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<title>Issue 2420: function&lt;void(ArgTypes...)&gt; does not discard the return value of the target object</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++17">C++17</a> status.</em></p>
<h3 id="2420"><a href="lwg-defects.html#2420">2420</a>. <code>function&lt;void(ArgTypes...)&gt;</code> does not discard the return value of the target object</h3>
<p><b>Section:</b> 22.10.17.3 <a href="https://wg21.link/func.wrap.func">[func.wrap.func]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Agust&iacute;n Berg&eacute; <b>Opened:</b> 2014-07-12 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>1
</p>
<p><b>View all other</b> <a href="lwg-index.html#func.wrap.func">issues</a> in [func.wrap.func].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>function&lt;void(ArgTypes...)&gt;</code> should discard the return value of the target object. This behavior was 
in the original proposal, and it was removed (accidentally?) by the resolution of LWG <a href="lwg-defects.html#870" title="Do unordered containers not support function pointers for predicate&#47;hasher? (Status: C++11)">870</a><sup><a href="https://cplusplus.github.io/LWG/issue870" title="Latest snapshot">(i)</a></sup>.
</p>

<p>
<strong>Previous resolution [SUPERSEDED]:</strong>
</p>
<blockquote class="note">
<ol>
<li><p>Edit 22.10.17.3 <a href="https://wg21.link/func.wrap.func">[func.wrap.func]</a> paragraph 2:</p>

<blockquote>
<p>
A callable object <code>f</code> of type <code>F</code> is <i>Callable</i> for argument types <code>ArgTypes</code> and return type 
<code>R</code> if the expression <code><i>INVOKE</i>(f, declval&lt;ArgTypes&gt;()...<del>, R</del>)</code>, considered as an unevaluated 
operand (Clause 5), is well formed (22.10.4 <a href="https://wg21.link/func.require">[func.require]</a>)<ins> and, if <code>R</code> is not <code>void</code>, implicitly 
convertible to <code>R</code></ins>.
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2014-10-05 Daniel comments]</i></p>

<p>
This side-effect was indeed not intended by <a href="lwg-defects.html#870" title="Do unordered containers not support function pointers for predicate&#47;hasher? (Status: C++11)">870</a><sup><a href="https://cplusplus.github.io/LWG/issue870" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[2015-05, Lenexa]</i></p>

<p>
STL provides improved wording. It replaces the current PR, and intentionally leaves 22.10.17.3 <a href="https://wg21.link/func.wrap.func">[func.wrap.func]</a> 
unchanged.
<p/>
Due to 7 <a href="https://wg21.link/expr">[expr]</a>/6, <code>static_cast&lt;void&gt;</code> is correct even when <code>R</code> is <code>const void</code>.
</p>


<p id="res-2420"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4431.</p>

<ol>
<li><p>Edit 22.10.4 <a href="https://wg21.link/func.require">[func.require]</a> as depicted:</p>

<blockquote>
<p>
-2- Define <code><i>INVOKE</i>(f, t1, t2, ..., tN, R)</code> as <ins><code>static_cast&lt;void&gt;(<i>INVOKE</i>(f, t1, t2, ..., tN))</code> 
if <code>R</code> is <i>cv</i> <code>void</code>, otherwise</ins> <code><i>INVOKE</i>(f, t1, t2, ..., tN)</code> implicitly converted 
to <code>R</code>.
</p>
</blockquote>
</li>

<li><p>Change 22.10.17.3.5 <a href="https://wg21.link/func.wrap.func.inv">[func.wrap.func.inv]</a> as depicted:</p>

<blockquote>
<pre>
R operator()(ArgTypes... args) const;
</pre>
<blockquote>
<p>
-1- <i><del>Effects</del><ins>Returns</ins></i>: <code><i>INVOKE</i>(f, std::forward&lt;ArgTypes&gt;(args)..., R)</code> (20.9.2), 
where <code>f</code> is the target object (20.9.1) of <code>*this</code>.
<p/>
<del>-2- <i>Returns</i>: Nothing if <code>R</code> is <code>void</code>, otherwise the return value of <code><i>INVOKE</i>(f, 
std::forward&lt;ArgTypes&gt;(args)..., R)</code>.</del>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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