaluation time. It’d be nice to do better.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="proposal"><span class="header-section-number">2</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>Supporting the reinterpret solution is expensive - since it requires the implementation to track multiple types for the same bytes. But supporting the union solution, because it’s more structured (and thus arguably better anyway), is very feasible. After all, our problem is localized to <code class="sourceCode cpp">has_value<span class="op">()</span></code>.</p>
<p>During runtime, we do need to read from <code class="sourceCode cpp">c</code>. But during constant evaluation time, we actually don’t. What we need <code class="sourceCode cpp">c</code> for is to know whether <code class="sourceCode cpp">b</code> is the active member of the union or not. But the compiler already knows whether <code class="sourceCode cpp">b</code> is the active member or not. Indeed, that is precisely <em>why</em> it’s rejecting this implementation. What if we could simply ask the compiler this question?</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">struct</span> OptBool <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a> <span class="kw">union</span> <span class="op">{</span> <span class="dt">bool</span> b; <span class="dt">char</span> c; <span class="op">}</span>;</span>
<span id="cb3-3"><a href="#cb3-3"></a></span>
<span id="cb3-4"><a href="#cb3-4"></a> <span class="kw">constexpr</span> OptBool<span class="op">()</span> <span class="op">:</span> c<span class="op">(</span><span class="dv">2</span><span class="op">)</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb3-5"><a href="#cb3-5"></a> <span class="kw">constexpr</span> OptBool<span class="op">(</span><span class="dt">bool</span> b<span class="op">)</span> <span class="op">:</span> b<span class="op">(</span>b<span class="op">)</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb3-6"><a href="#cb3-6"></a></span>
<span id="cb3-7"><a href="#cb3-7"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> has_value<span class="op">()</span> <span class="kw">const</span> <span class="op">-></span> <span class="dt">bool</span> <span class="op">{</span></span>
<span id="cb3-8"><a href="#cb3-8"></a> <span class="cf">if</span> <span class="kw">consteval</span> <span class="op">{</span></span>
<span id="cb3-9"><a href="#cb3-9"></a> <span class="cf">return</span> std<span class="op">::</span>is_active_member<span class="op">(&</span>b<span class="op">)</span>;</span>
<span id="cb3-10"><a href="#cb3-10"></a> <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb3-11"><a href="#cb3-11"></a> <span class="cf">return</span> c <span class="op">!=</span> <span class="dv">2</span>;</span>
<span id="cb3-12"><a href="#cb3-12"></a> <span class="op">}</span></span>
<span id="cb3-13"><a href="#cb3-13"></a> <span class="op">}</span></span>
<span id="cb3-14"><a href="#cb3-14"></a></span>
<span id="cb3-15"><a href="#cb3-15"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">*()</span> <span class="op">-></span> <span class="dt">bool</span><span class="op">&</span> <span class="op">{</span></span>
<span id="cb3-16"><a href="#cb3-16"></a> <span class="cf">return</span> b;</span>
<span id="cb3-17"><a href="#cb3-17"></a> <span class="op">}</span></span>
<span id="cb3-18"><a href="#cb3-18"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p>There’s no particular implementation burden here - the compiler already needs to know this information. No language change is necessary to support this change either.</p>
<h2 data-number="2.1" id="interesting-cases"><span class="header-section-number">2.1</span> Interesting Cases<a href="#interesting-cases" class="self-link"></a></h2>
<p>Let’s go over some interesting cases to flesh out how this facility should behave</p>
<h3 data-number="2.1.1" id="nested-anonymous-unions"><span class="header-section-number">2.1.1</span> Nested Anonymous Unions<a href="#nested-anonymous-unions" class="self-link"></a></h3>
<p>Consider:</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">union</span> Outer <span class="op">{</span></span>
<span id="cb4-2"><a href="#cb4-2"></a> <span class="kw">union</span> <span class="op">{</span></span>
<span id="cb4-3"><a href="#cb4-3"></a> <span class="dt">int</span> x;</span>
