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<title>Issue 884: shared_ptr swap</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#Resolved">Resolved</a> status.</em></p>
<h3 id="884"><a href="lwg-defects.html#884">884</a>. <code>shared_ptr</code> swap</h3>
<p><b>Section:</b> 20.3.2.2.5 <a href="https://wg21.link/util.smartptr.shared.mod">[util.smartptr.shared.mod]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2008-09-15 <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#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<blockquote><pre>
#include &lt;memory&gt;
#include &lt;cassert&gt;

struct A { };
struct B : A { };

int main()
{
    std::shared_ptr&lt;A&gt; pa(new A);
    std::shared_ptr&lt;B&gt; pb(new B);
    std::swap&lt;A&gt;(pa, pb);  // N.B. no argument deduction
    assert( pa.get() == pb.get() );
    return 0;
}
</pre></blockquote>

<p>
Is this behaviour correct (I believe it is) and if so, is it
unavoidable, or not worth worrying about?
</p>

<p>
This calls the lvalue/rvalue swap overload for <code>shared_ptr</code>:
</p>

<blockquote><pre>
template&lt;class T&gt; void swap( shared_ptr&lt;T&gt; &amp; a, shared_ptr&lt;T&gt; &amp;&amp; b );
</pre></blockquote>

<p>
silently converting the second argument from <code>shared_ptr&lt;B&gt;</code> to
<code>shared_ptr&lt;A&gt;</code> and binding the rvalue ref to the produced temporary.
</p>

<p>
This is not, in my opinion, a <code>shared_ptr</code> problem; it is a general issue
with the rvalue swap overloads. Do we want to prevent this code from
compiling? If so, how?
</p>

<p>
Perhaps we should limit rvalue args to swap to those types that would
benefit from the "swap trick".  Or, since we now have <code>shrink_to_fit()</code>, just
eliminate the rvalue swap overloads altogether.  The original motivation
was:
</p>

<blockquote><pre>
vector&lt;A&gt; v = ...;
...
swap(v, vector&lt;A&gt;(v));
</pre></blockquote>
<p>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1690.html#Improved%20swap%20Interface">N1690</a>.
</p>
<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote><p>
We agree with the proposed resolution.
Move to <del>NAD Editorial</del><ins>Resolved</ins>.
</p></blockquote>


<p id="res-884"><b>Proposed resolution:</b></p>
<p>
Recommend <del>NAD Editorial</del><ins>Resolved</ins>, fixed by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2844.html">N2844</a>.
</p>





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