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map
and multimap value_compare
overspecifiedSection: 24.4.4 [map] Status: NAD Submitter: Alisdair Meredith Opened: 2009-12-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The container class templates map
and multimap
both contain a
nested type called value_compare
, that is used to compare the
value_type pair
elements, an adaptor of the user-supplied comparison
function-like object.
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
map
and multimap value_compare
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.
If the value_compare
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
value_compare
types. This scenario is extremely unlikely outside
conformance suites.)
(Note that there are no similar problems for unordered maps, nor any of the set variants)
[ 2010-01-31 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
The value_compare
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.
Proposed resolution:
p2 24.4.4 [map]:
Above the declaration of class value_compare
in the map synopsis, add:
template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > class map { public: // types: ... // exposition only. class value_compare : public binary_function<value_type,value_type,bool> { ...
p2 24.4.5 [multimap]:
Above the declaration of class value_compare
in the map synopsis, add:
template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > class multimap { public: // types: ... // exposition only. class value_compare : public binary_function<value_type,value_type,bool> { ...