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<title>Issue 806: unique_ptr::reset effects incorrect, too permissive</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="806"><a href="lwg-defects.html#806">806</a>. <code>unique_ptr::reset</code> effects incorrect, too permissive</h3>
<p><b>Section:</b> 20.3.1.3.6 <a href="https://wg21.link/unique.ptr.single.modifiers">[unique.ptr.single.modifiers]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Peter Dimov <b>Opened:</b> 2008-03-13 <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#unique.ptr.single.modifiers">issues</a> in [unique.ptr.single.modifiers].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>void unique_ptr::reset(T* p = 0)</code> is currently specified as:
</p>

<blockquote><p>
<i>Effects:</i> If <code>p == get()</code> there are no effects. Otherwise <code>get_deleter()(get())</code>.
</p></blockquote>

<p>
There are two problems with this. One, if <code>get() == 0</code> and <code>p != 0</code>, the
deleter is called with a <code>NULL</code> pointer, and this is probably not what's
intended (the destructor avoids calling the deleter with 0.)
</p>

<p>
Two, the special check for <code>get() == p</code> is generally not needed and such a
situation usually indicates an error in the client code, which is being
masked. As a data point, <code>boost::shared_ptr</code> was changed to assert on such
self-resets in 2001 and there were no complaints.
</p>

<p>
One might think that self-resets are necessary for <code>operator=</code> to work; it's specified to perform
</p>

<blockquote><pre>
reset( u.release() );
</pre></blockquote>

<p>
and the self-assignment
</p>

<blockquote><pre>
p = move(p);
</pre></blockquote>

<p>
might appear to result in a self-reset. But it doesn't; the <code>release()</code> is
performed first, zeroing the stored pointer. In other words, <code>p.reset(
q.release() )</code> works even when <code>p</code> and <code>q</code> are the same <code>unique_ptr</code>, and there
is no need to special-case <code>p.reset( q.get() )</code> to work in a similar
scenario, as it definitely doesn't when <code>p</code> and <code>q</code> are separate.
</p>



<p id="res-806"><b>Proposed resolution:</b></p>

<p>
Change 20.3.1.3.6 <a href="https://wg21.link/unique.ptr.single.modifiers">[unique.ptr.single.modifiers]</a>:
</p>

<blockquote>
<pre>
void reset(T* p = 0);
</pre>
<blockquote><p>
-4- <i>Effects:</i> If <code><del>p ==</del> get()<ins> == 0</ins></code> there are no effects. Otherwise <code>get_deleter()(get())</code>.
</p></blockquote>
</blockquote>

<p>
Change 20.3.1.4.5 <a href="https://wg21.link/unique.ptr.runtime.modifiers">[unique.ptr.runtime.modifiers]</a>:
</p>

<blockquote>
<pre>
void reset(T* p = 0);
</pre>
<blockquote>
<p>...</p>
<p>
-2- <i>Effects:</i> If <code><del>p ==</del> get()<ins> == 0</ins></code> there are no effects. Otherwise <code>get_deleter()(get())</code>.
</p>
</blockquote>
</blockquote>






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