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<title>Issue 688: reference_wrapper, cref unsafe, allow binding to rvalues</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="688"><a href="lwg-defects.html#688">688</a>. reference_wrapper, cref unsafe, allow binding to rvalues</h3>
<p><b>Section:</b> 22.10.6.2 <a href="https://wg21.link/refwrap.const">[refwrap.const]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Peter Dimov <b>Opened:</b> 2007-05-10 <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#refwrap.const">issues</a> in [refwrap.const].</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>
A <code>reference_wrapper</code> can be constructed from an rvalue, either by using
the constructor, or via <code>cref</code> (and <code>ref</code> in some corner cases). This leads
to a dangling reference being stored into the <code>reference_wrapper</code> object.
Now that we have a mechanism to detect an rvalue, we can fix them to
disallow this source of undefined behavior.
</p>

<p>
Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.
</p>

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


<blockquote>
<p>
Now that <code>ref/cref</code> are constained that <code>T</code> must be an <code>ObjectType</code>, I do not
believe there is any risk of binding <code>ref</code> to a temporary (which would rely on
deducing <code>T</code> to be an rvalue reference type)
</p>
<p>
However, the problem for <code>cref</code> remains, so I recommend retaining that deleted
overload.
</p>
</blockquote>

<p><i>[
2009-05-10 Howard adds:
]</i></p>


<blockquote>
<p>
Without:
</p>

<blockquote><pre>
template &lt;class T&gt; void ref(const T&amp;&amp; t) = delete;
</pre></blockquote>
<p>
I believe this program will compile:
</p>

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

struct A {};

const A source() {return A();}

int main()
{
   std::reference_wrapper&lt;const A&gt; r = std::ref(source());
}
</pre></blockquote>
<p>
I.e. in:
</p>
<blockquote><pre>
template &lt;ObjectType T&gt; reference_wrapper&lt;T&gt; ref(T&amp; t);
</pre></blockquote>

<p>
this:
</p>

<blockquote><pre>
ref(source())
</pre></blockquote>
<p>
deduces <code>T</code> as <code>const A</code>, and so:
</p>

<blockquote><pre>
ref(const A&amp; t)
</pre></blockquote>

<p>
will bind to a temporary (tested with a pre-concepts rvalue-ref enabled compiler).
</p>
<p>
Therefore I think we still need the ref-protection.  I respectfully disagree with Alisdair's
comment and am in favor of the proposed wording as it stands.  Also, CWG 606
(noted below) has now been "favorably" resolved.
</p>

</blockquote>

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

<blockquote><p>
We agree with the proposed resolution.
Move to Tentatively Ready.
</p></blockquote>


<p id="res-688"><b>Proposed resolution:</b></p>
<p>
In 22.10 <a href="https://wg21.link/function.objects">[function.objects]</a>, add the following two signatures to the synopsis:
</p>

<blockquote><pre>
template &lt;class T&gt; void ref(const T&amp;&amp; t) = delete;
template &lt;class T&gt; void cref(const T&amp;&amp; t) = delete;
</pre></blockquote>



<p><i>[
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2292.html">N2292</a>
addresses the first part of the resolution but not the second.
]</i></p>


<p><i>[
Bellevue:  Doug noticed problems with the current wording.
]</i></p>


<p><i>[
post Bellevue:  Howard and Peter provided revised wording.
]</i></p>


<p><i>[
This resolution depends on a "favorable" resolution of CWG 606:  that is,
the "special deduction rule" is disabled with the const T&amp;&amp; pattern.
]</i></p>





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