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<title>Issue 1292: std::function should support all callable types</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="1292"><a href="lwg-defects.html#1292">1292</a>. <code>std::function</code> should support all callable types</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> Daniel Kr&uuml;gler <b>Opened:</b> 2009-12-19 <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>
Some parts of the specification of <code>std::function</code> is unnecessarily
restricted to a subset of all callable types (as defined in 22.10.3 <a href="https://wg21.link/func.def">[func.def]</a>/3), even though the intent clearly is to be usable for
<em>all</em> of them as described in 22.10.17.3 <a href="https://wg21.link/func.wrap.func">[func.wrap.func]</a>/1. This
argument becomes strengthened by the fact that current C++0x-compatible
compilers work fine with them:
</p>

<blockquote><pre>
#include &lt;functional&gt;
#include &lt;iostream&gt;

struct A
{
  int foo(int i) const {return i+1;}
};

struct B
{
  int mem;
};

int main()
{
  std::function&lt;int(const A&amp;, int)&gt; f(&amp;A::foo);
  A a;
  std::cout &lt;&lt; f(a, 1) &lt;&lt; '\n';
  std::cout &lt;&lt; f.target_type().name() &lt;&lt; '\n';
  typedef int (A::* target_t)(int) const;
  target_t* p = f.target&lt;target_t&gt;();
  std::cout &lt;&lt; (p != 0) &lt;&lt; '\n';
  std::function&lt;int(B&amp;)&gt; f2(&amp;B::mem);
  B b = { 42 };
  std::cout &lt;&lt; f2(b) &lt;&lt; '\n';
  std::cout &lt;&lt; f2.target_type().name() &lt;&lt; '\n';
  typedef int (B::* target2_t);
  target2_t* p2 = f2.target&lt;target2_t&gt;();
  std::cout &lt;&lt; (p2 != 0) &lt;&lt; '\n';
}
</pre></blockquote>

<p>
The problematic passages are 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a>/10:
</p>

<blockquote><pre>
template&lt;class F&gt; function(F f);
template &lt;class F, class A&gt; function(allocator_arg_t, const A&amp; a, F f);
</pre>
<blockquote>
<p>...</p>
<p>
10 <i>Postconditions:</i> <code>!*this</code> if any of the following hold:
</p>
<ul>
<li>
<code>f</code> is a <code>NULL</code> function pointer.
</li>
<li>
<code>f</code> is a <code>NULL</code> member function pointer.
</li>
<li>
<code>F</code> is an instance of the function class template, and <code>!f</code>
</li>
</ul>
</blockquote>
</blockquote>

<p>
because it does not consider pointer to data member and all constraints based on
<em>function objects</em> which like 22.10.17.3 <a href="https://wg21.link/func.wrap.func">[func.wrap.func]</a>/2 or 22.10.17.3.6 <a href="https://wg21.link/func.wrap.func.targ">[func.wrap.func.targ]</a>/3. The latter two will be resolved by the proposed
resolution of <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> and are therefore not handled here.
</p>

<p><i>[
Post-Rapperswil:
]</i></p>


<blockquote><p>
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
</p></blockquote>

<p><i>[
Adopted at 2010-11 Batavia
]</i></p>




<p id="res-1292"><b>Proposed resolution:</b></p>
<p>
Change 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a>/10+11 as indicated:
</p>

<blockquote><pre>
template&lt;class F&gt; function(F f);
template &lt;class F, class A&gt; function(allocator_arg_t, const A&amp; a, F f);
</pre>
<blockquote>
<p>...</p>
<p>
10 <i>Postconditions:</i> <code>!*this</code> if any of the following hold:
</p>
<ul>
<li>
<code>f</code> is a <code>NULL</code> function pointer.
</li>
<li>
<code>f</code> is a <code>NULL</code> <ins>pointer to</ins> member <del>function pointer</del>.
</li>
<li>
<code>F</code> is an instance of the function class template, and <code>!f</code>
</li>
</ul>

<p>
11 Otherwise, <code>*this</code> targets a copy of <code>f</code> <del>or</del><ins>,
initialized with</ins> <code>std::move(f)</code> <del>if <code>f</code> is not a pointer
to member function, and targets a copy of <code>mem_fn(f)</code> if <code>f</code> is a
pointer to member function</del>. [<i>Note:</i> implementations are encouraged
to avoid the use of dynamically allocated memory for small function objects, for
example, where <code>f</code>'s target is an object holding only a pointer or
reference to an object and a member function pointer. &mdash; <i>end note</i>]
</p>

</blockquote>
</blockquote>





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