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<title>Issue 3642: move_only_function assignment operators seem to be defined suboptimal</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="3642"><a href="lwg-active.html#3642">3642</a>. <code>move_only_function</code> assignment operators seem to be defined suboptimal</h3>
<p><b>Section:</b> 22.10.17.4.3 <a href="https://wg21.link/func.wrap.move.ctor">[func.wrap.move.ctor]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Alexander Guteniev <b>Opened:</b> 2021-11-20 <b>Last modified:</b> 2022-01-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#func.wrap.move.ctor">active issues</a> in [func.wrap.move.ctor].</p>
<p><b>View all other</b> <a href="lwg-index.html#func.wrap.move.ctor">issues</a> in [func.wrap.move.ctor].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
22.10.17.4.3 <a href="https://wg21.link/func.wrap.move.ctor">[func.wrap.move.ctor]</a>/22 and 22.10.17.4.3 <a href="https://wg21.link/func.wrap.move.ctor">[func.wrap.move.ctor]</a>/25 define the effects 
of assignment as following:
</p>
<blockquote>
<pre>
move_only_function&amp; operator=(move_only_function&amp;&amp; f);
</pre>
<blockquote>
<p>
<i>Effects:</i> Equivalent to: <code>move_only_function(std::move(f)).swap(*this);</code>
</p>
</blockquote>
[&hellip;]
<pre>
template&lt;class F&gt; move_only_function&amp; operator=(F&amp;&amp; f);
</pre>
<blockquote>
<p>
<i>Effects:</i> Equivalent to: <code>move_only_function(std::forward&lt;F&gt;(f)).swap(*this);</code>
</p>
</blockquote>
</blockquote>
<p>
The assignment via <code>swap</code> with temporary makes the implementation to do the following:
</p>
<ul>
<li><p>move out the previous target to a temporary location</p></li>
<li><p>move in the new target</p></li>
<li><p>finally destroy the previous target.</p></li>
</ul>
<p>
As everything is <code>noexcept</code> here, I think it can be short cut to just:
</p>
<ul>
<li><p>destroy the previous target.</p></li>
<li><p>move in the new target</p></li>
</ul>
<p>
Looks like the implementation cannot do such optimization in a generic case with small functor 
optimization enabled and non-trivial move constructor for the new target and with non-trivial 
destruction of the previous target, since the difference is observable.
<p/>
Apparently the optimization is precluded for no reason.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
Some suggestions for NAD, but others disagreed.
</p>



<p id="res-3642"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>.
</p>

<ol>

<li><p>Modify 22.10.17.4.3 <a href="https://wg21.link/func.wrap.move.ctor">[func.wrap.move.ctor]</a> as indicated:</p>

<blockquote>
<pre>
move_only_function&amp; operator=(move_only_function&amp;&amp; f);
</pre>
<blockquote>
<p>
-22- <i>Effects:</i> <ins>Sets the target object of <code>this</code> to the target object of <code>f</code> 
before the assignment and leaves <code>f</code> in a valid state with an unspecified value.</ins><del>Equivalent to: <code>move_only_function(std::move(f)).swap(*this);</code></del>
</p>
</blockquote>
[&hellip;]
<pre>
template&lt;class F&gt; move_only_function&amp; operator=(F&amp;&amp; f);
</pre>
<blockquote>
<p>
-25- <i>Effects:</i> Equivalent to: <code><ins>*this = </ins>move_only_function(std::forward&lt;F&gt;(f))<del>.swap(*this)</del>;</code>
</p>
</blockquote>
</blockquote>
</li>

</ol>





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