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<title>Using unknown pointers and references in constant expressions</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Using unknown pointers and references in constant expressions</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2280R4</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-04-08</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<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>
</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="#wait-why"><span class="toc-section-number">2.1</span> Wait, why?<span></span></a></li>
<li><a href="#other-examples"><span class="toc-section-number">2.2</span> Other Examples<span></span></a></li>
<li><a href="#the-this-pointer"><span class="toc-section-number">2.3</span> The <code class="sourceCode cpp"><span class="kw">this</span></code> pointer<span></span></a></li>
<li><a href="#other-pointers"><span class="toc-section-number">2.4</span> Other pointers<span></span></a>
<ul>
<li><a href="#what-about-nullptr"><span class="toc-section-number">2.4.1</span> What about <code class="sourceCode cpp"><span class="kw">nullptr</span></code>?<span></span></a></li>
</ul></li>
</ul></li>
<li><a href="#proposal"><span class="toc-section-number">3</span> Proposal<span></span></a>
<ul>
<li><a href="#implementation-experience"><span class="toc-section-number">3.1</span> Implementation Experience<span></span></a></li>
<li><a href="#other-not-quite-reference-examples"><span class="toc-section-number">3.2</span> Other not-quite-reference examples<span></span></a></li>
<li><a href="#lifetime-dilemma"><span class="toc-section-number">3.3</span> Lifetime Dilemma<span></span></a></li>
<li><a href="#still-further-cases"><span class="toc-section-number">3.4</span> Still further cases<span></span></a></li>
<li><a href="#wording"><span class="toc-section-number">3.5</span> Wording<span></span></a></li>
</ul></li>
<li><a href="#acknowledgments"><span class="toc-section-number">4</span> Acknowledgments<span></span></a></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="P2280R3">[<a href="#ref-P2280R3" role="doc-biblioref">P2280R3</a>]</span>, wording changes.</p>
<p>To help clarify what expressions were allowed in which versions, here is a table:</p>
<table>
<tr>
<th></th>
<th>
References to unknown
</th>
<th>
Accesses via <code class="sourceCode cpp"><span class="kw">this</span></code>
</th>
<th>
Pointers to unknown
</th>
</tr>
<tr>
<td>
<span class="citation" data-cites="P2280R0">[<a href="#ref-P2280R0" role="doc-biblioref">P2280R0</a>]</span>
</td>
<td>
✔️
</td>
<td>
❌
</td>
<td>
❌
</td>
</tr>
<tr>
<td>
<span class="citation" data-cites="P2280R1">[<a href="#ref-P2280R1" role="doc-biblioref">P2280R1</a>]</span>
</td>
<td>
✔️
</td>
<td>
✔️
</td>
<td>
❌
</td>
</tr>
<tr>
<td>
<span class="citation" data-cites="P2280R2">[<a href="#ref-P2280R2" role="doc-biblioref">P2280R2</a>]</span>
</td>
<td>
✔️
</td>
<td>
✔️
</td>
<td>
✔️
</td>
</tr>
<tr>
<td>
This paper (R3)
</td>
<td>
✔️
</td>
<td>
✔️
</td>
<td>
❌
</td>
</tr>
</table>
<p><span class="citation" data-cites="P2280R2">[<a href="#ref-P2280R2" role="doc-biblioref">P2280R2</a>]</span> extended R1 to also include pointers-to-unknown. R3 pares that extension back down, effectively reverting to R1 of this paper, following EWG discussion of R2.</p>
<p><span class="citation" data-cites="P2280R1">[<a href="#ref-P2280R1" role="doc-biblioref">P2280R1</a>]</span> extended R0 to also include <code class="sourceCode cpp"><span class="kw">this</span></code>. R2 extends that further to consider pointers-to-unknown in addition to references-to-unknown.</p>
<p><span class="citation" data-cites="P2280R0">[<a href="#ref-P2280R0" role="doc-biblioref">P2280R0</a>]</span> was discussed at the EWG telecon on Feb 3, 2021. The following polls were taken:</p>
<blockquote>
<p>The use cases presented in P2280 are problems in C++’s specification of constexpr, and we would like to fix these problems, ideally in C++23.</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>SF</strong>
</div></th>
<th><div style="text-align:center">
<strong>F</strong>
</div></th>
<th><div style="text-align:center">
<strong>N</strong>
</div></th>
<th><div style="text-align:center">
<strong>A</strong>
</div></th>
<th><div style="text-align:center">
<strong>SA</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td>3</td>
<td>14</td>
<td>2</td>
<td>0</td>
<td>0</td>
</tr>
</tbody>
</table>
<p>This should be a Defect Report against C++20, C++17, C++14, and C++11.</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>SF</strong>
</div></th>
<th><div style="text-align:center">
<strong>F</strong>
</div></th>
<th><div style="text-align:center">
<strong>N</strong>
</div></th>
<th><div style="text-align:center">
<strong>A</strong>
</div></th>
<th><div style="text-align:center">
<strong>SA</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td>3</td>
<td>11</td>
<td>4</td>
<td>0</td>
<td>0</td>
</tr>
</tbody>
</table>
<p>Send P2280 to Electronic Polling, with the intent of going to Core, after getting input from MSVC and GCC implementors.</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>SF</strong>
</div></th>
<th><div style="text-align:center">
