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<title>Issue 843:  Reference Closure</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#CD1">CD1</a> status.</em></p>
<h3 id="843"><a href="lwg-defects.html#843">843</a>.  Reference Closure</h3>
<p><b>Section:</b> 99 [func.referenceclosure.cons] <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Lawrence Crowl <b>Opened:</b> 2008-06-02 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The <code>std::reference_closure</code> type has a deleted copy assignment operator
under the theory that references cannot be assigned, and hence the
assignment of its reference member must necessarily be ill-formed.
</p>
<p>
However, other types, notably <code>std::reference_wrapper</code> and <code>std::function</code>
provide for the "copying of references", and thus the current definition
of <code>std::reference_closure</code> seems unnecessarily restrictive.  In particular,
it should be possible to write generic functions using both <code>std::function</code>
and <code>std::reference_closure</code>, but this generality is much harder when
one such type does not support assignment.
</p>
<p>
The definition of <code>reference_closure</code> does not necessarily imply direct
implementation via reference types.  Indeed, the <code>reference_closure</code> is
best implemented via a frame pointer, for which there is no standard
type.
</p>
<p>
The semantics of assignment are effectively obtained by use of the
default destructor and default copy assignment operator via
</p>

<blockquote><pre>
x.~reference_closure(); new (x) reference_closure(y);
</pre></blockquote>

<p>
So the copy assignment operator generates no significant real burden
to the implementation.
</p>


<p id="res-843"><b>Proposed resolution:</b></p>
<p>
In  [func.referenceclosure] Class template reference_closure,
replace the <code>=delete</code> in the copy assignment operator in the synopsis
with <code>=default</code>.
</p>

<blockquote><pre>
template&lt;class R , class... ArgTypes &gt; 
  class reference_closure&lt;R (ArgTypes...)&gt; { 
  public:
     ...
     reference_closure&amp; operator=(const reference_closure&amp;) = <del>delete</del> <ins>default</ins>;
     ...
</pre></blockquote>

<p>
In 99 [func.referenceclosure.cons] Construct, copy, destroy,
add the member function description
</p>

<blockquote>
<pre>
reference_closure&amp; operator=(const reference_closure&amp; f)
</pre>
<blockquote>
<p>
<i>Postcondition:</i> <code>*this</code> is a copy of <code>f</code>.
</p>
<p>
<i>Returns:</i> <code>*this</code>.
</p>
</blockquote>
</blockquote>







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