std::mem_fn

 

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Defined in header <functional>
template< class M, class T > /* unspecified */ mem_fn( M T::* pm ) noexcept;		(since C++11) (constexpr since C++20)

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

pm

-

pointer to member that will be wrapped

Return value

std::mem_fn returns a call wrapper fn 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) /* cvref-qualifiers */     noexcept(/* see below */);		(constexpr since C++20)

The expression fn(args) is equivalent to INVOKE(pmd, args), where pmd is the Callable object held by fn, it is of type M T::* and is direct-non-list-initialized with pm.

Thus, the return type of operator() is std::result_of<decltype(pm)(Args&&...)>::type or equivalently std::invoke_result_t<decltype(pm), Args&&...>, and the value in noexcept specifier is equal to std::is_nothrow_invocable_v<decltype(pm), Args&&...>)(since C++17).

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

Example

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

Run this code

#include <functional>
#include <iostream>
#include <memory>
 
struct Foo
{
    void display_greeting()
    {
        std::cout << "Hello, world.\n";
    }
 
    void display_number(int i)
    {
        std::cout << "number: " << i << '\n';
    }
 
    int add_xy(int x, int y)
    {
        return data + x + y;
    }
 
    template<typename... Args> int add_many(Args... args)
    {
        return data + (args + ...);
    }
 
    auto add_them(auto... args) // C++20 required
    {
        return data + (args + ...);
    }
 
    int data = 7;
};
 
int main()
{
    auto f = Foo{};
 
    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';
 
    auto add_xy = std::mem_fn(&Foo::add_xy);
    std::cout << "add_xy: " << add_xy(f, 1, 2) << '\n';
 
    auto u = std::make_unique<Foo>();
    std::cout << "access_data(u): " << access_data(u) << '\n';
    std::cout << "add_xy(u, 1, 2): " << add_xy(u, 1, 2) << '\n';
 
    auto add_many = std::mem_fn(&Foo::add_many<short, int, long>);
    std::cout << "add_many(u, ...): " << add_many(u, 1, 2, 3) << '\n';
 
    auto add_them = std::mem_fn(&Foo::add_them<short, int, float, double>);
    std::cout << "add_them(u, ...): " << add_them(u, 5, 7, 10.0f, 13.0) << '\n';
}

Output:

Hello, world.
number: 42
data: 7
add_xy: 10
access_data(u): 7
add_xy(u, 1, 2): 10
add_many(u, ...): 13
add_them(u, ...): 42

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
LWG 2489	C++11	noexcept not required	required

See also

function (C++11)	copyable wrapper of any copy constructible callable object (class template)
move_only_function (C++23)	move-only wrapper of any callable object that supports qualifiers in a given call signature (class template)
bind (C++11)	binds one or more arguments to a function object (function template)