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<title>Issue 2470: Allocator's destroy function should be allowed to fail to instantiate</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#C++17">C++17</a> status.</em></p>
<h3 id="2470"><a href="lwg-defects.html#2470">2470</a>. Allocator's <code>destroy</code> function should be allowed to fail to instantiate</h3>
<p><b>Section:</b> 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2015-03-22 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#allocator.requirements">active issues</a> in [allocator.requirements].</p>
<p><b>View all other</b> <a href="lwg-index.html#allocator.requirements">issues</a> in [allocator.requirements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
This issue is a spin-off of issue LWG <a href="lwg-defects.html#2447" title="Allocators and volatile-qualified value types (Status: C++17)">2447</a><sup><a href="https://cplusplus.github.io/LWG/issue2447" title="Latest snapshot">(i)</a></sup>: It focuses on the observation that
16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a> p9 (based on the numbering of working draft N4296) gives 
the template member <code>construct</code> more relaxations than the template member <code>destroy</code>:
</p>
<blockquote>
<p>
An allocator may constrain the types on which it can be instantiated and the arguments for which its
<code>construct</code> member may be called. If a type cannot be used with a particular allocator, the allocator class
or the call to <code>construct</code> may fail to instantiate.
</p>
</blockquote>
<p>
Construction and destruction of a type <code>T</code> are usually intimately related to each other, so it
seems similarly useful to allow the <code>destroy</code> member to fail to instantiate for a possible sub-set
of instantiation types.
</p>

<p><i>[2015-04-01 Library reflector vote]</i></p>

<p>
The issue has been identified as Tentatively Ready based on six votes in favour.
</p>


<p id="res-2470"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4296.</p>

<ol>
<li><p>Change 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a> p9 as indicated:</p>

<blockquote><p>
-8- An allocator may constrain the types on which it can be instantiated and the arguments for which its
<code>construct</code> <ins>or <code>destroy</code></ins> member<ins>s</ins> may be called. If a type cannot be 
used with a particular allocator, the allocator class or the call to <code>construct</code> <ins>or <code>destroy</code></ins> 
may fail to instantiate.
</p></blockquote>
</li>
</ol>





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