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Difference between revisions of "cpp/utility/functional/mem fn"

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< cpp‎ | utility‎ | functional
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Revision as of 17:57, 11 April 2023

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Function objects
Function invocation
(C++17)(C++23)
Identity function object
(C++20)
Transparent operator wrappers
(C++14)
(C++14)
(C++14)
(C++14)  
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)

Old binders and adaptors
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)  
(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
(until C++17*)(until C++17*)
(until C++17*)(until C++17*)

(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
 
Defined in header <functional>
template< class M, class T >
/* unspecified */ mem_fn(M T::* pm) noexcept;
 (since C++11)
(until C++20)
template< class M, class T >
constexpr /* unspecified */ mem_fn(M T::* pm) noexcept;
 (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.

Contents

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 */);
 (until C++20)
template<class... Args>

constexpr /* see below */ operator()(Args&&... args) /* cvref-qualifiers */

    noexcept(/* see below */);
 (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&&...)>::typeor 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)
    {
        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';
 
    // Working with smart pointer
    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';
 
    // Working with member function template with parameter pack
    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

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