<strong>F</strong>
</div></th>
<th><div style="text-align:center">
<strong>N</strong>
</div></th>
<th><div style="text-align:center">
<strong>A</strong>
</div></th>
<th><div style="text-align:center">
<strong>SA</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td>8</td>
<td>10</td>
<td>1</td>
<td>0</td>
<td>0</td>
</tr>
</tbody>
</table>
</blockquote>
<p>R1 updates wording. R1 also adds discussion of <a href="#the-this-pointer">the <code class="sourceCode cpp"><span class="kw">this</span></code> pointer</a>, and extends the proposal to additional cover <code class="sourceCode cpp"><span class="kw">this</span></code> (but not arbitrary pointers)</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>Let’s say I have an array and want to get its size as a constant expression. In C, I had to write a macro:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="pp">#define ARRAY_SIZE</span><span class="op">(</span>a<span class="op">)</span><span class="pp"> </span><span class="op">(</span><span class="kw">sizeof</span><span class="op">(</span>a<span class="op">)/</span><span class="kw">sizeof</span><span class="op">(</span>a<span class="op">[</span><span class="dv">0</span><span class="op">]))</span></span></code></pre></div>
<p>But in C++, we should be able to do better. We have <code class="sourceCode cpp"><span class="kw">constexpr</span></code> and templates, so we can use them:</p>
<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"><</span><span class="kw">typename</span> T, <span class="dt">size_t</span> N<span class="op">></span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="kw">constexpr</span> <span class="kw">auto</span> array_size<span class="op">(</span>T <span class="op">(&)[</span>N<span class="op">])</span> <span class="op">-></span> <span class="dt">size_t</span> <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="cf">return</span> N;</span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="op">}</span></span></code></pre></div>
<p>This seems like it should be a substantial improvement, yet it has surprising limitations:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="dt">void</span> check<span class="op">(</span><span class="dt">int</span> <span class="kw">const</span> <span class="op">(&</span>param<span class="op">)[</span><span class="dv">3</span><span class="op">])</span> <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a> <span class="dt">int</span> local<span class="op">[]</span> <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="cb3-3"><a href="#cb3-3"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> s0 <span class="op">=</span> array_size<span class="op">(</span>local<span class="op">)</span>; <span class="co">// ok</span></span>
<span id="cb3-4"><a href="#cb3-4"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> s1 <span class="op">=</span> array_size<span class="op">(</span>param<span class="op">)</span>; <span class="co">// error</span></span>
<span id="cb3-5"><a href="#cb3-5"></a><span class="op">}</span></span></code></pre></div>
<p>The goal of this paper is to make that second case, and others like it, valid.</p>
<h2 data-number="2.1" id="wait-why"><span class="header-section-number">2.1</span> Wait, why?<a href="#wait-why" class="self-link"></a></h2>
<p>The reason is that in order for <code class="sourceCode cpp">array_size<span class="op">(</span>param<span class="op">)</span></code> to work, we have to pass that reference to param into array_size - and that involves “reading” the reference. The specific rule we’re violating is <span>7.7 <a href="https://wg21.link/expr.const">[expr.const]</a></span>/5.12:</p>
<blockquote>
<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><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">(5.12)</a></span> an <em>id-expression</em> that refers to a variable or data member of reference type unless the reference has a preceding initialization and either
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">(5.12.1)</a></span> it is usable in constant expressions or</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">(5.12.2)</a></span> its lifetime began within the evaluation of <code class="sourceCode cpp">E</code>;</li>
</ul></li>
</ul>
</blockquote>
<p>The reason we violate the reference rule is due to the underlying principle that the constant evaluator has to reject all undefined behavior, so the compiler has to check that all references are valid.</p>
<p>This would be more obvious if our situation used pointers instead of references:</p>
<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">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="dt">size_t</span> N<span class="op">></span></span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="kw">constexpr</span> <span class="dt">size_t</span> array_size<span class="op">(</span>T <span class="op">(*)[</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> N;</span>
<span id="cb4-4"><a href="#cb4-4"></a><span class="op">}</span></span>
<span id="cb4-5"><a href="#cb4-5"></a></span>
<span id="cb4-6"><a href="#cb4-6"></a><span class="dt">void</span> check<span class="op">(</span><span class="dt">int</span> <span class="kw">const</span> <span class="op">(*</span>param<span class="op">)[</span><span class="dv">3</span><span class="op">])</span> <span class="op">{</span></span>
