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<title>Issue 1215: list::merge with unequal allocators</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="1215"><a href="lwg-defects.html#1215">1215</a>. <code>list::merge</code> with unequal allocators</h3>
<p><b>Section:</b> 23.3.11.5 <a href="https://wg21.link/list.ops">[list.ops]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Pablo Halpern <b>Opened:</b> 2009-09-24 <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#list.ops">issues</a> in [list.ops].</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>
In Bellevue (I think), we passed
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2525.pdf">N2525</a>,
which, among other things, specifies that the behavior of
<code>list::splice</code> is undefined if the allocators of the two lists
being spliced do not compare equal. The same rationale should apply to
<code>list::merge</code>. The intent of <code>list::merge</code> (AFAIK) is to
move nodes from one sorted <code>list</code> into another sorted
<code>list</code> without copying the elements. This is possible only if the
allocators compare equal.
</p>


<p id="res-1215"><b>Proposed resolution:</b></p>
<p>
Relative to the August 2009 WP,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2857.pdf">N2857</a>,
change 23.3.11.5 <a href="https://wg21.link/list.ops">[list.ops]</a>,
paragraphs 22-25 as follows:
</p>

<blockquote>
<pre>
void merge(list&amp;&amp; x);
template &lt;class Compare&gt; void merge(list&amp;&amp; x, Compare comp);
</pre>
<blockquote>
<p>
<i>Requires</i>: both the list and the argument list shall be sorted
according to operator&lt; or comp.
</p>
<p>
<i>Effects</i>: If <code>(&amp;x == this)</code> does nothing; otherwise, merges the
two sorted ranges <code>[begin(), end())</code> and <code>[x.begin(),
x.end())</code>. The result is a range in which the elements will be
sorted in non-decreasing order according to the ordering defined by
<code>comp</code>; that is, for every iterator <code>i</code>, in the range other than the
<code>first</code>, the condition <code>comp(*i, *(i - 1)<ins>)</ins></code> will be
<code>false</code>.
</p>
<p>
<i>Remarks</i>: Stable. If <code>(&amp;x != this)</code> the range <code>[x.begin(), x.end())</code> is
empty after the merge. <ins>No elements are copied by this operation.
The behavior is undefined if <code>this-&gt;get_allocator() !=
x.get_allocator()</code>.</ins>
</p>
<p>
<i>Complexity</i>: At most <code>size() + x.size() - 1</code> applications of <code>comp</code>
if <code>(&amp;x != this)</code>; otherwise, no applications of <code>comp</code> are performed. If an
exception is thrown other than by a comparison there are no effects.
</p>
</blockquote>
</blockquote>





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