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<title>Issue 881: shared_ptr conversion issue</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="881"><a href="lwg-defects.html#881">881</a>. <code>shared_ptr</code> conversion issue</h3>
<p><b>Section:</b> 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Peter Dimov <b>Opened:</b> 2008-08-30 <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#util.smartptr.shared.const">active issues</a> in [util.smartptr.shared.const].</p>
<p><b>View all other</b> <a href="lwg-index.html#util.smartptr.shared.const">issues</a> in [util.smartptr.shared.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>
We've changed <code>shared_ptr&lt;Y&gt;</code> to not convert to <code>shared_ptr&lt;T&gt;</code> when <code>Y*</code>
doesn't convert to <code>T*</code> by resolving issue <a href="lwg-defects.html#687" title="shared_ptr conversion constructor not constrained (Status: CD1)">687</a><sup><a href="https://cplusplus.github.io/LWG/issue687" title="Latest snapshot">(i)</a></sup>. This only fixed the
converting copy constructor though.
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2351.htm">N2351</a>
later added move support, and
the converting move constructor is not constrained.
</p>

<p><i>[
San Francisco:
]</i></p>


<blockquote><p>
We might be able to move this to NAD, Editorial once shared_ptr is
conceptualized, but we want to revisit this issue to make sure.
</p></blockquote>

<p><i>[
2009-07 Frankfurt
]</i></p>


<blockquote>
<p>
Moved to Ready.
</p>
<p>
This issue now represents the favored format for specifying constrained templates.
</p>
</blockquote>



<p id="res-881"><b>Proposed resolution:</b></p>
<p>
We need to change the Requires clause of the move constructor:
</p>

<blockquote><pre>
shared_ptr(shared_ptr&amp;&amp; r); 
template&lt;class Y&gt; shared_ptr(shared_ptr&lt;Y&gt;&amp;&amp; r); 
</pre>
<blockquote><p>
<i><del>Requires</del> <ins>Remarks</ins>:</i> <del>For the second constructor <code>Y*</code> shall be
convertible to <code>T*</code>.</del>
<ins>
The second constructor shall not participate in overload resolution
unless <code>Y*</code> is convertible to <code>T*</code>.
</ins>
</p></blockquote>
</blockquote>

<p>
in order to actually make the example in <a href="lwg-defects.html#687" title="shared_ptr conversion constructor not constrained (Status: CD1)">687</a><sup><a href="https://cplusplus.github.io/LWG/issue687" title="Latest snapshot">(i)</a></sup> compile
(it now resolves to the move constructor).
</p>






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