std::mem_fn
Defined in header <functional>
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template< class M, class T > /*unspecified*/ mem_fn(M T::* pm); |
(since C++11) (until C++17) |
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template< class M, class T > /*unspecified*/ mem_fn(M T::* pm) noexcept; |
(since C++17) | |
Function template std::mem_fn
generates wrapper objects for pointers to members, which can store, copy, and invoke a pointer to member. Both references and pointers (including smart pointers) to an object can be used when invoking a std::mem_fn
.
Parameters
pm | - | pointer to member that will be wrapped |
Return value
std::mem_fn
returns a call wrapper of unspecified type that has the following members:
std::mem_fn Return type
Member types
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(until C++20) |
Member function
template<class... Args> /* see below */ operator()(Args&&... args); |
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Let fn
be the call wrapper returned by a call to std::mem_fn
with a pointer to member pm
. Then the expression fn(t, a2, ..., aN) is equivalent to INVOKE(pm, t, a2, ..., aN), where INVOKE is the operation defined in Callable. (Thus, the return type of operator()
is std::result_of<decltype(pm)(Args&&...)>::type.)
Each argument in args
is perfectly forwarded, as if by std::forward<Args>(args)....
Example
Use mem_fn
to store and execute a member function and a member object:
#include <functional> #include <iostream> struct Foo { void display_greeting() { std::cout << "Hello, world.\n"; } void display_number(int i) { std::cout << "number: " << i << '\n'; } int data = 7; }; int main() { Foo f; 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'; }
Output:
Hello, world. number: 42 data: 7
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
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LWG 2048 | C++11 | unnecessary overloads provided | removed |
See also
(C++11) |
wraps callable object of any type with specified function call signature (class template) |
(C++11) |
binds one or more arguments to a function object (function template) |