<span id="cb4-7"><a href="#cb4-7"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> s2 <span class="op">=</span> array_size<span class="op">(</span>param<span class="op">)</span>; <span class="co">// error</span></span>
<span id="cb4-8"><a href="#cb4-8"></a><span class="op">}</span></span></code></pre></div>
<p>This case is perhaps more clear as to why it’s ill-formed: copying a function parameter during constant evaluation means having to read it in order to copy it. It has to itself be a constant expression, and function parameters are not constant expressions - even in <code class="sourceCode cpp"><span class="kw">constexpr</span></code> or <code class="sourceCode cpp"><span class="kw">consteval</span></code> functions.</p>
<p>But if the <code class="sourceCode cpp">param</code> case is ill-formed, why does the <code class="sourceCode cpp">local</code> case work? An unsatisfying answer is that… there just isn’t any rule in [expr.const] that we’re violating. There’s no lvalue-to-rvalue conversion (we’re not reading through the reference in any way yet) and we’re not referring to a reference (that’s the previous rule we ran afoul of). With the <code class="sourceCode cpp">param</code> case, the compiler cannot know whether the reference is valid, so it must reject. With the <code class="sourceCode cpp">local</code> case, the compiler can see for sure that the reference to <code class="sourceCode cpp">local</code> would be a valid reference, so it’s happy.</p>
<p>Notably, the rule we’re violating is only about <em>references</em>. We can’t write a function that takes an array by value, so let’s use the next-best thing: <code class="sourceCode cpp">std<span class="op">::</span>array</code> and use the standard library’s <code class="sourceCode cpp">std<span class="op">::</span>size</code> (cppref):</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="dt">void</span> check_arr_val<span class="op">(</span>std<span class="op">::</span>array<span class="op"><</span><span class="dt">int</span>, <span class="dv">3</span><span class="op">></span> <span class="kw">const</span> param<span class="op">)</span> <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a> std<span class="op">::</span>array<span class="op"><</span><span class="dt">int</span>, <span class="dv">3</span><span class="op">></span> local <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="cb5-3"><a href="#cb5-3"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> s3 <span class="op">=</span> std<span class="op">::</span>size<span class="op">(</span>local<span class="op">)</span>; <span class="co">// ok</span></span>
<span id="cb5-4"><a href="#cb5-4"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> s4 <span class="op">=</span> std<span class="op">::</span>size<span class="op">(</span>param<span class="op">)</span>; <span class="co">// ok</span></span>
<span id="cb5-5"><a href="#cb5-5"></a><span class="op">}</span></span></code></pre></div>
<p>If <code class="sourceCode cpp">param</code> were a reference, the initialization of <code class="sourceCode cpp">s4</code> would be ill-formed (for the same reason as previously), but because it’s a value, this is totally fine.</p>
<p>So as long as you pass all your containers around by value, you’re able to use get and use the size as a constant expression. Which is the kind of thing that’s intellectually interesting, but also wildly impractical because obviously nobody’s about to start passing all their containers around <em>by value</em>.</p>
<h2 data-number="2.2" id="other-examples"><span class="header-section-number">2.2</span> Other Examples<a href="#other-examples" class="self-link"></a></h2>
<p>Here are few other cases, which currently are ill-formed because of this reference-to-unknown rule.</p>
<p>From Andrzej Krzemienski:</p>
<blockquote>
<p>Another situation where being able to use a reference to a non-core-constant object is wen I am only interested in the type of the reference rather than the value of the object:</p>
<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"><</span><span class="kw">typename</span> T, <span class="kw">typename</span> U<span class="op">></span></span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="kw">constexpr</span> <span class="dt">bool</span> is_type<span class="op">(</span>U <span class="op">&&)</span></span>
<span id="cb6-3"><a href="#cb6-3"></a><span class="op">{</span></span>
<span id="cb6-4"><a href="#cb6-4"></a> <span class="cf">return</span> std<span class="op">::</span>is_same_v<span class="op"><</span>T, std<span class="op">::</span>decay_t<span class="op"><</span>U<span class="op">>></span>;</span>
<span id="cb6-5"><a href="#cb6-5"></a><span class="op">}</span></span></code></pre></div>
<p>So that I can use it like this:</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">auto</span> visitor <span class="op">=</span> <span class="op">[](</span><span class="kw">auto</span><span class="op">&&</span> v<span class="op">)</span> <span class="op">{</span></span>
