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  <title>Non-transient allocation with std::vector and
std::basic_string</title>
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<div class="wrapper">
<header id="title-block-header">
<h1 class="title" style="text-align:center">Non-transient allocation
with
<code class="sourceCode cpp">std<span class="op">::</span>vector</code>
and <code class="sourceCode cpp">std<span class="op">::</span>basic_string</code></h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3554R0 <a href="https://wg21.link/P3554">[Latest]</a> <a href="https://wg21.link/P3554/status">[Status]</a></td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2025-01-05</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>
      EWG<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>
      Peter Dimov<br>&lt;<a href="mailto:pdimov@gmail.com" class="email">pdimov@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="#introduction" id="toc-introduction"><span class="toc-section-number">1</span> Introduction<span></span></a></li>
<li><a href="#proposal" id="toc-proposal"><span class="toc-section-number">2</span> Proposal<span></span></a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">3</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>We’ve wanted to have non-transient constexpr allocation for quite
some time, and there have been multiple papers on this topic so far.
Three dealing with the general problem <span class="citation" data-cites="P0784R7">[<a href="https://wg21.link/p0784r7" role="doc-biblioref">P0784R7</a>]</span> <span class="citation" data-cites="P1974R0">[<a href="https://wg21.link/p1974r0" role="doc-biblioref">P1974R0</a>]</span> <span class="citation" data-cites="P2670R1">[<a href="https://wg21.link/p2670r1" role="doc-biblioref">P2670R1</a>]</span> and one more attempting to make
it work just in a narrow situation <span class="citation" data-cites="P3032R2">[<a href="https://wg21.link/p3032r2" role="doc-biblioref">P3032R2</a>]</span>.</p>
<p>During discussion of the latter paper (which only talked about
persisting
<code class="sourceCode cpp"><span class="kw">constexpr</span></code>
variables within
<code class="sourceCode cpp"><span class="kw">consteval</span></code>
functions) in St. Louis, we took a poll to change the paper to only
allow
<code class="sourceCode cpp">std<span class="op">::</span>vector</code>
and <code class="sourceCode cpp">std<span class="op">::</span>basic_string</code>,
even for persistent allocation:</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>4</td>
<td>9</td>
<td>8</td>
<td>3</td>
<td>0</td>
</tr>
</tbody>
</table>
<p>As of this writing (January 2025), we do not yet have consensus for a
general design for non-transient allocation. We have ideas (see above
papers), and we have a promising library workaround (<span class="citation" data-cites="P3491R0">[<a href="https://wg21.link/p3491r0" role="doc-biblioref">P3491R0</a>]</span>). But we know that regardless
of what the general solution will end up being, <code class="sourceCode cpp">std<span class="op">::</span>vector<span class="op">&lt;</span>T<span class="op">&gt;</span></code>
and <code class="sourceCode cpp">std<span class="op">::</span>basic_string<span class="op">&lt;</span>Char, Traits, std<span class="op">::</span>allocator<span class="op">&lt;</span>Char<span class="op">&gt;&gt;</span></code>
will work and allow their allocations to persist. These are the most
common dynamic containers. And while
<code class="sourceCode cpp">define_static_array</code> gets us some of
the way there, simply allowing these two containers to persist has a lot
of ergonomic value:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>With <code class="sourceCode cpp">define_static_array<span class="op">()</span></code></strong>
</div></th>
<th><div style="text-align:center">
<strong>Works By Fiat</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div>

<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> ints<span class="op">()</span> <span class="op">-&gt;</span> std<span class="op">::</span>vector<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span>;</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="co">// data is a span&lt;int const&gt;</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> data <span class="op">=</span> define_static_array<span class="op">(</span>ints<span class="op">())</span>;</span></code></pre></div>

</div></td>
<td><div>

<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> ints<span class="op">()</span> <span class="op">-&gt;</span> std<span class="op">::</span>vector<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span>;</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="co">// data is a vector&lt;int&gt;</span></span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> data <span class="op">=</span> ints<span class="op">()</span>;</span></code></pre></div>

</div></td>
</tr>
<tr class="even">
<td><div>

<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> strs<span class="op">()</span> <span class="op">-&gt;</span> std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>string<span class="op">&gt;</span>;</span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a><span class="co">// data is a span&lt;string_view const&gt;</span></span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> data <span class="op">=</span> define_static_array<span class="op">(</span></span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a>  strings<span class="op">()</span> <span class="op">|</span> std<span class="op">::</span>views<span class="op">::</span>transform<span class="op">([&amp;](</span><span class="kw">auto</span><span class="op">&amp;&amp;</span> str<span class="op">){</span></span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> std<span class="op">::</span>string_view<span class="op">(</span>define_static_string<span class="op">(</span>str<span class="op">))</span>;</span>
<span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a>  <span class="op">})</span></span>
<span id="cb3-8"><a href="#cb3-8" aria-hidden="true" tabindex="-1"></a><span class="op">)</span>;</span></code></pre></div>

