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<title>Issue 2431: Missing regular expression traits requirements</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#New">New</a> status.</em></p>
<h3 id="2431"><a href="lwg-active.html#2431">2431</a>. Missing regular expression traits requirements</h3>
<p><b>Section:</b> 28.6.2 <a href="https://wg21.link/re.req">[re.req]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2014-09-30 <b>Last modified:</b> 2020-04-16</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#re.req">active issues</a> in [re.req].</p>
<p><b>View all other</b> <a href="lwg-index.html#re.req">issues</a> in [re.req].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The requirements on the traits class in 28.6.2 <a href="https://wg21.link/re.req">[re.req]</a> do not say whether a
regular expression traits class is required to be <code>DefaultConstructible</code>, 
<code>CopyConstructible</code>, <code>CopyAssignable</code> etc.
<p/>
The <code>std::regex_traits</code> class appears to be all of the above, but can
<code>basic_regex</code> assume that for user-defined traits classes?
<p/>
Should the following statements all leave <code>u</code> in equivalent states?
</p>
<blockquote><pre>
X u{v};
X u; u = v;
X u; u.imbue(v.getloc();
</pre></blockquote>
<p>
Whether they are equivalent has implications for <code>basic_regex</code> copy construction and 
assignment.
</p>

<p><i>[2020-04-16,
Jonathan adds that 28.6.7.5 <a href="https://wg21.link/re.regex.locale">[re.regex.locale]</a> requires the traits type
to be default-initialized, despite no guarantee that the traits type is
default constructible.
]</i></p>




<p id="res-2431"><b>Proposed resolution:</b></p>





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