<functional>
Defines Standard Template Library (STL) functions that help construct function objects, also known as functors, and their binders.
For a list of all members of this header, see <functional> Members.
#include <functional>
Remarks
Algorithms require two types of function objects: unary and binary. Unary function objects require one argument, and binary function objects require two arguments. A function object and function pointers can be passed as a predicate to an algorithm, but function objects are also adaptable and increase the scope, flexibility, and efficiency of the STL. If, for example, a value needed to be bound to a function before being passed to an algorithm, then a function pointer could not be used. Function adaptors convert function pointers into adaptable function objects that can be bound to a value. The header <functional> also contains member function adaptors that allow member functions to be called as adaptable function objects. Functions are adaptable if they have nested type declarations specifying their argument and return types. The C++ Standard requires that this adaptability is implemented by having all standard object classes inherit from the unary_function or binary_function base classes. Function objects and their adaptors allow the STL to upgrade existing applications and help integrate the STL into the C++ programming environment.