</div></td>
<td><div>

<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> strs<span class="op">()</span> <span class="op">-&gt;</span> std<span class="op">::</span>vector<span class="op">&lt;</span>std<span class="op">::</span>string<span class="op">&gt;</span>;</span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a><span class="co">// data is a vector&lt;string&gt;</span></span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> data <span class="op">=</span> strings<span class="op">()</span>;</span></code></pre></div>

</div></td>
</tr>
</tbody>
</table>
<p>While with a single layer of wrapping, having to add an extra call to
<code class="sourceCode cpp">define_static_array</code> is tedious but
fine, once we get into more complicated data structures, the wrapping
itself gets more complicated. The good news here is that it is actually
<em>possible</em> to do. The fact that the left column can exist at all
after <span class="citation" data-cites="P3491R0">[<a href="https://wg21.link/p3491r0" role="doc-biblioref">P3491R0</a>]</span> is quite exciting.</p>
<p>But there are plenty of places where you might want persistent
constexpr allocation — template arguments, the initializer in an
expansion statement <span class="citation" data-cites="P1306R2">[<a href="https://wg21.link/p1306r2" role="doc-biblioref">P1306R2</a>]</span>, etc. Having to perform the
correct wrapping in all of these places would get old really fast. So we
think that even with the addition of <code class="sourceCode cpp">std<span class="op">::</span>define_static_array</code>
and <code class="sourceCode cpp">std<span class="op">::</span>define_static_string</code>,
we’d want a little bit more help.</p>
<p>Additionally, this avoids the awkward issue today where whether a
<code class="sourceCode cpp">std<span class="op">::</span>string</code>
is allowed to persist depends on the small string optimization strategy
of the given implementation. With this proposal, it would simply always
be allowed to persist.</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>The current rule is that any allocation within a constant expression
<code class="sourceCode cpp">E</code> <em>must</em> be deallocated
within <code class="sourceCode cpp">E</code>. That rule can be found in
<span>7.7 <a href="https://wg21.link/expr.const">[expr.const]</a></span>/10:</p>
<div class="std">
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">(10.18)</a></span>
a <em>new-expression</em>, unless either:
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">(10.18.1)</a></span>
the selected allocation function is a replaceable global allocation
function ([new.delete.single], [new.delete.array]) and the allocated
storage is deallocated within the evaluation of
<code class="sourceCode cpp">E</code>, or</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">(10.18.2)</a></span>
[…]</li>
</ul></li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">(10.19)</a></span>
a <em>delete-expression</em>, unless […]</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">(10.20)</a></span>
a call to an instance of <code class="sourceCode cpp">std​<span class="op">::</span>​allocator<span class="op">&lt;</span>T<span class="op">&gt;</span>​<span class="op">::</span>​allocate</code>
([allocator.members]), unless the allocated storage is deallocated
within the evaluation of <code class="sourceCode cpp">E</code>;</li>
</ul>
</blockquote>
</div>
<p>We instead propose introducing the concept of eligible for
persistence:</p>
<div class="std">
<blockquote>
<div class="addu">
<p>An allocation <code class="sourceCode cpp">A</code> that occurs
within the evaluation of a core constant expression
<code class="sourceCode cpp">E</code> is <em>constexpr-persistent</em>
if:</p>
<ul>
<li><code class="sourceCode cpp">E</code> is used to initialize an
object <code class="sourceCode cpp">O</code> that is potentially usable
in constant expressions,</li>
<li><code class="sourceCode cpp">A</code> is not deallocated within
<code class="sourceCode cpp">E</code>, and</li>
<li><code class="sourceCode cpp">A</code> is <em>eligible for
constexpr-persistence</em>.</li>
</ul>
</div>
</blockquote>
</div>
<p>And adjusting the wording as appropriate:</p>
<div class="std">
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">(10.18)</a></span>
a <em>new-expression</em>, unless either:
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">(10.18.1)</a></span>
the selected allocation function is a replaceable global allocation
function ([new.delete.single], [new.delete.array]) and the allocated
storage is <span class="addu">either constexpr-persistent or</span> is
deallocated within the evaluation of
<code class="sourceCode cpp">E</code>, or</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">(10.18.2)</a></span>
[…]</li>
</ul></li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_9" id="pnum_9">(10.19)</a></span>
