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<title>Issue 300: list::merge() specification incomplete</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="300"><a href="lwg-defects.html#300">300</a>. list::merge() specification incomplete</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#CD1">CD1</a>
 <b>Submitter:</b> John Pedretti <b>Opened:</b> 2001-01-23 <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#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The &quot;Effects&quot; clause for list::merge() (23.3.11.5 <a href="https://wg21.link/list.ops">[list.ops]</a>, p23)
appears to be incomplete: it doesn't cover the case where the argument
list is identical to *this (i.e., this == &amp;x). The requirement in the
note in p24 (below) is that x be empty after the merge which is surely
unintended in this case.
</p>


<p id="res-300"><b>Proposed resolution:</b></p>
<p>In 23.3.11.5 <a href="https://wg21.link/list.ops">[list.ops]</a>, replace paragraps 23-25 with:</p>
<blockquote>
<p>
23 Effects: if (&amp;x == this) does nothing; otherwise, merges the two
sorted ranges [begin(), end()) and [x.begin(), x.end()).  The result
is a range in which the elements will be sorted in non-decreasing
order according to the ordering defined by comp; that is, for every
iterator i in the range other than the first, the condition comp(*i,
*(i - 1)) will be false.
</p>

<p>
24 Notes: Stable: if (&amp;x != this), then for equivalent elements in the
two original ranges, the elements from the original range [begin(),
end()) always precede the elements from the original range [x.begin(),
x.end()).  If (&amp;x != this) the range [x.begin(), x.end()) is empty
after the merge.
</p>

<p>
25 Complexity: At most size() + x.size() - 1 applications of comp if
(&amp;x !  = this); otherwise, no applications of comp are performed.  If
an exception is thrown other than by a comparison there are no
effects.
</p>

</blockquote>

<p><i>[Copenhagen: The original proposed resolution did not fix all of
the problems in 23.3.11.5 <a href="https://wg21.link/list.ops">[list.ops]</a>, p22-25.  Three different
paragraphs (23, 24, 25) describe the effects of <code>merge</code>.
Changing p23, without changing the other two, appears to introduce
contradictions.  Additionally, "merges the argument list into the
list" is excessively vague.]</i></p>


<p><i>[Post-Cura&ccedil;ao: Robert Klarer provided new wording.]</i></p>







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