<span id="cb4-4"><a href="#cb4-4"></a> <span class="dt">unsigned</span> <span class="dt">int</span> y;</span>
<span id="cb4-5"><a href="#cb4-5"></a> <span class="op">}</span>;</span>
<span id="cb4-6"><a href="#cb4-6"></a> <span class="dt">float</span> f;</span>
<span id="cb4-7"><a href="#cb4-7"></a><span class="op">}</span>;</span>
<span id="cb4-8"><a href="#cb4-8"></a><span class="kw">consteval</span> <span class="dt">bool</span> f<span class="op">(</span>Outer <span class="op">*</span>p<span class="op">)</span> <span class="op">{</span></span>
<span id="cb4-9"><a href="#cb4-9"></a> <span class="cf">return</span> std<span class="op">::</span>is_consteval_active_member<span class="op">(&</span>p<span class="op">-></span>x<span class="op">)</span>;</span>
<span id="cb4-10"><a href="#cb4-10"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>If the active member of <code class="sourceCode cpp">p</code> is actually <code class="sourceCode cpp">f</code>, then not only is <code class="sourceCode cpp">p<span class="op">-></span>x</code> not the active member of <em>its</em> union but the union that it’s a member of isn’t even itself an active member. What should happen in this case?</p>
<p>Arguably, this should be valid and return <code class="sourceCode cpp"><span class="kw">false</span></code>. It doesn’t matter how many layers of inactivity there are - <code class="sourceCode cpp">p<span class="op">-></span>x</code> isn’t the active member and that’s what we’re asking about.</p>
<h3 data-number="2.1.2" id="subobject"><span class="header-section-number">2.1.2</span> Subobject<a href="#subobject" class="self-link"></a></h3>
<p>Consider:</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">struct</span> S <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a> <span class="kw">union</span> <span class="op">{</span></span>
<span id="cb5-3"><a href="#cb5-3"></a> <span class="kw">struct</span> <span class="op">{</span></span>
<span id="cb5-4"><a href="#cb5-4"></a> <span class="dt">int</span> i;</span>
<span id="cb5-5"><a href="#cb5-5"></a> <span class="op">}</span> n;</span>
<span id="cb5-6"><a href="#cb5-6"></a> <span class="op">}</span>;</span>
<span id="cb5-7"><a href="#cb5-7"></a><span class="op">}</span>;</span>
<span id="cb5-8"><a href="#cb5-8"></a></span>
<span id="cb5-9"><a href="#cb5-9"></a><span class="kw">consteval</span> <span class="dt">bool</span> f<span class="op">(</span>S s<span class="op">)</span> <span class="op">{</span></span>
<span id="cb5-10"><a href="#cb5-10"></a> <span class="cf">return</span> std<span class="op">::</span>is_consteval_active_member<span class="op">(&</span>s<span class="op">.</span>n<span class="op">.</span>i<span class="op">)</span>;</span>
<span id="cb5-11"><a href="#cb5-11"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Here, <code class="sourceCode cpp">s<span class="op">.</span>n<span class="op">.</span>i</code> is not a variant member of a union - it’s a subobject of <code class="sourceCode cpp">s<span class="op">.</span>n</code>. We could say this is ill-formed, requiring that the pointer into this function actually be a pointer to a variant member. But that’s seems overly strict. After all, the specific language rule is, emphasis mine:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">(5.9)</a></span> an lvalue-to-rvalue conversion that is applied to a glvalue that refers to a non-active member of a union <strong>or a subobject thereof;</strong></p>
</blockquote>
<p>The same rule that would reject using <code class="sourceCode cpp">s<span class="op">.</span>n</code> if that’s not an active member would reject <code class="sourceCode cpp">s<span class="op">.</span>n<span class="op">.</span>i</code> if it’s not the subobject of an active member. So we should just accept this case (returning true of <code class="sourceCode cpp"><span class="op">&</span>s<span class="op">.</span>n</code> is the active member), rather than rejecting it.</p>
<h3 data-number="2.1.3" id="other-rules"><span class="header-section-number">2.1.3</span> Other rules<a href="#other-rules" class="self-link"></a></h3>
<p>There’s a similar rule in this space, <a href="https://timsong-cpp.github.io/cppwp/n4868/expr.const#5.8">[expr.const]/5.8</a></p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">(5.8)</a></span> an lvalue-to-rvalue conversion unless it is applied to</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">(5.8.1)</a></span> a non-volatile glvalue that refers to an object that is usable in constant expressions, or</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">(5.8.2)</a></span> a non-volatile glvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;</li>
