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<title>Issue 541: shared_ptr template assignment and void</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#CD1">CD1</a> status.</em></p>
<h3 id="541"><a href="lwg-defects.html#541">541</a>. shared_ptr template assignment and void</h3>
<p><b>Section:</b> 20.3.2.2 <a href="https://wg21.link/util.smartptr.shared">[util.smartptr.shared]</a>, 99 [tr.util.smartptr.shared] <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2005-10-16 <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#util.smartptr.shared">issues</a> in [util.smartptr.shared].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Is the void specialization of the template assignment operator taking
a shared_ptr&lt;void&gt; as an argument supposed be well-formed?
</p>
<p>
I.e., is this snippet well-formed:
</p>
<blockquote><pre>
shared_ptr&lt;void&gt; p;
p.operator=&lt;void&gt;(p);
</pre></blockquote>

<p>
Gcc complains about auto_ptr&lt;void&gt;::operator*() returning a reference
to void. I suspect it's because shared_ptr has two template assignment
operators, one of which takes auto_ptr, and the auto_ptr template gets
implicitly instantiated in the process of overload resolution.
</p>

<p>
The only way I see around it is to do the same trick with auto_ptr&lt;void&gt;
operator*() as with the same operator in shared_ptr&lt;void&gt;.
</p>

<p>
PS Strangely enough, the EDG front end doesn't mind the code, even
though in a small test case (below) I can reproduce the error with
it as well.
</p>

<blockquote><pre>
template &lt;class T&gt;
struct A { T&amp; operator*() { return *(T*)0; } };

template &lt;class T&gt;
struct B {
    void operator= (const B&amp;) { }
    template &lt;class U&gt;
    void operator= (const B&lt;U&gt;&amp;) { }
    template &lt;class U&gt;
    void operator= (const A&lt;U&gt;&amp;) { }
};

int main ()
{
    B&lt;void&gt; b;
    b.operator=&lt;void&gt;(b);
}
</pre></blockquote>


<p id="res-541"><b>Proposed resolution:</b></p>
<p>
In [lib.memory] change:
</p>
<blockquote><pre>
template&lt;class X&gt; class auto_ptr;
<ins>template&lt;&gt; class auto_ptr&lt;void&gt;;</ins>
</pre></blockquote>

<p>
In [lib.auto.ptr]/2 add the following before the last closing brace:
</p>

<blockquote><pre>
template&lt;&gt; class auto_ptr&lt;void&gt;
{
public:
    typedef void element_type;
};
</pre></blockquote>






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