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<title>Permitting static constexpr variables in constexpr functions</title>
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<h1 class="title" style="text-align:center">Permitting <code class="sourceCode cpp"><span class="kw">static</span> <span class="kw">constexpr</span></code> variables in <code class="sourceCode cpp"><span class="kw">constexpr</span></code> functions</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2647R1</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-11-07</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>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
Jonathan Wakely<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="#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="#history"><span class="toc-section-number">2.1</span> History<span></span></a></li>
<li><a href="#workarounds"><span class="toc-section-number">2.2</span> Workarounds<span></span></a></li>
</ul></li>
<li><a href="#proposal"><span class="toc-section-number">3</span> Proposal<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="P2647R0">[<a href="#ref-P2647R0" role="doc-biblioref">P2647R0</a>]</span>, updated wording, added an example and feature-test macro.</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>Consider this example:</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="dt">char</span> xdigit<span class="op">(</span><span class="dt">int</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a> <span class="kw">static</span> <span class="kw">constexpr</span> <span class="dt">char</span> digits<span class="op">[]</span> <span class="op">=</span> <span class="st">"0123456789abcdef"</span>;</span>
<span id="cb1-3"><a href="#cb1-3"></a> <span class="cf">return</span> digits<span class="op">[</span>n<span class="op">]</span>;</span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This function is totally fine. But when we try to extend it to work at compile time too, we run into a problem:</p>
<blockquote>
<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">constexpr</span> <span class="dt">char</span> xdigit<span class="op">(</span><span class="dt">int</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a> <span class="kw">static</span> <span class="kw">constexpr</span> <span class="dt">char</span> digits<span class="op">[]</span> <span class="op">=</span> <span class="st">"0123456789abcdef"</span>;</span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="cf">return</span> digits<span class="op">[</span>n<span class="op">]</span>;</span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This is ill-formed. And it’s ill-formed for no good reason. We should make it valid.</p>
<h2 data-number="2.1" id="history"><span class="header-section-number">2.1</span> History<a href="#history" class="self-link"></a></h2>
<p>Originally, no <code class="sourceCode cpp"><span class="kw">static</span></code> variables could be declared in <code class="sourceCode cpp"><span class="kw">constexpr</span></code> functions at all. That was relaxed in <span class="citation" data-cites="P2242R3">[<a href="#ref-P2242R3" role="doc-biblioref">P2242R3</a>]</span>, which restructured the wording to be based on the actual control flow: rather than the function body not being allowed to contain a <code class="sourceCode cpp"><span class="kw">static</span></code> variable declaration, now the rule is that control flow needs to not pass through an initialization of a <code class="sourceCode cpp"><span class="kw">static</span></code> variable.</p>
<p>This makes sense for the perspective of a <code class="sourceCode cpp"><span class="kw">static</span></code> (or, worse, <code class="sourceCode cpp"><span class="kw">thread_local</span></code>) variable, whose initializer could run arbitrary code. But for a <code class="sourceCode cpp"><span class="kw">static</span> <span class="kw">constexpr</span></code> variable, which must, by definition, be constant-initialization, there’s no question about whether and when to run what initialization. It’s a constant.</p>
<h2 data-number="2.2" id="workarounds"><span class="header-section-number">2.2</span> Workarounds<a href="#workarounds" class="self-link"></a></h2>
<p>There are several workarounds for getting the above example to work. We could eschew the <code class="sourceCode cpp"><span class="kw">static</span></code> variable entirely and directly index the literal, but this only works if we need to use it exactly one time:</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">constexpr</span> <span class="dt">char</span> xdigit<span class="op">(</span><span class="dt">int</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a> <span class="cf">return</span> <span class="st">"0123456789abcdef"</span><span class="op">[</span>n<span class="op">]</span>;</span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>We could move the <code class="sourceCode cpp"><span class="kw">static</span></code> variable into non-local scope, though we wanted to make it local for a reason - it’s only relevant to this particular function:</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">static</span> <span class="kw">constexpr</span> <span class="dt">char</span> digits<span class="op">[]</span> <span class="op">=</span> <span class="st">"0123456789abcdef"</span>;</span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="kw">constexpr</span> <span class="dt">char</span> xdigit<span class="op">(</span><span class="dt">int</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb4-3"><a href="#cb4-3"></a> <span class="cf">return</span> digits<span class="op">[</span>n<span class="op">]</span>;</span>