</ul>
</blockquote>
<p>Should the facility in this paper address this case as well? At this point, this would become a “is this a pointer that I can read during constant evaluation?” facility. Would that actually be useful? It’s not clear what you could do with this information.</p>
<h2 data-number="2.2" id="alternative-naming"><span class="header-section-number">2.2</span> Alternative Naming<a href="#alternative-naming" class="self-link"></a></h2>
<p>Should the name reflect that this facility can only be used during constant evaluation (<code class="sourceCode cpp">std<span class="op">::</span>is_consteval_active_member</code>) or not (<code class="sourceCode cpp">std<span class="op">::</span>is_active_member</code>)?</p>
<p>It would help to make the name clearer from the call site that it has limited usage. But it is already a <code class="sourceCode cpp"><span class="kw">consteval</span></code> function, so it’s not like you could misuse it.</p>
<h2 data-number="2.3" id="implementation-experience"><span class="header-section-number">2.3</span> Implementation Experience<a href="#implementation-experience" class="self-link"></a></h2>
<p>Daveed Vandevoorde has implemented this proposal in EDG.</p>
<p>As pointed out by Johel Ernesto Guerrero Peña and Ed Catmur, this facility basically already works in <a href="https://godbolt.org/z/n4za9KzPr">gcc and clang</a>, which has a builtin function (<a href="https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fconstant_005fp"><code class="sourceCode cpp"><span class="fu">__builtin_constant_p</span></code></a>) that be used to achieve the same behavior.</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>21.3.3 <a href="https://wg21.link/meta.type.synop">[meta.type.synop]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb6"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb6-1"><a href="#cb6-1"></a>// all freestanding</span>
<span id="cb6-2"><a href="#cb6-2"></a>namespace std {</span>
<span id="cb6-3"><a href="#cb6-3"></a> // ...</span>
<span id="cb6-4"><a href="#cb6-4"></a></span>
<span id="cb6-5"><a href="#cb6-5"></a> // [meta.const.eval], constant evaluation context</span>
<span id="cb6-6"><a href="#cb6-6"></a> constexpr bool is_constant_evaluated() noexcept;</span>
<span id="cb6-7"><a href="#cb6-7"></a><span class="va">+ template<class T></span></span>
<span id="cb6-8"><a href="#cb6-8"></a><span class="va">+ consteval bool is_active_member(T*);</span></span>
<span id="cb6-9"><a href="#cb6-9"></a>}</span></code></pre></div>
</div>
</blockquote>
<p>And add to <span>21.3.11 <a href="https://wg21.link/meta.const.eval">[meta.const.eval]</a></span> (possibly this section should just change name, but it feels like these two should just go together):</p>
<blockquote>
<div class="sourceCode" id="cb7"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb7-1"><a href="#cb7-1"></a>constexpr bool is_constant_evaluated() noexcept;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">1</a></span> <em>Effects</em>: Equivalent to:</p>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="cf">if</span> <span class="kw">consteval</span> <span class="op">{</span></span>
<span id="cb8-2"><a href="#cb8-2"></a> <span class="cf">return</span> <span class="kw">true</span>;</span>
<span id="cb8-3"><a href="#cb8-3"></a><span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb8-4"><a href="#cb8-4"></a> <span class="cf">return</span> <span class="kw">false</span>;</span>
<span id="cb8-5"><a href="#cb8-5"></a><span class="op">}</span></span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">2</a></span> [<em>Example 1</em>:</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb9-1"><a href="#cb9-1"></a><span class="kw">constexpr</span> <span class="dt">void</span> f<span class="op">(</span><span class="dt">unsigned</span> <span class="dt">char</span> <span class="op">*</span>p, <span class="dt">int</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb9-2"><a href="#cb9-2"></a> <span class="cf">if</span> <span class="op">(</span>std<span class="op">::</span>is_constant_evaluated<span class="op">())</span> <span class="op">{</span> <span class="co">// should not be a constexpr if statement</span></span>