<span id="cb7-2"><a href="#cb7-2"></a> <span class="cf">if</span> <span class="kw">constexpr</span><span class="op">(</span>is_type<span class="op"><</span>Alternative1<span class="op">>(</span>v<span class="op">))</span> <span class="op">{</span></span>
<span id="cb7-3"><a href="#cb7-3"></a> <span class="co">// ...</span></span>
<span id="cb7-4"><a href="#cb7-4"></a> <span class="op">}</span> <span class="cf">else</span> <span class="cf">if</span> <span class="kw">constexpr</span><span class="op">(</span>is_type<span class="op"><</span>Alternative2<span class="op">>(</span>v<span class="op">))</span> <span class="op">{</span></span>
<span id="cb7-5"><a href="#cb7-5"></a> <span class="co">// ...</span></span>
<span id="cb7-6"><a href="#cb7-6"></a> <span class="op">}</span></span>
<span id="cb7-7"><a href="#cb7-7"></a><span class="op">}</span>;</span></code></pre></div>
<p>I can do it with a macro:</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="pp">#define IS_TYPE</span><span class="op">(</span>TYPE,<span class="pp"> </span>EXPR<span class="op">)</span><span class="pp"> </span><span class="op">(</span>std<span class="op">::</span>is_same_v<span class="op"><</span>TYPE,<span class="pp"> </span>std<span class="op">::</span>decay_t<span class="op"><</span><span class="kw">decltype</span><span class="op">(</span>EXPR<span class="op">)>>)</span></span></code></pre></div>
</blockquote>
<p>From Jonathan Wakely:</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="kw">auto</span> rando<span class="op">(</span>std<span class="op">::</span>uniform_random_bit_generator <span class="kw">auto</span><span class="op">&</span> g<span class="op">)</span></span>
<span id="cb9-2"><a href="#cb9-2"></a><span class="op">{</span></span>
<span id="cb9-3"><a href="#cb9-3"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>std<span class="op">::</span>has_single_bit<span class="op">(</span>g<span class="op">.</span>max<span class="op">()</span> <span class="op">-</span> g<span class="op">.</span>min<span class="op">()))</span></span>
<span id="cb9-4"><a href="#cb9-4"></a> <span class="co">// ...</span></span>
<span id="cb9-5"><a href="#cb9-5"></a> <span class="cf">else</span></span>
<span id="cb9-6"><a href="#cb9-6"></a> <span class="co">// ...</span></span>
<span id="cb9-7"><a href="#cb9-7"></a><span class="op">}</span></span></code></pre></div>
<p>The concept requires that <code class="sourceCode cpp">g<span class="op">.</span>max<span class="op">()</span></code> and <code class="sourceCode cpp">g<span class="op">.</span>min<span class="op">()</span></code> are constexpr static member functions, so this should work. And if I did it with an object of that type, it would work. But because <code class="sourceCode cpp">g</code> is a reference, it’s not usable in a constant expression. That makes it awkward to refactor code into a function (or function template), because what worked on the object itself doesn’t work in a function that binds a reference to that object.</p>
<p>I can rewrite it as something like:</p>
<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">using</span> G <span class="op">=</span> remove_reference_t<span class="op"><</span><span class="kw">decltype</span><span class="op">(</span>g<span class="op">)></span>;</span>
<span id="cb10-2"><a href="#cb10-2"></a><span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>std<span class="op">::</span>has_single_bit<span class="op">(</span>G<span class="op">::</span>max<span class="op">()</span> <span class="op">-</span> G<span class="op">::</span>min<span class="op">()))</span></span></code></pre></div>
<p>Or avoid abbreviated function syntax so I have a name for the type:</p>
<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"><</span>std<span class="op">::</span>uniform_random_bit_generator G<span class="op">></span></span>
<span id="cb11-2"><a href="#cb11-2"></a><span class="kw">auto</span> rando<span class="op">(</span>G<span class="op">&</span> g<span class="op">)</span></span>
<span id="cb11-3"><a href="#cb11-3"></a><span class="op">{</span></span>
<span id="cb11-4"><a href="#cb11-4"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>std<span class="op">::</span>has_single_bit<span class="op">(</span>G<span class="op">::</span>max<span class="op">()</span> <span class="op">-</span> G<span class="op">::</span>min<span class="op">()))</span></span>
<span id="cb11-5"><a href="#cb11-5"></a><span class="op">}</span></span></code></pre></div>
<p>But it’s awkward that the first version doesn’t Just Work.</p>
</blockquote>
<p>Another from <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101937">gcc bug 101937</a>:</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">struct</span> S <span class="op">{</span> <span class="kw">enum</span> <span class="op">{</span> e <span class="op">=</span> <span class="dv">4</span> <span class="op">}</span>; <span class="op">}</span></span>
<span id="cb12-2"><a href="#cb12-2"></a></span>
<span id="cb12-3"><a href="#cb12-3"></a><span class="dt">void</span> f<span class="op">(</span>S<span class="op">&</span> s<span class="op">)</span> <span class="op">{</span></span>
<span id="cb12-4"><a href="#cb12-4"></a> <span class="kw">constexpr</span> <span class="dt">int</span> i <span class="op">=</span> s<span class="op">.</span>e;</span>
