std::mem_fn

Defined in header `<functional>`
template< class M, class T > /unspecified/ mem_fn(M T::* pm);		(since C++11) (until C++17)
template< class M, class T > /unspecified/ mem_fn(M T::* pm) noexcept;		(since C++17)

Function template std::mem_fn generates wrapper objects for pointers to members, which can store, copy, and invoke a pointer to member. Both references and pointers (including smart pointers) to an object can be used when invoking a std::mem_fn.

Parameters

pointer to member that will be wrapped

Return value

std::mem_fn returns a call wrapper of unspecified type that has the following members:

std::mem_fn Return type

Member types

type	definition
`result_type`(deprecated in C++17)	the return type of `pm` if `pm` is a pointer to member function, not defined for pointer to member object
`argument_type`(deprecated in C++17)	`T*`, possibly cv-qualified, if `pm` is a pointer to member function taking no arguments
`first_argument_type`(deprecated in C++17)	`T*` if `pm` is a pointer to member function taking one argument
`second_argument_type`(deprecated in C++17)	`T1` if `pm` is a pointer to member function taking one argument of type `T1`

(until C++20)

Member function

template<class... Args>
/* see below */ operator()(Args&&... args);

Let fn be the call wrapper returned by a call to std::mem_fn with a pointer to member pm. Then the expression fn(t, a2, ..., aN) is equivalent to INVOKE(pm, t, a2, ..., aN), where INVOKE is the operation defined in Callable. (Thus, the return type of operator() is std::result_of<decltype(pm)(Args&&...)>::type.).

Each argument in args is perfectly forwarded, as if by std::forward<Args>(args)....

Example

Use mem_fn to store and execute a member function and a member object:

#include <functional>
#include <iostream>
 
struct Foo {
    void display_greeting() {
        std::cout << "Hello, world.\n";
    }
    void display_number(int i) {
        std::cout << "number: " << i << '\n';
    }
    int data = 7;
};
 
int main() {
    Foo f;
 
    auto greet = std::mem_fn(&Foo::display_greeting);
    greet(f);
 
    auto print_num = std::mem_fn(&Foo::display_number);
    print_num(f, 42);
 
    auto access_data = std::mem_fn(&Foo::data);
    std::cout << "data: " << access_data(f) << '\n';
}

Output:

Hello, world.
number: 42
data: 7

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2048	C++11	unnecessary overloads provided	removed

function (C++11)	wraps callable object of any type with specified function call signature (class template)
bind (C++11)	binds one or more arguments to a function object (function template)