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  <title>Cleaning up the trivial relocation APIs in C++26</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Cleaning up the trivial
relocation APIs in C++26</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>
      P3631R0
      [<a href="https://wg21.link/P3631">Latest</a>]
      [<a href="https://wg21.link/P3631/status">Status</a>]
    </td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2025-05-13</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      LEWG, LWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Louis Dionne<br>&lt;<a href="mailto:ldionne.2@gmail.com" class="email">ldionne.2@gmail.com</a>&gt;<br>
      Giuseppe D’Angelo<br>&lt;<a href="mailto:giuseppe.dangelo@kdab.com" class="email">giuseppe.dangelo@kdab.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="#wording-changes" id="toc-wording-changes"><span class="toc-section-number">3</span> Wording changes<span></span></a>
<ul>
<li><a href="#synopsis-of-relocation-functions-20.2.2-memory.syn" id="toc-synopsis-of-relocation-functions-20.2.2-memory.syn"><span class="toc-section-number">3.1</span> Synopsis of relocation functions
20.2.2 [memory.syn]<span></span></a></li>
<li><a href="#definitions-for-relocation-functions-20.2.6-obj.lifetime" id="toc-definitions-for-relocation-functions-20.2.6-obj.lifetime"><span class="toc-section-number">3.2</span> Definitions for relocation
functions (20.2.6 [obj.lifetime])<span></span></a></li>
</ul></li>
</ul>
</div>
<h1 data-number="1" id="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p><a href="https://wg21.link/P3516">P3516</a> was design-approved by
LEWG in Hagenberg. As part of that discussion, we also discussed
cleaning up the relocation-related APIs provided in the library (since
<a href="https://wg21.link/P2786">P2786</a> also added some). There was
a fair amount of consensus in the room that the introduction of <a href="https://wg21.link/P3516">P3516</a> removed the need for some of
the APIs added by <a href="https://wg21.link/P2786">P2786</a>. However,
we decided to punt it to a separate paper to allow P3516 to progress.
This is the cleanup paper.</p>
<h1 data-number="2" id="proposal"><span class="header-section-number">2</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>The current state of the APIs that have been voted into the draft or
design approved in Hagenberg is this:</p>
<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="co">// P2786 APIs (in draft)</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>T<span class="op">*</span> trivially_relocate<span class="op">(</span>T<span class="op">*</span> first, T<span class="op">*</span> last, T<span class="op">*</span> result<span class="op">)</span>; <span class="co">// freestanding</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> T<span class="op">*</span> relocate<span class="op">(</span>T<span class="op">*</span> first, T<span class="op">*</span> last, T<span class="op">*</span> result<span class="op">)</span>; <span class="co">// freestanding</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a><span class="co">// P3516 APIs (design approved)</span></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a> <span class="kw">requires</span> <em>relocatable-from</em><span class="op">&lt;</span>T, T<span class="op">&gt;</span></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a>  <span class="kw">constexpr</span> T<span class="op">*</span> relocate_at<span class="op">(</span>T<span class="op">*</span> dest, T<span class="op">*</span> source<span class="op">)</span></span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a>    <span class="kw">noexcept</span><span class="op">(</span>is_nothrow_relocatable_v<span class="op">&lt;</span>T<span class="op">&gt;)</span>;</span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> NoThrowForwardIterator1, <span class="kw">class</span> NoThrowForwardIterator2<span class="op">&gt;</span></span>
<span id="cb1-15"><a href="#cb1-15" aria-hidden="true" tabindex="-1"></a>  <span class="kw">constexpr</span> NoThrowForwardIterator2</span>
<span id="cb1-16"><a href="#cb1-16" aria-hidden="true" tabindex="-1"></a>    uninitialized_relocate<span class="op">(</span>NoThrowForwardIterator1 first,</span>
