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<title>Issue 3957: &sect;[container.alloc.reqmts] The value category of v should be claimed</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#WP">WP</a> status.</em></p>
<h3 id="3957"><a href="lwg-defects.html#3957">3957</a>. &sect;[container.alloc.reqmts] The value category of <code>v</code> should be claimed</h3>
<p><b>Section:</b> 23.2.2.5 <a href="https://wg21.link/container.alloc.reqmts">[container.alloc.reqmts]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> jim x <b>Opened:</b> 2023-07-10 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#container.alloc.reqmts">issues</a> in [container.alloc.reqmts].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
23.2.2.5 <a href="https://wg21.link/container.alloc.reqmts">[container.alloc.reqmts]</a> p2 says:
</p>
<blockquote><p>
[&hellip;] an expression <code>v</code> of type <code>T</code> or <code>const T</code>, [&hellip;]
</p></blockquote>
<p>
Then 23.2.2.5 <a href="https://wg21.link/container.alloc.reqmts">[container.alloc.reqmts]</a> bullet (2.4) says:
</p>
<blockquote><p>
<code>T</code> is <i>Cpp17CopyInsertable into <code>X</code></i> means that, in addition to <code>T</code> being 
<i>Cpp17MoveInsertable</i> into <code>X</code>, the following expression is well-formed:
</p>
<blockquote><pre>
allocator_traits&lt;A&gt;::construct(m, p, v)
</pre></blockquote>
</blockquote>
<p>
So, what is the value category of the expression <code>v</code>? We didn't explicitly phrase the wording. 
The intent may be that the value category of <code>v</code> is any defined value category in 7.2.1 <a href="https://wg21.link/basic.lval">[basic.lval]</a>, 
however, the intent is not clear in the current wording. Maybe, we can say:
</p>
<blockquote><p>
[&hellip;] the following expression is well-formed:
</p>
<blockquote><pre>
allocator_traits&lt;A&gt;::construct(m, p, v)
</pre></blockquote>
<p>
for <code>v</code> of any value category.
</p>
</blockquote>
<p>
which can make the intent meaning clearer.
</p>

<p><i>[2023-10-27; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>

<p><i>[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3957"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4950" title=" Working Draft, Standard for Programming Language C++">N4950</a>.
</p>

<ol>

<li><p>Modify 23.2.2.5 <a href="https://wg21.link/container.alloc.reqmts">[container.alloc.reqmts]</a> as indicated:</p>

<blockquote>
<p>
-2- Given an allocator type <code>A</code> and given a container type <code>X</code> having a <code>value_type</code> 
identical to <code>T</code> and an <code>allocator_type</code> identical to <code>allocator_traits&lt;A&gt;::rebind_alloc&lt;T&gt;</code> 
and given an lvalue <code>m</code> of type <code>A</code>, a pointer <code>p</code> of type <code>T*</code>, an expression 
<code>v</code> <ins>that denotes an lvalue</ins> of type <code>T</code> or <code>const T</code> <ins>or an rvalue of type <code>const T</code></ins>, 
and an rvalue <code>rv</code> of type <code>T</code>, the following terms are defined. [&hellip;]
</p>
<ol style="list-style-type: none">
<li><p>[&hellip;]</p></li>
<li><p>(2.3) &mdash; <code>T</code> is <i>Cpp17MoveInsertable into <code>X</code></i> means that the following expression is well-formed:</p>
<blockquote><pre>
allocator_traits&lt;A&gt;::construct(m, p, rv)
</pre></blockquote>
<p>and its evaluation causes the following postcondition to hold: The value of <code>*p</code> is equivalent to the value
of <code>rv</code> before the evaluation.
<p/>
[<i>Note 1</i>: <code>rv</code> remains a valid object. Its state is unspecified &mdash; <i>end note</i>]</p></li>
<li><p>(2.4) &mdash; <code>T</code> is <i>Cpp17CopyInsertable into <code>X</code></i> means that, in addition to <code>T</code> being 
<i>Cpp17MoveInsertable</i> into <code>X</code>, the following expression is well-formed:</p>
<blockquote><pre>
allocator_traits&lt;A&gt;::construct(m, p, v)
</pre></blockquote>
<p>and its evaluation causes the following postcondition to hold: The value of <code>v</code> is unchanged and is
equivalent to <code>*p</code>.</p>
</li>
<li><p>[&hellip;]</p></li>
</ol>
</blockquote>

</li>

</ol>





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