From cppreference.com
classvalue_compare;std::map::value_compare is a function object that compares objects of type std::map::value_type (key-value pairs) by comparing of the first components of the pairs.
Member types
Type Definition result_type(deprecated in C++17)(removed in C++20)boolfirst_argument_type(deprecated in C++17)(removed in C++20)value_typesecond_argument_type(deprecated in C++17)(removed in C++20)value_typeThese member types are obtained via publicly inheriting std::binary_function<value_type,value_type,bool>.
(until C++11)Protected member objects
Compare comp
the stored comparator
(protected member object)Member functions
std::map<Key,T,Compare,Alloc>::value_compare::value_compare
protected:value_compare(Comparec);Initializes the internal instance of the comparator to c.
Parameters
c - comparator to assign std::map<Key,T,Compare,Alloc>::value_compare::operator()
booloperator()(constvalue_type&lhs,constvalue_type&rhs)const;Compares lhs.first and rhs.first by calling the stored comparator.
Parameters
lhs, rhs - values to compare Return value
comp(lhs.first,rhs.first)
Exceptions
May throw implementation-defined exceptions.