<span id="cb4-4"><a href="#cb4-4"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>We could make the variable non-<code class="sourceCode cpp"><span class="kw">static</span></code>, but compilers have <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99091">difficulty optimizing this</a>, leading to <a href="https://godbolt.org/z/dvzTqbhc8">much worse code-gen</a>:</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">constexpr</span> <span class="dt">char</span> xdigit<span class="op">(</span><span class="dt">int</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a> <span class="kw">constexpr</span> <span class="dt">char</span> digits<span class="op">[]</span> <span class="op">=</span> <span class="st">"0123456789abcdef"</span>;</span>
<span id="cb5-3"><a href="#cb5-3"></a> <span class="cf">return</span> digits<span class="op">[</span>n<span class="op">]</span>;</span>
<span id="cb5-4"><a href="#cb5-4"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Or we could wrap the variable into a <code class="sourceCode cpp"><span class="kw">consteval</span></code> lambda, which is the most general workaround for this problem, whose only downside is that… we’re writing a <code class="sourceCode cpp"><span class="kw">consteval</span></code> lambda because we can’t write a variable:</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">constexpr</span> <span class="dt">char</span> xdigit<span class="op">(</span><span class="dt">int</span> n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-2"><a href="#cb6-2"></a> <span class="kw">auto</span> get_digits <span class="op">=</span> <span class="op">[]()</span> <span class="kw">consteval</span> <span class="op">{</span> <span class="cf">return</span> <span class="st">"0123456789abcdef"</span>; <span class="op">}</span>;</span>
<span id="cb6-3"><a href="#cb6-3"></a> <span class="cf">return</span> get_digits<span class="op">()[</span>n<span class="op">]</span>;</span>
<span id="cb6-4"><a href="#cb6-4"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Having a local <code class="sourceCode cpp"><span class="kw">static</span> <span class="kw">constexpr</span></code> variable is simply the obvious, most direct solution to the problem. We should permit it.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="proposal"><span class="header-section-number">3</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>Change <span>7.7 <a href="https://wg21.link/expr.const">[expr.const]</a></span>/5.2:</p>
<blockquote>
<ul>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">5</a></span> An expression <code class="sourceCode cpp">E</code> is a <em>core constant expression</em> unless the evaluation of <code class="sourceCode cpp">E</code>, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following:</p>
<ul>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">(5.1)</a></span> …</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">(5.2)</a></span> a control flow that passes through a declaration of a variable with static ([basic.stc.static]) or thread ([basic.stc.thread]) storage duration<span class="addu">, unless that variable is usable in constant expressions</span>;</p></li>
</ul></li>
</ul>
<div class="addu">
<p>[<em>Example</em>:</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb7-1"><a href="#cb7-1"></a>constexpr char test()</span>
<span id="cb7-2"><a href="#cb7-2"></a>{</span>
<span id="cb7-3"><a href="#cb7-3"></a> static const int x = 5;</span>
<span id="cb7-4"><a href="#cb7-4"></a> static constexpr char c[] = "Hello World";</span>
<span id="cb7-5"><a href="#cb7-5"></a> return *(c+x);</span>
<span id="cb7-6"><a href="#cb7-6"></a>}</span>
<span id="cb7-7"><a href="#cb7-7"></a>static_assert(' ' == test());</span></code></pre></div>
<p>-<em>end example</em>]</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>Change <span>15.11 <a href="https://wg21.link/cpp.predefined">[cpp.predefined]</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><span class="st">- __cpp_constexpr <span class="diffdel">202207L</span></span></span>
<span id="cb8-2"><a href="#cb8-2"></a><span class="va">+ __cpp_constexpr <span class="diffins">20XXXXL</span></span></span></code></pre></div>
</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-P2242R3">
<p>[P2242R3] Ville Voutilainen. 2021-07-13. Non-literal variables (and labels and gotos) in constexpr functions. <br />
<a href="https://wg21.link/p2242r3">https://wg21.link/p2242r3</a></p>
</div>
<div id="ref-P2647R0">
<p>[P2647R0] Barry Revzin, Jonathan Wakely. 2022-10-06. Permitting static constexpr variables in constexpr functions. <br />
<a href="https://wg21.link/p2647r0">https://wg21.link/p2647r0</a></p>
</div>
</div>
</div>
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