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Defined in header <functional> | ||
template<class M,class T> /* unspecified */ mem_fn( M T::* pm)noexcept; | (since C++11) (constexpr since C++20) | |
Function templatestd::mem_fn generates wrapper objects for pointers to members, which can store, copy, and invoke apointer to member. Both references and pointers (including smart pointers) to an object can be used when invoking astd::mem_fn.
Contents |
| pm | - | pointer to member that will be wrapped |
std::mem_fn returns a call wrapperfn of unspecified type that has the following members:
Member types
| (until C++20) |
template<class...Args> /* see below */ operator()(Args&&...args)/* cvref-qualifiers */ | (constexpr since C++20) | |
The expressionfn(args) is equivalent toINVOKE(pmd, args), wherepmd is theCallable object held byfn, it is of typeM T::* and is direct-non-list-initialized withpm.
Thus, the return type ofoperator() isstd::result_of<decltype(pm)(Args&&...)>::typeor equivalentlystd::invoke_result_t<decltype(pm), Args&&...>, and the value innoexcept specifier is equal tostd::is_nothrow_invocable_v<decltype(pm), Args&&...>)(since C++17).
Each argument inargs is perfectly forwarded, as if bystd::forward<Args>(args)....
Usestd::mem_fn to store and execute a member function and a member object:
#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)// C++20 required{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'; 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'; 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: 42data: 7add_xy: 10access_data(u): 7add_xy(u, 1, 2): 10add_many(u, ...): 13add_them(u, ...): 42
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 |
(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] |
