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<title>Issue 885: pair 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#C++11">C++11</a> status.</em></p>
<h3 id="885"><a href="lwg-defects.html#885">885</a>. <code>pair</code> assignment</h3>
<p><b>Section:</b> 22.3 <a href="https://wg21.link/pairs">[pairs]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2008-09-15 <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#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<blockquote><pre>
20.2.3 pairs
Missing assignemnt operator:
template&lt;class U , class V&gt;
  requires CopyAssignable&lt;T1, U&gt; &amp;&amp; CopyAssignable&lt;T2, V&gt;
    pair&amp; operator=(pair&lt;U , V&gt; const &amp; p );
</pre></blockquote>

<p>
Well, that's interesting. This assignment operator isn't in the
current working paper, either. Perhaps we deemed it acceptable to
build a temporary of type <code>pair</code> from <code>pair&lt;U, V&gt;</code>, then move-assign
from that temporary?
</p>
<p>
It sounds more like an issue waiting to be opened, unless you want to plug
it now.  As written we risk moving from lvalues.
</p>

<p><i>[
San Francisco:
]</i></p>


<blockquote>
<p>
Would be NAD if better ctors fixed it.
</p>
<p>
Related to <a href="lwg-defects.html#811" title="pair of pointers no longer works with literal 0 (Status: C++11)">811</a><sup><a href="https://cplusplus.github.io/LWG/issue811" title="Latest snapshot">(i)</a></sup>.
</p>
</blockquote>

<p><i>[
post San Francisco:
]</i></p>


<blockquote><p>
Possibly NAD Editorial, solved by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2770.pdf">N2770</a>.
</p></blockquote>

<p><i>[
2009-05-25 Alisdair adds:
]</i></p>


<blockquote><p>
Issue <a href="lwg-defects.html#885" title="pair assignment (Status: C++11)">885</a><sup><a href="https://cplusplus.github.io/LWG/issue885" title="Latest snapshot">(i)</a></sup> was something I reported while reviewing the library concepts
documents ahead of San Francisco.  The missing operator was added as part of
the paper adopted at that meeting
(<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2770.pdf">N2770</a>)
and I can confirm this operator is
present in the current working paper.  I recommend NAD.
</p></blockquote>

<p><i>[
2009-07 Frankfurt
]</i></p>


<blockquote><p>
We agree with the intent, but we need to wait for the dust to settle on concepts.
</p></blockquote>

<p><i>[
2010-03-11 Stefanus provided wording.
]</i></p>


<p><i>[
2010 Pittsburgh:  Moved to Ready for Pittsburgh.
]</i></p>




<p id="res-885"><b>Proposed resolution:</b></p>
<p>
Add the following declaration 22.3.2 <a href="https://wg21.link/pairs.pair">[pairs.pair]</a>, before the
declaration of <code>pair&amp; operator=(pair&amp;&amp; p);</code>:
</p>
 
<blockquote><pre>
template&lt;class U, class V&gt; pair&amp; operator=(const pair&lt;U, V&gt;&amp; p);
</pre></blockquote>

<p>
Add the following description to 22.3.2 <a href="https://wg21.link/pairs.pair">[pairs.pair]</a> after paragraph 11 (before
the description of <code>pair&amp; operator=(pair&amp;&amp; p);)</code>:
</p>
 
<blockquote><pre>
template&lt;class U, class V&gt; pair&amp; operator=(const pair&lt;U, V&gt;&amp; p);
</pre>
<blockquote>
<p>
<i>Requires:</i> <code>T1</code> shall satisfy the requirements of
<code>CopyAssignable</code> from <code>U</code>. <code>T2</code> shall
satisfy the requirements of <code>CopyAssignable</code> from <code>V</code>.
</p>
<p>
<i>Effects:</i> Assigns <code>p.first</code> to <code>first</code> and 
<code>p.second</code> to <code>second</code>.
</p>
<p>
<i>Returns:</i> <code>*this</code>.
</p>
</blockquote>
</blockquote>






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