<span id="cb12-5"><a href="#cb12-5"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Here, <code class="sourceCode cpp">S<span class="op">::</span>e</code> is an enum, so it’s about as constant as constant can get, but because we’re accessing through <code class="sourceCode cpp">s</code> this is invalid. libstdc++ was inadvertently relying on this being valid.</p>
<p>Another from me:</p>
<blockquote>
<p>I have a project that has a structure like:</p>
<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"><</span><span class="kw">typename</span><span class="op">...</span> Types<span class="op">></span></span>
<span id="cb13-2"><a href="#cb13-2"></a><span class="kw">struct</span> Widget <span class="op">{</span></span>
<span id="cb13-3"><a href="#cb13-3"></a> <span class="kw">struct</span> Config <span class="op">:</span> Types<span class="op">::</span>config<span class="op">...</span> <span class="op">{</span></span>
<span id="cb13-4"><a href="#cb13-4"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T<span class="op">></span></span>
<span id="cb13-5"><a href="#cb13-5"></a> <span class="kw">static</span> <span class="kw">constexpr</span> <span class="kw">auto</span> sends<span class="op">(</span>T<span class="op">)</span> <span class="op">-></span> <span class="dt">bool</span> <span class="op">{</span></span>
<span id="cb13-6"><a href="#cb13-6"></a> <span class="cf">return</span> std<span class="op">::</span>is_base_of_v<span class="op"><</span><span class="kw">typename</span> T<span class="op">::</span>config, Config<span class="op">></span>;</span>
<span id="cb13-7"><a href="#cb13-7"></a> <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>
<span id="cb13-10"><a href="#cb13-10"></a> Config config;</span>
<span id="cb13-11"><a href="#cb13-11"></a><span class="op">}</span>;</span></code></pre></div>
<p>With the intent that this function makes for a nice and readable way of doing dispatch:</p>
<div class="sourceCode" id="cb14"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1"></a><span class="dt">void</span> do_configuration<span class="op">(</span><span class="kw">auto</span><span class="op">&</span> config<span class="op">)</span> <span class="op">{</span></span>
<span id="cb14-2"><a href="#cb14-2"></a> <span class="co">// the actual type of config is... complicated</span></span>
<span id="cb14-3"><a href="#cb14-3"></a></span>
<span id="cb14-4"><a href="#cb14-4"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>config<span class="op">.</span>sends<span class="op">(</span>Goomba<span class="op">{}))</span> <span class="op">{</span></span>
<span id="cb14-5"><a href="#cb14-5"></a> <span class="co">// do something</span></span>
<span id="cb14-6"><a href="#cb14-6"></a> <span class="op">}</span></span>
<span id="cb14-7"><a href="#cb14-7"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>config<span class="op">.</span>sends<span class="op">(</span>Paratroopa<span class="op">{}))</span> <span class="op">{</span></span>
<span id="cb14-8"><a href="#cb14-8"></a> <span class="co">// do something else</span></span>
<span id="cb14-9"><a href="#cb14-9"></a> <span class="op">}</span></span>
<span id="cb14-10"><a href="#cb14-10"></a><span class="op">}</span></span></code></pre></div>
<p>Except this doesn’t work, and I have to write:</p>
<div class="sourceCode" id="cb15"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb15-1"><a href="#cb15-1"></a><span class="dt">void</span> do_configuration<span class="op">(</span><span class="kw">auto</span><span class="op">&</span> config<span class="op">)</span> <span class="op">{</span></span>
<span id="cb15-2"><a href="#cb15-2"></a> <span class="kw">using</span> Config <span class="op">=</span> std<span class="op">::</span>remove_cvref_t<span class="op"><</span><span class="kw">decltype</span><span class="op">(</span>config<span class="op">)></span>;</span>
<span id="cb15-3"><a href="#cb15-3"></a></span>
<span id="cb15-4"><a href="#cb15-4"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>Config<span class="op">::</span>sends<span class="op">(</span>Goomba<span class="op">{}))</span> <span class="op">{</span></span>
<span id="cb15-5"><a href="#cb15-5"></a> <span class="co">// ...</span></span>
<span id="cb15-6"><a href="#cb15-6"></a> <span class="op">}</span></span></code></pre></div>
<p>Which is not really “better.”</p>
</blockquote>
<p>What all of these examples have in common is that they are using a reference to an object of type <code class="sourceCode cpp">T</code> but do not care at all about the identity of that object. We’re either querying properties of the type, invoking static member functions, or even when invoking a non-static member function (as in <code class="sourceCode cpp">std<span class="op">::</span>array<span class="op">::</span>size</code>), not actually accessing any non-static data members. The result would be the same for every object of type <code class="sourceCode cpp">T</code>… so if the identity doesn’t change the result, why does the lack of identity cause the result to be non-constant? It’s very much constant.</p>
<h2 data-number="2.3" id="the-this-pointer"><span class="header-section-number">2.3</span> The <code class="sourceCode cpp"><span class="kw">this</span></code> pointer<a href="#the-this-pointer" class="self-link"></a></h2>
