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<title>Issue 925: shared_ptr's explicit conversion from unique_ptr</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="925"><a href="lwg-defects.html#925">925</a>. <code>shared_ptr</code>'s explicit conversion from <code>unique_ptr</code></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> Rodolfo Lima <b>Opened:</b> 2008-10-12 <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>
The current working draft
(<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2798.pdf">N2798</a>),
section 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> declares
<code>shared_ptr</code>'s constructor that takes a rvalue reference to <code>unique_ptr</code> and
<code>auto_ptr</code> as being explicit, affecting several valid smart pointer use
cases that would take advantage of this conversion being implicit, for
example:
</p>

<blockquote><pre>
class A;
std::unique_ptr&lt;A&gt; create();
void process(std::shared_ptr&lt;A&gt; obj);

int main()
{
   process(create());                  // use case #1
   std::unique_ptr&lt;A&gt; uobj = create();
   process(std::move(uobj));           // use case #2
   return 0;
}
</pre></blockquote>

<p>
If <code>unique_ptr</code> to <code>shared_ptr</code> conversions are explicit, the above lines
should be written:
</p>

<blockquote><pre>
process(std::shared_ptr&lt;A&gt;(create()));        // use case #1
process(std::shared_ptr&lt;A&gt;(std::move(uobj))); // use case #2
</pre></blockquote>

<p>
The extra cast required doesn't seems to give any benefits to the user,
nor protects him of any unintended conversions, this being the raison
d'etre of explicit constructors.
</p>

<p>
It seems that this constructor was made explicit to mimic the conversion
from <code>auto_ptr</code> in pre-rvalue reference days, which accepts both lvalue and
rvalue references. Although this decision was valid back then, C++0x
allows the user to express in a clear and non verbose manner when he wants
move semantics to be employed, be it implicitly (use case 1) or explicitly
(use case 2).
</p>

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

<blockquote>
<p>
Howard and Alisdair like the motivating use cases
and the proposed resolution.
</p>
<p>
Move to Tentatively Ready.
</p>
</blockquote>


<p id="res-925"><b>Proposed resolution:</b></p>
<p>
In both 20.3.2.2 <a href="https://wg21.link/util.smartptr.shared">[util.smartptr.shared]</a> paragraph 1 and 
20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> change:
</p>

<blockquote><pre>
template &lt;class Y&gt; <del>explicit</del> shared_ptr(auto_ptr&lt;Y&gt; &amp;&amp;r);
template &lt;class Y, class D&gt; <del>explicit</del> shared_ptr(unique_ptr&lt;Y, D&gt; &amp;&amp;r);
</pre></blockquote>






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