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<title>Issue 353: std::pair missing template assignment</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="353"><a href="lwg-defects.html#353">353</a>. <code>std::pair</code> missing template assignment</h3>
<p><b>Section:</b> 22.3 <a href="https://wg21.link/pairs">[pairs]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2001-12-02 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#pairs">issues</a> in [pairs].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The class template <code>std::pair</code> defines a template ctor (20.2.2, p4) but
no template assignment operator. This may lead to inefficient code since
assigning an object of <code>pair&lt;C, D&gt;</code> to <code>pair&lt;A, B&gt;</code>
where the types <code>C</code> and <code>D</code> are distinct from but convertible to
<code>A</code> and <code>B</code>, respectively, results in a call to the template copy
ctor to construct an unnamed temporary of type <code>pair&lt;A, B&gt;</code>
followed by an ordinary (perhaps implicitly defined) assignment operator,
instead of just a straight assignment.
</p>


<p id="res-353"><b>Proposed resolution:</b></p>
<p>
Add the following declaration to the definition of <code>std::pair</code>:
</p>
<pre>
    template&lt;class U, class V&gt;
    pair&amp; operator=(const pair&lt;U, V&gt; &amp;p);
</pre>
<p>
And also add a paragraph describing the effects of the function template to the
end of 20.2.2:
</p>
<pre>
    template&lt;class U, class V&gt;
    pair&amp; operator=(const pair&lt;U, V&gt; &amp;p);
</pre>
<p>
    <b>Effects</b>: <code>first = p.first;</code>
                    <code>second = p.second;</code>
    <b>Returns</b>: <code>*this</code>
</p>

<p><i>[Cura&ccedil;ao: There is no indication this is was anything other than
a design decision, and thus NAD.&nbsp; May be appropriate for a future
standard.]</i></p>


<p><i>[
Pre Bellevue:  It was recognized that this was taken care of by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1856.html">N1856</a>,
and thus moved from NAD Future to <del>NAD Editorial</del><ins>Resolved</ins>.
]</i></p>






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