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<title>Issue 1288: std::function assignment from rvalues</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="1288"><a href="lwg-defects.html#1288">1288</a>. <code>std::function</code> assignment from rvalues</h3>
<p><b>Section:</b> 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2009-12-13 <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#func.wrap.func.con">issues</a> in [func.wrap.func.con].</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>
<p>
In 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a>
</p>

<blockquote><pre>
template&lt;class F&gt; function&amp; operator=(F f);
</pre>
<blockquote>
<p>
20        <i>Effects:</i> <code>function(f).swap(*this);</code>
</p>
<p>
21        <i>Returns:</i> <code>*this</code>
</p>
</blockquote>
</blockquote>

<p>
This assignment operator can be called such that <code>F</code> is an rvalue-reference e.g.
</p>

<blockquote><pre>
func.operator=&lt;F&amp;&amp;&gt;(f);
</pre></blockquote>

<p>
There are two issues with this.
</p>

<ol>
<li>
the effects mean that <code>f</code> is passed as an lvalue and so there will be an
unnecessary copy. The argument should be forwarded, so that the copy can be
avoided.
</li>
<li>
It should not be necessary to use that syntax to pass an rvalue. As <code>F</code>
is a deduced context it can be made to work with either lvalues or rvalues.
</li>
</ol>

<p>
The same issues apply to <code>function::assign</code>.
</p>

<p>
N.B. this issue is not related to <a href="lwg-defects.html#1287" title="std::function requires CopyConstructible target object (Status: C++11)">1287</a><sup><a href="https://cplusplus.github.io/LWG/issue1287" title="Latest snapshot">(i)</a></sup> and applies whether that
issue is resolved or not. The wording below assumes the resolution of LWG <a href="lwg-defects.html#1258" title="std::function Effects clause impossible to satisfy (Status: Resolved)">1258</a><sup><a href="https://cplusplus.github.io/LWG/issue1258" title="Latest snapshot">(i)</a></sup> has been applied.
</p>

<p><i>[
2009-12-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]</i></p>


<p><i>[
201002-11 Opened by Alisdair for the purpose of merging <a href="lwg-defects.html#1258" title="std::function Effects clause impossible to satisfy (Status: Resolved)">1258</a><sup><a href="https://cplusplus.github.io/LWG/issue1258" title="Latest snapshot">(i)</a></sup> into
this issue as there is a minor conflict.
]</i></p>


<p><i>[
2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]</i></p>




<p id="res-1288"><b>Proposed resolution:</b></p>
<p>
In 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a>
</p>

<blockquote><pre>
template&lt;class F&gt; function&amp; operator=(F<ins>&amp;&amp;</ins> f);
</pre>
<blockquote>
<p>
20 <i>Effects:</i>
<code>function(<ins>std::forward&lt;F&gt;(</ins>f<ins>)</ins>).swap(*this);</code>
</p>
<p>
21        <i>Returns:</i> <code>*this</code>
</p>
</blockquote>
</blockquote>

<p>
In 22.10.17.3.3 <a href="https://wg21.link/func.wrap.func.mod">[func.wrap.func.mod]</a>
</p>

<blockquote><pre>
template&lt;class F, <del>Allocator Alloc</del><ins>class A</ins>&gt;
  void assign(F<ins>&amp;&amp; f</ins>, const A<del>lloc</del>&amp; a);
</pre>
<blockquote>
<p>
<ins>3</ins> <i>Effects:</i> <code>function(<del>f, a</del><ins>allocator_arg, a,
std::forward&lt;F&gt;(f)</ins>).swap(*this);</code>
</p>
</blockquote>
</blockquote>

<p>
Update member function signature for class template in 22.10.17.3 <a href="https://wg21.link/func.wrap.func">[func.wrap.func]</a>
</p>

<blockquote><pre>
template&lt;class F&gt; function&amp; operator=(F<ins>&amp;&amp;</ins>);

template&lt;class F, class A&gt; void assign(F<ins>&amp;&amp;</ins>, const A&amp;);
</pre></blockquote>







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