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<title>Issue 403: basic_string::swap should not throw exceptions</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#CD1">CD1</a> status.</em></p>
<h3 id="403"><a href="lwg-defects.html#403">403</a>. basic_string::swap should not throw exceptions</h3>
<p><b>Section:</b> 27.4.3.7.8 <a href="https://wg21.link/string.swap">[string.swap]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Beman Dawes <b>Opened:</b> 2003-03-25 <b>Last modified:</b> 2016-11-12</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#string.swap">issues</a> in [string.swap].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>

<p>
std::basic_string, 27.4.3 <a href="https://wg21.link/basic.string">[basic.string]</a> paragraph 2 says that
basic_string "conforms to the requirements of a Sequence, as specified
in (23.1.1)." The sequence requirements specified in (23.1.1) to not
include any prohibition on swap members throwing exceptions.
</p>

<p>
Section 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> paragraph 10 does limit conditions under
which exceptions may be thrown, but applies only to "all container
types defined in this clause" and so excludes basic_string::swap
because it is defined elsewhere.
</p>

<p>
Eric Niebler points out that 27.4.3 <a href="https://wg21.link/basic.string">[basic.string]</a> paragraph 5 explicitly
permits basic_string::swap to invalidates iterators, which is
disallowed by 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> paragraph 10. Thus the standard would
be contradictory if it were read or extended to read as having
basic_string meet 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> paragraph 10 requirements.
</p>

<p>
Yet several LWG members have expressed the belief that the original
intent was that basic_string::swap should not throw exceptions as
specified by 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> paragraph 10, and that the standard is
unclear on this issue. The complexity of basic_string::swap is
specified as "constant time", indicating the intent was to avoid
copying (which could cause a bad_alloc or other exception). An
important use of swap is to ensure that exceptions are not thrown in
exception-safe code.
</p>

<p>
Note: There remains long standing concern over whether or not it is
possible to reasonably meet the 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> paragraph 10 swap
requirements when allocators are unequal. The specification of
basic_string::swap exception requirements is in no way intended to
address, prejudice, or otherwise impact that concern.
</p>







<p id="res-403"><b>Proposed resolution:</b></p>
<p>
In 27.4.3.7.8 <a href="https://wg21.link/string.swap">[string.swap]</a>, add a throws clause:
</p>

<p>
Throws: Shall not throw exceptions.
</p>





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