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<title>Issue 1294: Difference between callable wrapper and forwarding  call wrapper unclear</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="1294"><a href="lwg-defects.html#1294">1294</a>. Difference between callable wrapper and forwarding  call wrapper unclear</h3>
<p><b>Section:</b> 22.10.4 <a href="https://wg21.link/func.require">[func.require]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Jens Maurer <b>Opened:</b> 2009-12-21 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#func.require">active issues</a> in [func.require].</p>
<p><b>View all other</b> <a href="lwg-index.html#func.require">issues</a> in [func.require].</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>
The current wording in the standard makes it hard to discriminate the difference
between a "call wrapper" as defined in 22.10.3 <a href="https://wg21.link/func.def">[func.def]</a>/5+6:
</p>

<blockquote>
<p>
5 A <i>call wrapper type</i> is a type that holds a callable object and supports
a call operation that forwards to that object.
</p>
<p>
6 A <i>call wrapper</i> is an object of a call wrapper type.
</p>
</blockquote>

<p>
and a "forwarding call wrapper" as defined in 22.10.4 <a href="https://wg21.link/func.require">[func.require]</a>/4:
</p>

<blockquote>
<p>
4 [..] A <i>forwarding call wrapper</i> is a call wrapper that can be called
with an argument list. [<i>Note:</i> in a typical implementation forwarding call
wrappers have an overloaded function call operator of the form
</p>

<blockquote><pre>
template&lt;class... ArgTypes&gt;
R operator()(ArgTypes&amp;&amp;... args) <i>cv-qual</i>;
</pre></blockquote>

<p>
&mdash; <i>end note</i>]
</p>
</blockquote>

<p>
Reason for this lack of clear difference seems to be that the wording adaption
to variadics and rvalues that were applied after it's original proposal in <a
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1673.html#call%20wrapper">N1673</a>:
</p>

<blockquote>
<p>
[..] A <b>forwarding call wrapper</b> is a call wrapper that can be called
with an argument list <code>t1, t2, ..., tN</code> where each <code>ti</code> is an lvalue.
The effect of calling a forwarding call wrapper with one or more
arguments that are rvalues is implementation defined. [<i>Note:</i> in
a typical implementation forwarding call wrappers have overloaded
function call operators of the form
</p>

<blockquote><pre>
template&lt;class T1, class T2, ..., class TN&gt;
R operator()(T1&amp; t1, T2&amp; t2, ..., TN&amp; tN) <i>cv-qual</i>;
</pre></blockquote>

<p>
&mdash; <i>end note</i>]
</p>
</blockquote>

<p>
combined with the fact that the word "forward" has two different meanings in
this context. This issue attempts to clarify the difference better.
</p>

<p><i>[
2010-09-14 Daniel provides improved wording and verified that it is correct against N3126. Previous resolution is shown here:
]</i></p>


<blockquote>
<p>
4 [..] A <i>forwarding call wrapper</i> is a call wrapper that can be called
with an <ins>arbitrary</ins> argument list<ins> and uses perfect forwarding to
deliver the arguments to the wrapped callable object</ins>. [<i>Note:</i> in a
typical implementation forwarding call wrappers have an overloaded function call
operator of the form
</p>

<blockquote><pre>
template&lt;class... ArgTypes&gt;
R operator()(ArgTypes&amp;&amp;... args) <i>cv-qual</i>;
</pre></blockquote>

<p>
&mdash; <i>end note</i>]
</p>
</blockquote>

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




<p id="res-1294"><b>Proposed resolution:</b></p>
<p>
Change 22.10.4 <a href="https://wg21.link/func.require">[func.require]</a>/4 as indicated:
</p>

<blockquote><p>
[..] A <em>forwarding call wrapper</em> is a call wrapper that can be called with an <ins>arbitrary</ins> argument 
list <ins>and delivers the arguments as references to the wrapped callable object. This forwarding step shall ensure
that rvalue arguments are delivered as rvalue-references and lvalue arguments are delivered as lvalue-references</ins>. 
[<em>Note</em>: in a typical implementation forwarding call wrappers have an overloaded function call operator of the 
form
</p>

<blockquote><pre>
template&lt;class... UnBoundArgs&gt;
R operator()(UnBoundArgs&amp;&amp;... unbound_args) <em>cv-qual</em>;
</pre></blockquote>
<p>
&mdash; <em>end note</em> ]
</p>
</blockquote>






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