<p>Consider the following example, very similar to one I shared earlier. Here, we need to read a constant through a member, so we write our member function two different ways (the latter using <span class="citation" data-cites="P0847R7">[<a href="#ref-P0847R7" role="doc-biblioref">P0847R7</a>]</span>):</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Regular non-static member function</strong>
</div></th>
<th><div style="text-align:center">
<strong>With deducing this</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><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">template</span> <span class="op"><</span><span class="dt">bool</span> V<span class="op">></span></span>
<span id="cb16-2"><a href="#cb16-2"></a><span class="kw">struct</span> Widget <span class="op">{</span></span>
<span id="cb16-3"><a href="#cb16-3"></a> <span class="kw">struct</span> Config <span class="op">{</span></span>
<span id="cb16-4"><a href="#cb16-4"></a> <span class="kw">static</span> <span class="kw">constexpr</span> <span class="dt">bool</span> value <span class="op">=</span> V;</span>
<span id="cb16-5"><a href="#cb16-5"></a> <span class="op">}</span> config;</span>
<span id="cb16-6"><a href="#cb16-6"></a></span>
<span id="cb16-7"><a href="#cb16-7"></a> <span class="dt">void</span> f<span class="op">()</span> <span class="op">{</span></span>
<span id="cb16-8"><a href="#cb16-8"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>config<span class="op">.</span>value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb16-9"><a href="#cb16-9"></a> <span class="co">// ...</span></span>
<span id="cb16-10"><a href="#cb16-10"></a> <span class="op">}</span></span>
<span id="cb16-11"><a href="#cb16-11"></a> <span class="op">}</span></span>
<span id="cb16-12"><a href="#cb16-12"></a><span class="op">}</span>;</span></code></pre></div></td>
<td><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"><</span><span class="dt">bool</span> V<span class="op">></span></span>
<span id="cb17-2"><a href="#cb17-2"></a><span class="kw">struct</span> Widget <span class="op">{</span></span>
<span id="cb17-3"><a href="#cb17-3"></a> <span class="kw">struct</span> Config <span class="op">{</span></span>
<span id="cb17-4"><a href="#cb17-4"></a> <span class="kw">static</span> <span class="kw">constexpr</span> <span class="dt">bool</span> value <span class="op">=</span> V;</span>
<span id="cb17-5"><a href="#cb17-5"></a> <span class="op">}</span> config;</span>
<span id="cb17-6"><a href="#cb17-6"></a></span>
<span id="cb17-7"><a href="#cb17-7"></a> <span class="dt">void</span> f<span class="op">(</span><span class="kw">this</span> Widget<span class="op">&</span> self<span class="op">)</span> <span class="op">{</span></span>
<span id="cb17-8"><a href="#cb17-8"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>self<span class="op">.</span>config<span class="op">.</span>value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb17-9"><a href="#cb17-9"></a> <span class="co">// ...</span></span>
<span id="cb17-10"><a href="#cb17-10"></a> <span class="op">}</span></span>
<span id="cb17-11"><a href="#cb17-11"></a> <span class="op">}</span></span>
<span id="cb17-12"><a href="#cb17-12"></a><span class="op">}</span>;</span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>Even if we drop the restriction on using references-to-unknown (the extent of the R0 proposal of this paper), the example on the left is still ill-formed. Because we don’t even have a reference here exactly, we’re accessing through <code class="sourceCode cpp"><span class="kw">this</span></code>, and one of the things we’re not allowed to evaluate as part of constant evaluation is the first bullet from [expr.const]/5:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">(5.1)</a></span> <code class="sourceCode cpp"><span class="kw">this</span></code>, except in a constexpr function that is being evaluated as part of <code class="sourceCode cpp">E</code>;</li>
</ul>
</blockquote>
<p>And here, <code class="sourceCode cpp">Widget<span class="op"><</span>V<span class="op">>::</span>f</code> is not a <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function.</p>
<p>However, the example on the right is valid with the suggested rule change. Here, <code class="sourceCode cpp">self</code> is a reference-to-unknown and <code class="sourceCode cpp">value</code> ends up being a constexpr variable that we can read. So this works. This example wasn’t exactly what we had in mind when we wrote that paper though, and while we would be happy to keep dumping motivating use-cases into that paper… it doesn’t exactly seem like a meaningful solution to the problem. It seems pretty unsatisfactory that <code class="sourceCode cpp">self<span class="op">.</span>config<span class="op">.</span>value</code> is okay while <code class="sourceCode cpp"><span class="op">(*</span><span class="kw">this</span><span class="op">).</span>config<span class="op">.</span>value</code> is not, when <code class="sourceCode cpp">self</code> and <code class="sourceCode cpp"><span class="op">(*</span><span class="kw">this</span><span class="op">)</span></code> mean the same thing in this context.</p>
