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<title>Issue 2151: basic_string&lt;&gt;::swap semantics ignore allocators</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#Resolved">Resolved</a> status.</em></p>
<h3 id="2151"><a href="lwg-defects.html#2151">2151</a>. <code>basic_string&lt;&gt;::swap</code> semantics ignore allocators</h3>
<p><b>Section:</b> 27.4.3.2 <a href="https://wg21.link/string.require">[string.require]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Robert Shearer <b>Opened:</b> 2012-04-13 <b>Last modified:</b> 2018-11-25</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#string.require">issues</a> in [string.require].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In C++11, <code>basic_string</code> is not described as a "container", and is not governed by the allocator-aware 
container semantics described in sub-clause 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a>; as a result, and 
requirements or contracts for the <code>basic_string</code> interface must be documented in Clause 
27 <a href="https://wg21.link/strings">[strings]</a>.
<p/>
Sub-clause 27.4.3.7.8 <a href="https://wg21.link/string.swap">[string.swap]</a> defines the <code>swap</code> member function with no requirements, and
with guarantees to execute in constant time without throwing. Fulfilling such a contract is not reasonable 
in the presence of unequal non-propagating allocators.
<p/>
In contrast, 23.2.2 <a href="https://wg21.link/container.requirements.general">[container.requirements.general]</a> p7 declares the behavior of member <code>swap</code> 
for containers with unequal non-propagating allocators to be undefined.
<p/>
Resolution proposal:
<p/>
Additional language from Clause 23 <a href="https://wg21.link/containers">[containers]</a> should probably be copied to Clause 
27 <a href="https://wg21.link/strings">[strings]</a>. I will refrain from an exactly recommendation, however, as I am raising further
issues related to the language in Clause 23 <a href="https://wg21.link/containers">[containers]</a>.
</p>

<p><i>[2013-03-15 Issues Teleconference]</i></p>

<p>
Moved to Open.
</p>
<p>
Alisdair has offered to provide wording.
</p>
<p>
Telecon notes that 23.2.2 <a href="https://wg21.link/container.requirements.general">[container.requirements.general]</a>p13 says that <code>string</code> is an
allocator-aware container.
</p>
<p>Resolved by the adoption of <a href="https://wg21.link/P1148">P1148</a> in San Diego.</p>


<p id="res-2151"><b>Proposed resolution:</b></p>





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