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std::mem_fun

From cppreference.com
< cpp‎ | utility‎ | functional
 
 
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Defined in header <functional>
template< class Res, class T >
std::mem_fun_t<Res,T> mem_fun( Res (T::*f)() );
(1) (deprecated in C++11)
(removed in C++17)
template< class Res, class T >
std::const_mem_fun_t<Res,T> mem_fun( Res (T::*f)() const );
(1) (deprecated in C++11)
(removed in C++17)
template< class Res, class T, class Arg >
std::mem_fun1_t<Res,T,Arg> mem_fun( Res (T::*f)(Arg) );
(2) (deprecated in C++11)
(removed in C++17)
template< class Res, class T, class Arg >
std::const_mem_fun1_t<Res,T,Arg> mem_fun( Res (T::*f)(Arg) const );
(2) (deprecated in C++11)
(removed in C++17)

Creates a member function wrapper object, deducing the target type from the template arguments. The wrapper object expects a pointer to an object of type T as the first parameter to its operator().

1) Effectively calls std::mem_fun_t<Res,T>(f) or std::const_mem_fun_t<Res,T>(f).
2) Effectively calls std::mem_fun1_t<Res,T,Arg>(f) or std::const_mem_fun1_t<Res,T,Arg>(f).

This function and the related types were deprecated in C++11 and removed in C++17 in favor of the more general std::mem_fn and std::bind, both of which create callable adapter-compatible function objects from member functions.

Contents

[edit] Parameters

f - pointer to a member function to create a wrapper for

[edit] Return value

A function object wrapping f.

[edit] Exceptions

May throw implementation-defined exceptions.

[edit] Notes

The difference between std::mem_fun and std::mem_fun_ref is that the former produces an function wrapper that expects a pointer to an object, whereas the latter -- a reference.

[edit] Example

demonstrates std::mem_fun usage and compares it with std::mem_fn. C++11/14 compatible compilation mode might be necessary (e.g. g++/clang++ with -std=c++11, cl with /std:c++11).

#include <iostream>
#include <functional>
 
struct S {
    int get_data() const { return data; }
    void no_args() const { std::cout << "void S::no_args() const\n"; }
    void one_arg(int) { std::cout << "void S::one_arg()\n"; }
    void two_args(int, int) { std::cout << "void S::two_args(int, int)\n"; }
    int data{42};
};
 
int main() {
    S s;
 
    auto p = std::mem_fun(&S::get_data);
    std::cout << "s.get_data(): " << p(&s) << '\n';
 
    auto p0 = std::mem_fun(&S::no_args);
    p0(&s);
    auto p1 = std::mem_fun(&S::one_arg);
    p1(&s, 1);
 
//  auto p2 = std::mem_fun(&S::two_args); // Error: mem_fun supports only member functions
                                          // without parameters or with only one parameter.
                                          // Thus, std::mem_fn is a better alternative:
    auto p2 = std::mem_fn(&S::two_args);
    p2(s, 1, 2);
 
//  auto pd = std::mem_fun(&S::data); // Error: pointers to data members are not supported.
                                      // Use std::mem_fn instead:
    auto pd = std::mem_fn(&S::data);
    std::cout << "s.data = " << pd(s) << '\n';
}

Possible output:

s.get_data(): 42
void S::no_args() const
void S::one_arg(int)
void S::two_args(int, int)
s.data = 42

[edit] See also

(C++11)
creates a function object out of a pointer to a member
(function template) [edit]
(deprecated in C++11)(removed in C++17)
creates a wrapper from a pointer to member function, callable with a reference to object
(function template) [edit]