<p>So that’s also fairly unsatisfying. It would be nice to simply support this use-case as well. <code class="sourceCode cpp"><span class="kw">this</span></code>, after all, is a reference (practically speaking).</p>
<h2 data-number="2.4" id="other-pointers"><span class="header-section-number">2.4</span> Other pointers<a href="#other-pointers" class="self-link"></a></h2>
<p>The thing is though: why just the <code class="sourceCode cpp"><span class="kw">this</span></code> pointer and not all pointers? For that matter, is there really a meaningful distinction between pointers and references?</p>
<p>Is there a meaningful distinction between supporting these examples?</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>References</strong>
</div></th>
<th><div style="text-align:center">
<strong>Pointers</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><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"><</span><span class="kw">typename</span> T, <span class="dt">size_t</span> N<span class="op">></span></span>
<span id="cb18-2"><a href="#cb18-2"></a><span class="kw">constexpr</span> <span class="kw">auto</span> array_size<span class="op">(</span>T <span class="op">(&)[</span>N<span class="op">])</span> <span class="op">-></span> <span class="dt">size_t</span> <span class="op">{</span></span>
<span id="cb18-3"><a href="#cb18-3"></a> <span class="cf">return</span> N;</span>
<span id="cb18-4"><a href="#cb18-4"></a><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="dt">void</span> check<span class="op">(</span><span class="dt">int</span> <span class="kw">const</span> <span class="op">(&</span>param<span class="op">)[</span><span class="dv">3</span><span class="op">])</span> <span class="op">{</span></span>
<span id="cb18-7"><a href="#cb18-7"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> s <span class="op">=</span> array_size<span class="op">(</span>param<span class="op">)</span>;</span>
<span id="cb18-8"><a href="#cb18-8"></a><span class="op">}</span></span></code></pre></div></td>
<td><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">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="dt">size_t</span> N<span class="op">></span></span>
<span id="cb19-2"><a href="#cb19-2"></a><span class="kw">constexpr</span> <span class="kw">auto</span> array_size<span class="op">(</span>T <span class="op">(*)[</span>N<span class="op">])</span> <span class="op">-></span> <span class="dt">size_t</span> <span class="op">{</span></span>
<span id="cb19-3"><a href="#cb19-3"></a> <span class="cf">return</span> N;</span>
<span id="cb19-4"><a href="#cb19-4"></a><span class="op">}</span></span>
<span id="cb19-5"><a href="#cb19-5"></a></span>
<span id="cb19-6"><a href="#cb19-6"></a><span class="dt">void</span> check<span class="op">(</span><span class="dt">int</span> <span class="kw">const</span> <span class="op">(*</span>param<span class="op">)[</span><span class="dv">3</span><span class="op">])</span> <span class="op">{</span></span>
<span id="cb19-7"><a href="#cb19-7"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> s <span class="op">=</span> array_size<span class="op">(</span>param<span class="op">)</span>;</span>
<span id="cb19-8"><a href="#cb19-8"></a><span class="op">}</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>Pointers require a lot more specification effort, since pointers allow more operations, and we’d have to define what all of those things mean. For instance:</p>
<div class="sourceCode" id="cb20"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb20-1"><a href="#cb20-1"></a><span class="dt">void</span> f<span class="op">(</span>std<span class="op">::</span>array<span class="op"><</span><span class="dt">int</span>, <span class="dv">3</span><span class="op">>&</span> r, std<span class="op">::</span>array<span class="op"><</span><span class="dt">int</span>, <span class="dv">4</span><span class="op">>*</span> p<span class="op">)</span> <span class="op">{</span></span>
<span id="cb20-2"><a href="#cb20-2"></a> <span class="kw">static_assert</span><span class="op">(</span>r<span class="op">.</span>size<span class="op">()</span> <span class="op">==</span> <span class="dv">3</span><span class="op">)</span>; <span class="co">// #1</span></span>
<span id="cb20-3"><a href="#cb20-3"></a> <span class="kw">static_assert</span><span class="op">(</span>p<span class="op">-></span>size<span class="op">()</span> <span class="op">==</span> <span class="dv">4</span><span class="op">)</span>; <span class="co">// #2</span></span>
<span id="cb20-4"><a href="#cb20-4"></a> <span class="kw">static_assert</span><span class="op">(</span>p<span class="op">[</span><span class="dv">3</span><span class="op">].</span>size<span class="op">()</span> <span class="op">==</span> <span class="dv">4</span><span class="op">)</span>; <span class="co">// #3</span></span>
<span id="cb20-5"><a href="#cb20-5"></a> <span class="kw">static_assert</span><span class="op">(&</span>r <span class="op">==</span> <span class="op">&</span>r<span class="op">)</span>; <span class="co">// #4</span></span>
<span id="cb20-6"><a href="#cb20-6"></a><span class="op">}</span></span></code></pre></div>
