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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="1189"><a href="lwg-defects.html#1189">1189</a>. Awkward interface for changing the number of buckets in an unordered associative container</h3>
<p><b>Section:</b> 23.2.8 <a href="https://wg21.link/unord.req">[unord.req]</a>, 23.5 <a href="https://wg21.link/unord">[unord]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Matt Austern <b>Opened:</b> 2009-08-10 <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#unord.req">active issues</a> in [unord.req].</p>
<p><b>View all other</b> <a href="lwg-index.html#unord.req">issues</a> in [unord.req].</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>
Consider a typical use case: I create an <code>unordered_map</code> and then start
adding elements to it one at a time. I know that it will eventually need
to store a few million elements, so, for performance reasons, I would
like to reserve enough capacity that none of the calls to <code>insert</code> will
trigger a rehash.
</p>

<p>
Unfortunately, the existing interface makes this awkward. The user
naturally sees the problem in terms of the number of elements, but the
interface presents it as buckets. If <code>m</code> is the map and <code>n</code> is the expected
number of elements, this operation is written <code>m.rehash(n /
m.max_load_factor())</code> &mdash; not very novice friendly.
</p>

<p><i>[
2009-09-30 Daniel adds:
]</i></p>


<blockquote><p>
I recommend to replace "<code>resize</code>" by a different name like
"<code>reserve</code>", because that would better match the intended
use-case. Rational: Any existing resize function has the on-success
post-condition that the provided size is equal to <code>size()</code>, which
is not satisfied for the proposal. Reserve seems to fit the purpose of
the actual renaming suggestion.
</p></blockquote>

<p><i>[
2009-10-28 Ganesh summarizes alternative resolutions and expresses a
strong preference for the second (and opposition to the first):
]</i></p>


<blockquote>
<ol>
<li>
<p>
In the unordered associative container requirements (23.2.8 <a href="https://wg21.link/unord.req">[unord.req]</a>),
remove the row for
rehash and replace it with:
</p>

<blockquote>
<table border="1">
<caption>Table 87 &mdash; Unordered associative container requirements
(in addition to container)</caption>

<tr>
<th>Expression</th><th>Return type</th><th>Assertion/note pre-/post-condition</th>
<th>Complexity</th>
</tr>
<tr>
<td><code>a.<del>rehash</del><ins>reserve</ins>(n)</code></td>
<td><code>void</code></td>
<td>
Post: <code>a.bucket_count &gt; <ins>max(</ins>a.size()<ins>, n)</ins>
/ a.max_load_factor()</code><del> and <code>a.bucket_count()
&gt;= n</code></del>.
</td>
<td>
Average case linear in <code>a.size()</code>, worst case quadratic.
</td>
</tr>
</table>
</blockquote>

<p>
Make the corresponding change in the class synopses in 23.5.3 <a href="https://wg21.link/unord.map">[unord.map]</a>, 23.5.4 <a href="https://wg21.link/unord.multimap">[unord.multimap]</a>,  23.5.6 <a href="https://wg21.link/unord.set">[unord.set]</a>, and 23.5.7 <a href="https://wg21.link/unord.multiset">[unord.multiset]</a>.
</p>
</li>
<li>

<p>
In 23.2.8 <a href="https://wg21.link/unord.req">[unord.req]</a>/9, table 98, append a new row after the last one:
</p>

<blockquote>
<table border="1">
<caption>Table 87 &mdash; Unordered associative container requirements
(in addition to container)</caption>

<tr>
<th>Expression</th><th>Return type</th><th>Assertion/note pre-/post-condition</th>
<th>Complexity</th>
</tr>
<tr>
<td><code>a.rehash(n)</code></td>
<td><code>void</code></td>
<td>
Post: <code>a.bucket_count &gt; a.size()
/ a.max_load_factor()</code> and <code>a.bucket_count()
&gt;= n</code>.
</td>
<td>
Average case linear in <code>a.size()</code>, worst case quadratic.
</td>
</tr>
<tr>
<td><ins>
<code>a.reserve(n)</code>
</ins></td>
<td><ins>
<code>void</code>
</ins></td>
<td><ins>
Same as <code>a.rehash(ceil(n / a.max_load_factor()))</code>
</ins></td>
<td><ins>
Average case linear in <code>a.size()</code>, worst case quadratic.
</ins></td>
</tr>
</table>
</blockquote>

