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<title>Issue 144: Deque constructor complexity wrong </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#TC1">TC1</a> status.</em></p>
<h3 id="144"><a href="lwg-defects.html#144">144</a>. Deque constructor complexity wrong </h3>
<p><b>Section:</b> 23.3.5.2 <a href="https://wg21.link/deque.cons">[deque.cons]</a> <b>Status:</b> <a href="lwg-active.html#TC1">TC1</a>
 <b>Submitter:</b> Herb Sutter <b>Opened:</b> 1999-05-09 <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#deque.cons">issues</a> in [deque.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TC1">TC1</a> status.</p>
<p><b>Discussion:</b></p>
<p>In 23.3.5.2 <a href="https://wg21.link/deque.cons">[deque.cons]</a> paragraph 6, the deque ctor that takes an iterator range appears
to have complexity requirements which are incorrect, and which contradict the
complexity requirements for insert(). I suspect that the text in question,
below, was taken from vector:</p>
<blockquote>
  <p>Complexity: If the iterators first and last are forward iterators,
  bidirectional iterators, or random access iterators the constructor makes only
  N calls to the copy constructor, and performs no reallocations, where N is
  last - first.</p>
</blockquote>
<p>The word &quot;reallocations&quot; does not really apply to deque. Further,
all of the following appears to be spurious:</p>
<blockquote>
  <p>It makes at most 2N calls to the copy constructor of T and log N
  reallocations if they are input iterators.1)</p>
  <p>1) The complexity is greater in the case of input iterators because each
  element must be added individually: it is impossible to determine the distance
  between first abd last before doing the copying.</p>
</blockquote>
<p>This makes perfect sense for vector, but not for deque. Why should deque gain
an efficiency advantage from knowing in advance the number of elements to
insert?</p>


<p id="res-144"><b>Proposed resolution:</b></p>
<p>In 23.3.5.2 <a href="https://wg21.link/deque.cons">[deque.cons]</a> paragraph 6, replace the Complexity description, including the
footnote, with the following text (which also corrects the &quot;abd&quot;
typo):</p>
<blockquote>
  <p>Complexity: Makes last - first calls to the copy constructor of T.</p>
</blockquote>




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