a <em>delete-expression</em>, unless […]</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_10" id="pnum_10">(10.20)</a></span>
a call to an instance of <code class="sourceCode cpp">std​<span class="op">::</span>​allocator<span class="op">&lt;</span>T<span class="op">&gt;</span>​<span class="op">::</span>​allocate</code>
([allocator.members]), unless the allocated storage is <span class="addu">either constexpr-persistent or</span> is deallocated within
the evaluation of <code class="sourceCode cpp">E</code>;</li>
</ul>
</blockquote>
</div>
<p>Mark <code class="sourceCode cpp">vector</code> allocations as
eligible for constexpr-persistence in <span>23.3.11 <a href="https://wg21.link/vector">[vector]</a></span>:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_11" id="pnum_11">2</a></span>
A vector meets all of the requirements of a container
([container.reqmts]), of a reversible container
([container.rev.reqmts]), of an allocator-aware container
([container.alloc.reqmts]), of a sequence container, including most of
the optional sequence container requirements ([sequence.reqmts]), and,
for an element type other than bool, of a contiguous container. The
exceptions are the push_front, prepend_range, pop_front, and
emplace_front member functions, which are not provided. Descriptions are
provided here only for operations on vector that are not described in
one of these tables or for operations where there is additional semantic
information.</p>
<div class="addu">
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_12" id="pnum_12">x</a></span>
Any storage allocated for the elements of a specialization of the
<code class="sourceCode cpp">vector</code> primary template is eligible
for constexpr-persistence ([expr.const]).</p>
</div>
</blockquote>
</div>
<p>And the same in <span>23.3.12 <a href="https://wg21.link/vector.bool">[vector.bool]</a></span>:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_13" id="pnum_13">1</a></span>
To optimize space allocation, a partial specialization of vector for
bool elements is provided:</p>
<div class="addu">
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_14" id="pnum_14">2</a></span>
Any storage allocated for the elements of the partial specialization
defined in this subclause is eligible for constexpr-persistence
([expr.const]).</p>
</div>
</blockquote>
</div>
<p>Mark <code class="sourceCode cpp">basic_string</code> allocations as
eligible for constexpr-persistence in <span>27.4.3.1 <a href="https://wg21.link/basic.string.general">[basic.string.general]</a></span>:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_15" id="pnum_15">2</a></span>
A specialization of <code class="sourceCode cpp">basic_string</code> is
a contiguous container ([container.reqmts]).</p>
<div class="addu">
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_16" id="pnum_16">x</a></span>
Any storage allocated for the elements of a specialization of the
<code class="sourceCode cpp">basic_string</code> primary template is
eligible for constexpr-persistence ([expr.const]).</p>
</div>
</blockquote>
</div>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">3</span>
References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references csl-bib-body hanging-indent" data-entry-spacing="1" role="doc-bibliography">
<div id="ref-P0784R7" class="csl-entry" role="doc-biblioentry">
[P0784R7] Daveed Vandevoorde, Peter Dimov,Louis Dionne, Nina Ranns,
Richard Smith, Daveed Vandevoorde. 2019-07-22. More constexpr
containers. <a href="https://wg21.link/p0784r7"><div class="csl-block">https://wg21.link/p0784r7</div></a>
</div>
<div id="ref-P1306R2" class="csl-entry" role="doc-biblioentry">
[P1306R2] Dan Katz, Andrew Sutton, Sam Goodrick, Daveed Vandevoorde.
2024-05-07. Expansion statements. <a href="https://wg21.link/p1306r2"><div class="csl-block">https://wg21.link/p1306r2</div></a>
</div>
<div id="ref-P1974R0" class="csl-entry" role="doc-biblioentry">
[P1974R0] Jeff Snyder, Louis Dionne, Daveed Vandevoorde. 2020-05-15.
Non-transient constexpr allocation using propconst. <a href="https://wg21.link/p1974r0"><div class="csl-block">https://wg21.link/p1974r0</div></a>
</div>
<div id="ref-P2670R1" class="csl-entry" role="doc-biblioentry">
[P2670R1] Barry Revzin. 2023-02-03. Non-transient constexpr allocation.
<a href="https://wg21.link/p2670r1"><div class="csl-block">https://wg21.link/p2670r1</div></a>
</div>
<div id="ref-P3032R2" class="csl-entry" role="doc-biblioentry">
[P3032R2] Barry Revzin. 2024-04-16. Less transient constexpr allocation.
<a href="https://wg21.link/p3032r2"><div class="csl-block">https://wg21.link/p3032r2</div></a>
</div>
<div id="ref-P3491R0" class="csl-entry" role="doc-biblioentry">
[P3491R0] Barry Revzin, Peter Dimov, Daveed Vandevoorde, Dan Katz.
2024-12-15. define_static_{string,object,array}. <a href="https://wg21.link/p3491r0"><div class="csl-block">https://wg21.link/p3491r0</div></a>
</div>
</div>
</div>
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