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<title>Issue 1246: vector::resize() missing efficiency guarantee</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#NAD">NAD</a> status.</em></p>
<h3 id="1246"><a href="lwg-closed.html#1246">1246</a>. <code>vector::resize()</code> missing efficiency guarantee</h3>
<p><b>Section:</b> 23.3.13.3 <a href="https://wg21.link/vector.capacity">[vector.capacity]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> David Abrahams <b>Opened:</b> 2009-10-24 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#vector.capacity">active issues</a> in [vector.capacity].</p>
<p><b>View all other</b> <a href="lwg-index.html#vector.capacity">issues</a> in [vector.capacity].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
If <code>v</code> is a <code>vector</code>, I think repeated calls to
<code>v.resize( v.size() + 1 )</code> should be amortized O(1), but it's not
clear that's true from the text of the standard:
</p>

<blockquote><pre>
void resize(size_type sz);
</pre>
<blockquote><p>
<i>Effects:</i> If <code>sz &lt; size()</code>, equivalent to <code>erase(begin() + sz, end());</code>. If
<code>size() &lt; sz</code>, appends <code>sz - size()</code> default constructed elements to the
sequence.
</p></blockquote>
</blockquote>

<p>
Seems to me if we used <code>push_back</code> instead of appends, we might be giving
the guarantee I'd like.  Thoughts?
</p>

<p><i>[
2009-11-10 Howard adds:
]</i></p>


<blockquote><p>
Moved to Tentatively NAD after 5 positive votes on c++std-lib.  Rationale added
below.
</p></blockquote>



<p id="res-1246"><b>Proposed resolution:</b></p>
<p>
In 23.3.13.3 <a href="https://wg21.link/vector.capacity">[vector.capacity]</a>/10, change
</p>

<blockquote><pre>
void resize(size_type sz);
</pre>
<blockquote><p>
<i>Effects:</i> If <code>sz &lt; size()</code>, equivalent to <code>erase(begin() + sz, end());</code>. If
<code>size() &lt; sz</code>, <del>appends <code>sz - size()</code> default constructed elements to the
sequence</del>
<ins>equivalent to <code>sz - size()</code> consecutive evaluations of <code>push_back(T())</code></ins>.
</p></blockquote>
</blockquote>



<p><b>Rationale:</b></p>
<p>
The description in terms of <code>push_back</code> led some to believe that
one could expect the exact same growth pattern from both <code>resize</code> and
<code>push_back</code> (e.g.) which could lead to sub-optimal implementations.
Additionally, 23.3.13 <a href="https://wg21.link/vector">[vector]</a>, p1 includes a statement that this container
"supports (amortized) constant time insert and erase operations at the end;",
therefore addressing the concern of this issue.
</p>





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