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<title>Issue 1296: map and multimap value_compare overspecified</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="1296"><a href="lwg-closed.html#1296">1296</a>. <code>map</code> and <code>multimap value_compare</code> overspecified</h3>
<p><b>Section:</b> 23.4.3 <a href="https://wg21.link/map">[map]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-12-22 <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#map">issues</a> in [map].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The container class templates <code>map</code> and <code>multimap</code> both contain a
nested type called <code>value_compare</code>, that is used to compare the
<code>value_type pair</code> elements, an adaptor of the user-supplied comparison
function-like object.
</p>

<p>
I believe these types are over-specified, as we require a distinct type for each
template, even though the allocator plays no part in the comparator, and
<code>map</code> and <code>multimap value_compare</code> classes could easily be shared.
 The benefits are similar to the SCARY iterator proposal (although on a much
smaller scale!) but unlike SCARY, this is not a QoI issue today but actively
prohibited.
</p>

<p>
If the <code>value_compare</code> classes were marked 'exposition only', a vendor
would be free to experiment with implementations that do not produce so many
template instantiations with negligible impact on conforming programs.  (There
is a minor risk that programs could no longer portably overload functions taking
<code>value_compare</code> types.  This scenario is extremely unlikely outside
conformance suites.)
</p>

<p>
(Note that there are no similar problems for unordered maps, nor any of the set
variants)
</p>

<p><i>[
2010-01-31 Moved to Tentatively NAD after 5 positive votes on c++std-lib.
Rationale added below.
]</i></p>


<p><b>Rationale:</b></p>
<p>
The <code>value_compare</code> specification is an unfortunate bit from the past
that we have to live with.  Fortunately vendors can work around the problems
mentioned in this issue.
</p>




<p id="res-1296"><b>Proposed resolution:</b></p>
<p>
p2 23.4.3 <a href="https://wg21.link/map">[map]</a>:
Above the declaration of class <code>value_compare</code> in the map synopsis, add:
</p>

<blockquote><pre>
template &lt;class Key, class T, class Compare = less&lt;Key&gt;,
          class Allocator = allocator&lt;pair&lt;const Key, T&gt; &gt; &gt;
class map {
public:
  // types:
  ...
  <ins>// exposition only.</ins>
  class value_compare
    : public binary_function&lt;value_type,value_type,bool&gt; {
    ...
</pre></blockquote>



<p>
p2 23.4.4 <a href="https://wg21.link/multimap">[multimap]</a>:
Above the declaration of class <code>value_compare</code> in the map synopsis, add:
</p>

<blockquote><pre>
template &lt;class Key, class T, class Compare = less&lt;Key&gt;,
          class Allocator = allocator&lt;pair&lt;const Key, T&gt; &gt; &gt;
class multimap {
public:
  // types:
  ...
  <ins>// exposition only.</ins>
  class value_compare
    : public binary_function&lt;value_type,value_type,bool&gt; {
    ...
</pre></blockquote>





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