std::multimap<Key,T,Compare,Allocator>::value_compare
Min standard notice:
std::multimap::value_compare is a function object that compares objects of type std::multimap::value_type (key-value pairs) by comparing of the first components of the pairs.
# Declarations
class value_compare;
# Parameters
c: comparator to assign
# Return value
comp(lhs.first, rhs.first)