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<title>Issue 1287: std::function requires CopyConstructible 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++11">C++11</a> status.</em></p>
<h3 id="1287"><a href="lwg-defects.html#1287">1287</a>. <code>std::function</code> requires <code>CopyConstructible</code> target object</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>
I think <code>std::function</code> should require <code>CopyConstructible</code> for the
target object.
</p>

<p>
I initially thought that <code>MoveConstructible</code> was enough, but it's not. If
<code>F</code> is move-only then function's copy constructor cannot be called, but
because function uses type erasure, <code>F</code> is not known and so the copy
constructor cannot be disabled via <code>enable_if</code>.  One option would be to
throw an exception if you try to copy a function with a non-copyable target
type, but I think that would be a terrible idea.
</p>

<p>
So although the constructors require that the target be initialised by
<code>std::move(f)</code>, that's only an optimisation, and a copy constructor is
required.
</p>

<p><i>[
2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]</i></p>




<p id="res-1287"><b>Proposed resolution:</b></p>
<p>
Add to 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a> paragraph 9:
</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>
9  <i>Requires:</i> <ins><code>F</code> shall be <code>CopyConstructible</code>.</ins>
<code>f</code> shall be callable for argument types <code>ArgTypes</code> and return
type <code>R</code>. The copy constructor and destructor of <code>A</code> shall not
throw exceptions.
</p></blockquote>
</blockquote>






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