std::mem_fun

 

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Old binders and adaptors
unary_function (until C++17*) binary_function (until C++17*) ptr_fun (until C++17*) pointer_to_unary_function (until C++17*) pointer_to_binary_function (until C++17*)   mem_fun (until C++17*) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) not1 (until C++20*) not2 (until C++20*)	binder1stbinder2nd (until C++17*)(until C++17*) bind1stbind2nd (until C++17*)(until C++17*) mem_fun_ref (until C++17*) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) unary_negate (until C++20*) binary_negate (until C++20*)

 

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 adaptor-compatible function objects from member functions.

Parameters

f

-

pointer to a member function to create a wrapper for

Return value

A function object wrapping f.

Exceptions

May throw implementation-defined exceptions.

Notes

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

Example

Demonstrates std::mem_fun usage and compares it with std::mem_fn. C++11/14 compatible compilation mode might be necessary: g++/clang++ with -std=c++11, cl with /std:c++11, etc. On recent compilers, e.g. gcc-12, might issue "deprecated declaration" warnings if not compiled in C++98 mode.

Run this code

#include <functional>
#include <iostream>
 
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"; }
#if __cplusplus > 201100
    int data{42};
#else
    int data;
    S() : data(42) {}
#endif
};
 
int main()
{
    S s;
 
    std::const_mem_fun_t<int, S> p = std::mem_fun(&S::get_data);
    std::cout << "s.get_data(): " << p(&s) << '\n';
 
    std::const_mem_fun_t<void, S> p0 = std::mem_fun(&S::no_args);
    p0(&s);
 
    std::mem_fun1_t<void, S, int> p1 = std::mem_fun(&S::one_arg);
    p1(&s, 1);
 
#if __cplusplus > 201100
//  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';
#endif
}

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

See also

mem_fn (C++11)	creates a function object out of a pointer to a member (function template)
mem_fun_ref (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)