<p>
In 23.5.3 <a href="https://wg21.link/unord.map">[unord.map]</a>/3 in the definition of class template <code>unordered_map</code>, in
23.5.4 <a href="https://wg21.link/unord.multimap">[unord.multimap]</a>/3 in the definition of class template <code>unordered_multimap</code>, in
23.5.6 <a href="https://wg21.link/unord.set">[unord.set]</a>/3 in the definition of class template <code>unordered_set</code> and in
23.5.7 <a href="https://wg21.link/unord.multiset">[unord.multiset]</a>/3 in the definition of class template <code>unordered_multiset</code>, add the
following line after member function <code>rehash()</code>:
</p>

<blockquote><pre>
void reserve(size_type n);
</pre></blockquote>

</li>
</ol>
</blockquote>

<p><i>[
2009-10-28 Howard:
]</i></p>


<blockquote>
<p>
Moved to Tentatively Ready after 5 votes in favor of Ganesh's option 2 above.
The original proposed wording now appears here:
</p>

<blockquote>
<p>
Informally: instead of providing <code>rehash(n)</code> provide <code>resize(n)</code>, with the
semantics "make the container a good size for <code>n</code> elements".
</p>

<p>
In the unordered associative container requirements (23.2.8 <a href="https://wg21.link/unord.req">[unord.req]</a>),
remove the row for
rehash and replace it with:
</p>

<blockquote>
<table border="1">
<caption>Table 87 &mdash; Unordered associative container requirements
(in addition to container)</caption>

<tr>
<th>Expression</th><th>Return type</th><th>Assertion/note pre-/post-condition</th>
<th>Complexity</th>
</tr>
<tr>
<td><code>a.<del>rehash</del><ins>resize</ins>(n)</code></td>
<td><code>void</code></td>
<td>
Post: <code>a.bucket_count &gt; <ins>max(</ins>a.size()<ins>, n)</ins>
/ a.max_load_factor()</code><del> and <code>a.bucket_count()
&gt;= n</code></del>.
</td>
<td>
Average case linear in <code>a.size()</code>, worst case quadratic.
</td>
</tr>
</table>
</blockquote>

<p>
Make the corresponding change in the class synopses in 23.5.3 <a href="https://wg21.link/unord.map">[unord.map]</a>, 23.5.4 <a href="https://wg21.link/unord.multimap">[unord.multimap]</a>,  23.5.6 <a href="https://wg21.link/unord.set">[unord.set]</a>, and 23.5.7 <a href="https://wg21.link/unord.multiset">[unord.multiset]</a>.
</p>

</blockquote>
</blockquote>


<p id="res-1189"><b>Proposed resolution:</b></p>
<p>
In 23.2.8 <a href="https://wg21.link/unord.req">[unord.req]</a>/9, table 98, append a new row after the last one:
</p>

<blockquote>
<table border="1">
<caption>Table 87 &mdash; Unordered associative container requirements
(in addition to container)</caption>

<tr>
<th>Expression</th><th>Return type</th><th>Assertion/note pre-/post-condition</th>
<th>Complexity</th>
</tr>
<tr>
<td><code>a.rehash(n)</code></td>
<td><code>void</code></td>
<td>
Post: <code>a.bucket_count &gt; a.size()
/ a.max_load_factor()</code> and <code>a.bucket_count()
&gt;= n</code>.
</td>
<td>
Average case linear in <code>a.size()</code>, worst case quadratic.
</td>
</tr>
<tr>
<td><ins>
<code>a.reserve(n)</code>
</ins></td>
<td><ins>
<code>void</code>
</ins></td>
<td><ins>
Same as <code>a.rehash(ceil(n / a.max_load_factor()))</code>
</ins></td>
<td><ins>
Average case linear in <code>a.size()</code>, worst case quadratic.
</ins></td>
</tr>
</table>
</blockquote>

<p>
In 23.5.3 <a href="https://wg21.link/unord.map">[unord.map]</a>/3 in the definition of class template <code>unordered_map</code>, in
23.5.4 <a href="https://wg21.link/unord.multimap">[unord.multimap]</a>/3 in the definition of class template <code>unordered_multimap</code>, in
23.5.6 <a href="https://wg21.link/unord.set">[unord.set]</a>/3 in the definition of class template <code>unordered_set</code> and in
23.5.7 <a href="https://wg21.link/unord.multiset">[unord.multiset]</a>/3 in the definition of class template <code>unordered_multiset</code>, add the
following line after member function <code>rehash()</code>:
</p>

<blockquote><pre>
void reserve(size_type n);
</pre></blockquote>





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