Function Object
Base class for creating binary function objects.
#include <functional>
template <class Arg1, class Arg2, class Result>
struct binary_function{
typedef Arg1 first_argument_type;
typedef Arg2 second_argument_type;
typedef Result result_type;
};
Function objects are objects with an operator() defined. They are important for the effective use of the standard library's generic algorithms, because the interface for each algorithmic template can accept either an object with an operator() defined or a pointer to a function. The Standard C++ Library provides both a standard set of function objects, and a pair of classes that you can use as the base for creating your own function objects.
Function objects that take two arguments are called binary function objects. Binary function objects are required to provide the typedefs first_argument_type, second_argument_type, and result_type. The binary_function class makes the task of creating templated binary function objects easier by providing the necessary typedefs for a binary function object. You can create your own binary function objects by inheriting from binary_function.
Function Objects, unary_function, the Function Objects section of the User's Guide.