NAME¶

std::mem_fun - std::mem_fun

Synopsis¶

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

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