[boost] [container] [flat_map] Sub-optimal behavior when utilized with std::less<>

Subject: [boost] [container] [flat_map] Sub-optimal behavior when utilized with std::less<>
From: Viktor Sehr (viktor.sehr_at_[hidden])
Date: 2018-06-15 10:22:33

It seems to me that neither boost::flat_map nor boost:flat_set with a
custom comparator such as std::less<> behaves like std::set/std::map.
This results in unnecessary allocation, in difference from std::map/set
when used with a std::string as key for example

Example:
// Please note that I pretend that "dummy" is large enough to make the
std::string utilize the heap.
auto set = std::set<std::string, std::less<> >{};
auto n = set.count("dummy"); // No conversion from const char* to
std::string

auto flatset = boost::container::flat_set<std::string, std::less<> >{};
auto n = flatset.count("dummy"); // const char* is converted to a
std::string before comparison, thus resulting in an unnecessary allocation.

Is this a known issue intended to be resolved?

Looking at the code of STL (which ever version ships with Visual Studio),
find, count etc has been extended with template key types in addition to
the regular key_type using an "is_transparent" trait:
template<class _Other, class _Mycomp = key_compare, class = typename
_Mycomp::is_transparent>
size_type count(const _Other& _Keyval) const {
  _Keyval _Paircc _Ans = equal_range(_Keyval);
   return (_STD distance(_Ans.first, _Ans.second));
 }

Best regards
/Viktor Sehr