<span id="cb9-3"><a href="#cb9-3"></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>; k<span class="op"><</span>n; <span class="op">++</span>k<span class="op">)</span> p<span class="op">[</span>k<span class="op">]</span> <span class="op">=</span> <span class="dv">0</span>;</span>
<span id="cb9-4"><a href="#cb9-4"></a> <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb9-5"><a href="#cb9-5"></a> memset<span class="op">(</span>p, <span class="dv">0</span>, n<span class="op">)</span>; <span class="co">// not a core constant expression</span></span>
<span id="cb9-6"><a href="#cb9-6"></a> <span class="op">}</span></span>
<span id="cb9-7"><a href="#cb9-7"></a><span class="op">}</span></span></code></pre></div>
<p>— <em>end example</em>]</p>
<div class="addu">
<div class="sourceCode" id="cb10"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb10-1"><a href="#cb10-1"></a>template<class T></span>
<span id="cb10-2"><a href="#cb10-2"></a> consteval bool is_active_member(T* p);</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">3</a></span> <em>Returns</em>: <code class="sourceCode cpp"><span class="kw">true</span></code> if <code class="sourceCode cpp">p</code> is a pointer to the active member of a union or a subobject thereof; otherwise, <code class="sourceCode cpp"><span class="kw">false</span></code>.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">4</a></span> <em>Remarks</em>: A call to this function is not a core constant expression ([expr.const]) unless <code class="sourceCode cpp">p</code> is a pointer to a variant member of a union or a subobject thereof.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_9" id="pnum_9">5</a></span> [<em>Example 2</em>:</p>
<div class="sourceCode" id="cb11"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb11-1"><a href="#cb11-1"></a>struct OptBool {</span>
<span id="cb11-2"><a href="#cb11-2"></a> union { bool b; char c; };</span>
<span id="cb11-3"><a href="#cb11-3"></a></span>
<span id="cb11-4"><a href="#cb11-4"></a> constexpr OptBool() : c(2) { }</span>
<span id="cb11-5"><a href="#cb11-5"></a> constexpr OptBool(bool b) : b(b) { }</span>
<span id="cb11-6"><a href="#cb11-6"></a></span>
<span id="cb11-7"><a href="#cb11-7"></a> constexpr auto has_value() const -> bool {</span>
<span id="cb11-8"><a href="#cb11-8"></a> if consteval {</span>
<span id="cb11-9"><a href="#cb11-9"></a> return std::is_active_member(&b); // during constant evaluation, cannot read from c</span>
<span id="cb11-10"><a href="#cb11-10"></a> } else {</span>
<span id="cb11-11"><a href="#cb11-11"></a> return c != 2; // during runtime, must read from c</span>
<span id="cb11-12"><a href="#cb11-12"></a> }</span>
<span id="cb11-13"><a href="#cb11-13"></a> }</span>
<span id="cb11-14"><a href="#cb11-14"></a></span>
<span id="cb11-15"><a href="#cb11-15"></a> constexpr auto operator*() -> bool& {</span>
<span id="cb11-16"><a href="#cb11-16"></a> return b;</span>
<span id="cb11-17"><a href="#cb11-17"></a> }</span>
<span id="cb11-18"><a href="#cb11-18"></a>};</span>
<span id="cb11-19"><a href="#cb11-19"></a></span>
<span id="cb11-20"><a href="#cb11-20"></a>constexpr OptBool disengaged;</span>
<span id="cb11-21"><a href="#cb11-21"></a>constexpr OptBool engaged(true);</span>
<span id="cb11-22"><a href="#cb11-22"></a>static_assert(!disengaged.has_value());</span>
<span id="cb11-23"><a href="#cb11-23"></a>static_assert(engaged.has_value());</span>
<span id="cb11-24"><a href="#cb11-24"></a>static_assert(*engaged);</span></code></pre></div>
<ul>
<li><em>end example</em>]</li>
</ul>
</div>
</blockquote>
<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-N4868">
<p>[N4868] Richard Smith. 2020-10-18. Working Draft, Standard for Programming Language C++. <br />
<a href="https://wg21.link/n4868">https://wg21.link/n4868</a></p>
</div>
</div>
<section class="footnotes" role="doc-endnotes">
<hr />
<ol>
<li id="fn1" role="doc-endnote"><p>All references to this paper are to <span class="citation" data-cites="N4868">[<a href="#ref-N4868" role="doc-biblioref">N4868</a>]</span>, C++20 for convenient and stable linking.<a href="#fnref1" class="footnote-back" role="doc-backlink">↩︎</a></p></li>
</ol>
</section>
</div>
</div>
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