Multimap/set with predicates of derived types.

It looks as though in the MS VS2010 implementation the stored function
object for the compare on a multimap is a copy of the supplied object,
rather than the object itself. This means that the supplied object to a
constructor taking a compare can???t be of a class derived from the
template, but must be of the same class. Is this correct behaviour? If
so, why?

Andy

#include <set>
#include <algorithm>
#include <iostream>

int main()
{
   // compare class derives from less<int>,
   // but behaves like greater
   class KC: public std::less<int>
   {
   public:
     bool operator()(const int& _Left, const int& _Right) const
     {
       // inverted from normal
       return (_Left > _Right);
     }
   };

   // This class has a compare _type_ of less<int>
   // but a specific object similar to greater
   auto M = std::set<int>(KC());

   M.insert(1);
   M.insert(3);
   M.insert(2);

   // Output shows sort is by less not greater
   std::for_each(M.begin(), M.end(), [&](int i){std::cout << i;});
}

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