<span id="cb1-17"><a href="#cb1-17" aria-hidden="true" tabindex="-1"></a>                           NoThrowForwardIterator1 last,</span>
<span id="cb1-18"><a href="#cb1-18" aria-hidden="true" tabindex="-1"></a>                           NoThrowForwardIterator2 result<span class="op">)</span>; <span class="co">// freestanding</span></span>
<span id="cb1-19"><a href="#cb1-19" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-20"><a href="#cb1-20" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> NoThrowForwardIterator1, <span class="kw">class</span> Size, <span class="kw">class</span> NoThrowForwardIterator2<span class="op">&gt;</span></span>
<span id="cb1-21"><a href="#cb1-21" aria-hidden="true" tabindex="-1"></a>  <span class="kw">constexpr</span> pair<span class="op">&lt;</span>NoThrowForwardIterator1, NoThrowForwardIterator2<span class="op">&gt;</span></span>
<span id="cb1-22"><a href="#cb1-22" aria-hidden="true" tabindex="-1"></a>    uninitialized_relocate_n<span class="op">(</span>NoThrowForwardIterator1 first,</span>
<span id="cb1-23"><a href="#cb1-23" aria-hidden="true" tabindex="-1"></a>                             Size n,</span>
<span id="cb1-24"><a href="#cb1-24" aria-hidden="true" tabindex="-1"></a>                             NoThrowForwardIterator2 result<span class="op">)</span>; <span class="co">// freestanding</span></span>
<span id="cb1-25"><a href="#cb1-25" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-26"><a href="#cb1-26" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> NoThrowBidirectionalIterator1, <span class="kw">class</span> NoThrowBidirectionalIterator2<span class="op">&gt;</span></span>
<span id="cb1-27"><a href="#cb1-27" aria-hidden="true" tabindex="-1"></a>  <span class="kw">constexpr</span> NoThrowBidirectionalIterator2</span>
<span id="cb1-28"><a href="#cb1-28" aria-hidden="true" tabindex="-1"></a>    uninitialized_relocate_backward<span class="op">(</span>NoThrowBidirectionalIterator1 first,</span>
<span id="cb1-29"><a href="#cb1-29" aria-hidden="true" tabindex="-1"></a>                                    NoThrowBidirectionalIterator1 last,</span>
<span id="cb1-30"><a href="#cb1-30" aria-hidden="true" tabindex="-1"></a>                                    NoThrowBidirectionalIterator2 result<span class="op">)</span>; <span class="co">// freestanding</span></span>
<span id="cb1-31"><a href="#cb1-31" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-32"><a href="#cb1-32" aria-hidden="true" tabindex="-1"></a><span class="co">// ... also std::execution_policy overloads ...</span></span>
<span id="cb1-33"><a href="#cb1-33" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-34"><a href="#cb1-34" aria-hidden="true" tabindex="-1"></a><span class="kw">namespace</span> ranges <span class="op">{</span></span>
<span id="cb1-35"><a href="#cb1-35" aria-hidden="true" tabindex="-1"></a>  <span class="co">// ... similar overloads here ...</span></span>
<span id="cb1-36"><a href="#cb1-36" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>To clean things up, we propose removing <code class="sourceCode cpp">std<span class="op">::</span>relocate<span class="op">(...)</span></code>.
It was introduced before we had proper relocation algorithms, and it’s
basically a less capable version of the uninitialized algorithms above
since it doesn’t support iterators and potentially-throwing move
constructors. One difference is that <code class="sourceCode cpp">std<span class="op">::</span>relocate<span class="op">(...)</span></code>
does detect overlaps and decides to relocate forward or backward, which
<a href="https://wg21.link/P3516">P3516</a> handles by having a
<code class="sourceCode cpp">_backward</code> variant instead (which is
consistent with existing uninitialized algorithms).</p>
<p>Note that we <em>do not</em> propose removing <code class="sourceCode cpp">std<span class="op">::</span>trivially_relocate</code>
or making it an exposition-only helper, although the idea was explored.
While we don’t want to encourage users to call it, we decided that the
function had a useful role as a low level object manipulation function,
similarly to
<code class="sourceCode cpp">std<span class="op">::</span>memmove</code>.
