std::mem_fun

 

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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 > 201100int data{42};#elseint 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)