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<title>Issue 1193: default_delete cannot be instantiated with incomplete types</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="1193"><a href="lwg-defects.html#1193">1193</a>. <code>default_delete</code> cannot be instantiated with incomplete types</h3>
<p><b>Section:</b> 20.3.1.2 <a href="https://wg21.link/unique.ptr.dltr">[unique.ptr.dltr]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2009-08-18 <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#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p>
According to the general rules of 16.4.5.8 <a href="https://wg21.link/res.on.functions">[res.on.functions]</a> p 2 b 5 the effects
are undefined, if an incomplete type is used to instantiate a library template. But neither in
20.3.1.2 <a href="https://wg21.link/unique.ptr.dltr">[unique.ptr.dltr]</a> nor
in any other place of the standard such explicit allowance is given.
Since this template is intended to be instantiated with incomplete
types, this must be fixed.
</p>

<p><i>[
2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]</i></p>


<p><i>[
2009-11-17 Alisdair Opens:
]</i></p>


<blockquote>
<p>
LWG 1193 tries to support <code>unique_ptr</code> for incomplete types.  I believe the
proposed wording goes too far:
</p>

<blockquote><p>
The template parameter <code>T</code> of <code>default_delete</code> may be an
incomplete type.
</p></blockquote>

<p>
Do we really want to support <code>cv-void</code>?  Suggested ammendment:
</p>

<blockquote><p>
The template parameter <code>T</code> of <code>default_delete</code> may be an
incomplete type <ins>other than <code>cv-void</code></ins>.
</p></blockquote>

<p>
We might also consider saying something about arrays of incomplete types.
</p>

<p>
Did we lose support for <code>unique_ptr&lt;function-type&gt;</code> when the
concept-enabled work was shelved?  If so, we might want a
<code>default_delete</code> partial specialization for function types that does
nothing.  Alternatively, function types should <em>not</em> be supported by
default, but there is no reason a user cannot support them via their own
deletion policy.
</p>

<p>
Function-type support might also lead to conditionally supporting a
function-call operator in the general case, and that seems way too inventive at
this stage to me, even if we could largely steal wording directly from
<code>reference_wrapper</code>.  <code>shared_ptr</code> would have similar problems
too.
</p>

</blockquote>

<p><i>[
2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]</i></p>




<p id="res-1193"><b>Proposed resolution:</b></p>
<p>
Add two new paragraphs directly to 20.3.1.2 <a href="https://wg21.link/unique.ptr.dltr">[unique.ptr.dltr]</a> (before
20.3.1.2.2 <a href="https://wg21.link/unique.ptr.dltr.dflt">[unique.ptr.dltr.dflt]</a>) with the following
content:
</p>

<blockquote>
<p><ins>
The class template <code>default_delete</code> serves as the default deleter (destruction policy) for
the class template <code>unique_ptr</code>.
</ins></p>

<p><ins>
The template parameter <code>T</code> of <code>default_delete</code> may be an incomplete type.
</ins></p>
</blockquote>





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