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<title>Issue 1023: Unclear inheritance relation for std::function</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#NAD_Editorial">NAD Editorial</a> status.</em></p>
<h3 id="1023"><a href="lwg-closed.html#1023">1023</a>. Unclear inheritance relation for <code>std::function</code></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#NAD_Editorial">NAD Editorial</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-03-11 <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">issues</a> in [func.wrap.func].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Editorial">NAD Editorial</a> status.</p>
<p><b>Discussion:</b></p>

<p><b>Addresses DE 22 [CD1]</b></p>

<p>Related to issue <a href="lwg-defects.html#1114" title="Type traits underspecified (Status: C++11)">1114</a><sup><a href="https://cplusplus.github.io/LWG/issue1114" title="Latest snapshot">(i)</a></sup>.</p>

<p>
The conditions for deriving from <code>std::unary_function</code> and
<code>std::binary_function</code> are unclear: The condition would also be satisfied if
<code>ArgTypes</code> were <code>std::vector&lt;T1&gt;</code>, because it (arguably)
"contains" <code>T1</code>.
</p>

<p><i>[
Summit:
]</i></p>


<blockquote><p>
Agree. <code>std::reference_wrapper</code> has the same structure, and we
suggest that <code>std::function</code> be presented in the same way as
<code>std::reference_wrapper</code>.
</p></blockquote>

<p><i>[
2009-05-09 Alisdair adds:
]</i></p>


<blockquote><p>
Phrasing should be "publicly and unambiguously derived from" and probably back in 
<code>reference_wrapper</code> too.  Updated wording supplied.
</p></blockquote>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote><p>
We agree with the proposed wording.
Move to NAD Editorial.
</p></blockquote>


<p id="res-1023"><b>Proposed resolution:</b></p>
<p>
(no changes to <code>&lt;functional&gt;</code> synopsis required)
</p>

<p>
Change synopsis in Class template function 22.10.17.3 <a href="https://wg21.link/func.wrap.func">[func.wrap.func]</a>:
</p>

<blockquote><pre>
template&lt;Returnable R, CopyConstructible... ArgTypes&gt; 
class function&lt;R(ArgTypes...)&gt; 
  : public unary_function&lt;T1, R&gt;      // <del><i>iff</i> sizeof...(ArgTypes) == 1 <i>and</i></del> <ins><i>see below</i></ins>
                                      <del>// ArgTypes <i>contains</i> T1</del>
  : public binary_function&lt;T1, T2, R&gt; // <del><i>iff</i> sizeof...(ArgTypes) == 2 <i>and</i></del> <ins><i>see below</i></ins>
                                      <del>// ArgTypes <i>contains</i> T1 <i>and</i> T2</del>
{
   ...
</pre></blockquote>

<p>
Add new p1/p2 before 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a>:
</p>

<blockquote>
<p><ins>
The template instantiation <code>function&lt;R(T1)&gt;</code> shall be publicly and
unambiguously derived from 
<code>std::unary_function&lt;T1,R&gt;</code> if and only if the template type parameter
is a function type taking one argument of type <code>T1</code> and returning <code>R</code>.
</ins></p>

<p><ins>
The template instantiation <code>function&lt;R(T1,T2)&gt;</code> shall be publicly and
unambiguously derived from 
<code>std::binary_function&lt;T1,T2,R&gt;</code> if and only if the template type
parameter is a function type taking two arguments of type <code>T1</code> and <code>T2</code> and
returning <code>R</code>.
</ins></p>

<pre>
explicit function();
</pre>
</blockquote>






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