This will be a simple wrapper over the compiler builtin that all
compilers are expected to provide, which may be helpful to write some
low level APIs portably. General users are still expected to use the
higher level <code class="sourceCode cpp">std<span class="op">::</span>uninitialized_xxx</code>
algorithms instead.</p>
<h1 data-number="3" id="wording-changes"><span class="header-section-number">3</span> Wording changes<a href="#wording-changes" class="self-link"></a></h1>
<h2 data-number="3.1" id="synopsis-of-relocation-functions-20.2.2-memory.syn"><span class="header-section-number">3.1</span> Synopsis of relocation
functions 20.2.2 [memory.syn]<a href="#synopsis-of-relocation-functions-20.2.2-memory.syn" class="self-link"></a></h2>
<p>Remove <code class="sourceCode cpp">std<span class="op">::</span>relocate</code> from
the synopsis:</p>
<div class="rm" style="color: #bf0303">

<blockquote>
<div class="sourceCode" id="cb2"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>template &lt;class T&gt;</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>  constexpr T* relocate(T* first, T* last, T* result);                        // freestanding</span></code></pre></div>
</blockquote>

</div>
<h2 data-number="3.2" id="definitions-for-relocation-functions-20.2.6-obj.lifetime"><span class="header-section-number">3.2</span> Definitions for relocation
functions (20.2.6 [obj.lifetime])<a href="#definitions-for-relocation-functions-20.2.6-obj.lifetime" class="self-link"></a></h2>
<p>Remove <code class="sourceCode cpp">std<span class="op">::</span>relocate</code>. In
<em>Explicit lifetime management</em> <span>20.2.6
<a href="https://wg21.link/obj.lifetime">[obj.lifetime]</a></span>:</p>
<div class="rm" style="color: #bf0303">

<blockquote>
<div class="sourceCode" id="cb3"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>template &lt;class T&gt;</span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a> constexpr T* relocate(T* first, T* last, T* result);</span></code></pre></div>
</blockquote>
<p><em>Mandates</em>: <code class="sourceCode default">is_nothrow_relocatable_v&lt;T&gt; &amp;&amp; !is_const_v&lt;T&gt;</code>
is <code class="sourceCode default">true</code>.
<code class="sourceCode default">T</code> is not an array of unknown
bound.</p>
<p><em>Preconditions</em>:</p>
<ul>
<li><code class="sourceCode default">[first, last)</code> is a valid
range.</li>
<li><code class="sourceCode default">[result, result + (last - first))</code>
denotes a region of storage that is a subset of the region reachable
through result (6.8.4 [basic.compound]) and suitably aligned for the
type <code class="sourceCode default">T</code>.</li>
<li>No element in the range
<code class="sourceCode default">[first, last)</code> is a
potentially-overlapping subobject.</li>
</ul>
<p><em>Effects</em>:</p>
<ul>
<li>If <code class="sourceCode default">result == first</code> is
<code class="sourceCode default">true</code>, no effect;</li>
<li>otherwise, if not called during constant evaluation and <code class="sourceCode default">is_trivially_relocatable_v&lt;T&gt;</code> is
<code class="sourceCode default">true</code>, then has effects
equivalent to: <code class="sourceCode default">trivially_relocate(first, last, result);</code></li>
<li>otherwise, for each integer
<code class="sourceCode default">i</code> in
<code class="sourceCode default">[0, last - first)</code>,
<ul>
<li>if <code class="sourceCode default">T</code> is an array type,
equivalent to: <code class="sourceCode default">relocate(begin(first[i]), end(first[i]), *start_lifetime_as&lt;T&gt;(result + i));</code></li>
<li>otherwise, equivalent to: <code class="sourceCode default">construct_at(result + i, std::move(first[i])); destroy_at(first + i);</code></li>
</ul></li>
</ul>
<p><em>Returns</em>:
<code class="sourceCode default">result + (last-first)</code>.</p>
<p><em>Throws</em>: Nothing.</p>
<p><span class="note"><span>[ <em>Note:</em> </span>Overlapping ranges
are supported.<span> — <em>end note</em> ]</span></span></p>
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