Function Objects

Collaboration diagram for Function Objects:

Modules
	Adaptors for pointers to functions
	Adaptors for pointers to members
	Arithmetic Function Object Classes
	Binder Classes
	Boolean Operations Classes
	Comparison Classes
	Hashes
	Negators

Classes
struct	std::binary_function< _Arg1, _Arg2, _Result >
class	std::function< _Res(_ArgTypes...)>
class	std::reference_wrapper< _Tp >
struct	std::unary_function< _Arg, _Result >

Functions
template<typename _Tp , typename _Class >
constexpr _Mem_fn< _Tp _Class::* >	std::mem_fn (_Tp _Class::*__pm) noexcept

Detailed Description

Function objects, or functors, are objects with an operator() defined and accessible. They can be passed as arguments to algorithm templates and used in place of a function pointer. Not only is the resulting expressiveness of the library increased, but the generated code can be more efficient than what you might write by hand. When we refer to functors, then, generally we include function pointers in the description as well.

Often, functors are only created as temporaries passed to algorithm calls, rather than being created as named variables.

Two examples taken from the standard itself follow. To perform a by-element addition of two vectors a and b containing double, and put the result in a, use

transform (a.begin(), a.end(), b.begin(), a.begin(), plus<double>());

To negate every element in a, use

transform(a.begin(), a.end(), a.begin(), negate<double>());

The addition and negation functions will usually be inlined directly.

An adaptable function object is one which provides nested typedefs result_type and either argument_type (for a unary function) or first_argument_type and second_argument_type (for a binary function). Those typedefs are used by function object adaptors such as bind2nd. The standard library provides two class templates, unary_function and binary_function, which define those typedefs and so can be used as base classes of adaptable function objects.

Since C++11 the use of function object adaptors has been superseded by more powerful tools such as lambda expressions, function<>, and more powerful type deduction (using auto and decltype). The helpers for defining adaptable function objects are deprecated since C++11, and no longer part of the standard library since C++17. However, they are still defined and used by libstdc++ after C++17, as a conforming extension.

Function Documentation

mem_fn()

template<typename _Tp , typename _Class >

constexpr _Mem_fn< _Tp _Class::* > std::mem_fn ( _Tp _Class::*  __pm )

inlineconstexprnoexcept

Returns a function object that forwards to the member pointer pm.

Definition at line 179 of file functional.