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<title>Issue 2081: Allocator requirements should include CopyConstructible</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++14">C++14</a> status.</em></p>
<h3 id="2081"><a href="lwg-defects.html#2081">2081</a>. <code>Allocator</code> requirements should include <code>CopyConstructible</code></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++14">C++14</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2011-08-30 <b>Last modified:</b> 2016-01-28</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++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>

<p>
As discussed in c++std-lib-31054 and c++std-lib-31059, the <code>Allocator</code>
requirements implicitly require <code>CopyConstructible</code> because
<code>a.select_on_container_copy_construction()</code> and
<code>container.get_allocator()</code> both return a copy by value, but the
requirement is not stated explicitly anywhere.
<p/>
In order to clarify that allocators cannot have 'explicit' copy
constructors, the requirements should include <code>CopyConstructible</code>.
</p>

<p><i>[2012, Kona]</i></p>

<p>
Move to Ready.
</p>

<p><i>[2012, Portland: applied to WP]</i></p>




<p id="res-2081"><b>Proposed resolution:</b></p>
<p>This wording is relative to the FDIS.</p>

<ol>
<li><p>Change Table 28 &mdash; Allocator requirements in 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a>:</p>

<table border="1">
<caption>Table 28 &mdash; Allocator requirements</caption>
<tr>
<th>Expression</th>
<th>Return type</th>
<th>Assertion&#47;note pre-&#47;post-condition</th>
<th>Default</th>
</tr> 
<tr>
<td>
<code>X a1(a);<br/>
<ins>X a1 = a;</ins></code>
</td>
<td>
</td>
<td>
Shall not exit via an exception.<br/>
post: <code>a1 == a</code>
</td>
<td>
</td>
</tr>

<tr>
<td colspan="4" align="center">
<code>&hellip;</code>
</td>
</tr>

<tr>
<td>
<code>X a1(move(a));<br/>
<ins>X a1 = move(a);</ins></code>
</td>
<td>
</td>
<td>
Shall not exit via an exception.<br/>
post: <code>a1</code> equals the prior value<br/>
of <code>a</code>.
</td>
<td>
</td>
</tr>
</table>

</li>

<li><p>Change 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a> paragraph 4:</p>

<blockquote><p>
<ins>An allocator type <code>X</code> shall satisfy the requirements of <code>CopyConstructible</code> 
(16.4.4.2 <a href="https://wg21.link/utility.arg.requirements">[utility.arg.requirements]</a>).</ins> The <code>X::pointer</code>, <code>X::const_pointer</code>, 
<code>X::void_pointer</code>, and <code>X::const_void_pointer</code> types shall satisfy the requirements of 
<code>NullablePointer</code> (16.4.4.4 <a href="https://wg21.link/nullablepointer.requirements">[nullablepointer.requirements]</a>). No constructor, comparison 
operator, copy operation, move operation, or swap operation on these types shall exit via an 
exception. <code>X::pointer</code> and <code>X::const_pointer</code> shall also satisfy the requirements 
for a random access iterator (24.3 <a href="https://wg21.link/iterator.requirements">[iterator.requirements]</a>).
</p></blockquote>
</li>
</ol>





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