<p><code class="sourceCode cpp"><span class="pp">#1</span></code> is one of the motivating examples in the paper. <code class="sourceCode cpp"><span class="pp">#2</span></code> would require dereferencing a pointer, which is similar to accessing through a reference yet isn’t exactly the same. <code class="sourceCode cpp"><span class="pp">#3</span></code> additionally requires array access and we have no idea if <code class="sourceCode cpp">p</code> actually points to an array, much less what the size of that array would be. But both <code class="sourceCode cpp"><span class="pp">#2</span></code> and <code class="sourceCode cpp"><span class="pp">#3</span></code> generally fit the notion that these are expressions that either have a particular constant value or are undefined behavior, although <code class="sourceCode cpp"><span class="pp">#2</span></code> only requires that <code class="sourceCode cpp">p</code> be a pointer to unknown object while <code class="sourceCode cpp"><span class="pp">#3</span></code> requires <code class="sourceCode cpp">p</code> be a pointer to an unknown array of objects.</p>
<p><code class="sourceCode cpp"><span class="pp">#4</span></code> is interesting in a different way: here this actually has to be true, but in order support that, rather than simply tracking that <code class="sourceCode cpp"><span class="op">&</span>r</code> is “pointer to known <code class="sourceCode cpp">array<span class="op"><</span><span class="dt">int</span>, <span class="dv">3</span><span class="op">></span></code>”, we have to additionally track that it is specifically a pointer to <code class="sourceCode cpp">r</code>. This, at least in EDG, is a much bigger change (with much less commensurate value).</p>
<p>The problem is, while changing the specification to support <code class="sourceCode cpp"><span class="pp">#1</span></code> is largely around <em>not</em> rejecting the case, supporting <code class="sourceCode cpp"><span class="pp">#2</span></code> is a much more involved process. We not only have to introduce the concept of pointer-to-unknown but we also have to specify what all the operations mean. We have to say what a pointer-to-unknown means. That it dereferences into a reference-to-unknown and likewise that taking the address of a reference-to-unknown yields a pointer-to-unknown.</p>
<p>But then we also have to define what the various other operations on pointers to references are. What about addition and subtraction and indexing (i.e. <code class="sourceCode cpp"><span class="pp">#3</span></code>)? Equality (i.e. <code class="sourceCode cpp"><span class="pp">#4</span></code>)? Ordering? If we reject indexing, what about <code class="sourceCode cpp">p<span class="op">[</span><span class="dv">0</span><span class="op">]</span></code>?</p>
<p>Supporting references-to-unknown is largely about <em>not</em> rejecting those cases that are currently rejected. Similarly, supporting <code class="sourceCode cpp"><span class="kw">this</span></code> in the context of (implicit or explicit) class member access is likewise simply about not rejecting. In order to support pointers-to-unknown, we likewise try to push rejecting cases as far as possible. That is, indirecting through a <code class="sourceCode cpp">T<span class="op">*</span></code> with unknown value just gives you some unknown <code class="sourceCode cpp">T</code>.</p>
<p>But what about the other operations? Comparing two pointers, where at least one is a pointer-to-unknown, cannot be a constant expression so will have to be rejected. There is a notable exception here in doing something like <code class="sourceCode cpp">p <span class="op">==</span> p</code> which could potentially be <code class="sourceCode cpp"><span class="kw">true</span></code> but seems exceedingly narrow. What about pointer arithmetic? Should the <code class="sourceCode cpp"><span class="pp">#3</span></code> example above work or not? Would your answer change if instead of a pointer we had an array of unknown bound (there’s an example of such later in this paper)? What if it were <code class="sourceCode cpp">p<span class="op">[</span><span class="dv">0</span><span class="op">]</span></code> instead of <code class="sourceCode cpp">p<span class="op">[</span><span class="dv">3</span><span class="op">]</span></code>?</p>
<h3 data-number="2.4.1" id="what-about-nullptr"><span class="header-section-number">2.4.1</span> What about <code class="sourceCode cpp"><span class="kw">nullptr</span></code>?<a href="#what-about-nullptr" class="self-link"></a></h3>
<p>Consider this example from David Stone:</p>
<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="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="dt">int</span> N<span class="op">></span></span>
<span id="cb21-2"><a href="#cb21-2"></a><span class="kw">struct</span> array <span class="op">{</span></span>
<span id="cb21-3"><a href="#cb21-3"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> size<span class="op">()</span> <span class="kw">const</span> <span class="op">-></span> <span class="dt">int</span> <span class="op">{</span> <span class="cf">return</span> N; <span class="op">}</span></span>
<span id="cb21-4"><a href="#cb21-4"></a><span class="op">}</span>;</span>
<span id="cb21-5"><a href="#cb21-5"></a></span>