std::mem_fn
| Defined in header
<functional>
|
||
| template< class R, class T >
/*unspecified*/ mem_fn(R T::* pm); |
(1) | (since C++11) |
| template< class R, class T, class... Args >
/*unspecified*/ mem_fn(R (T::* pm)(Args...)); |
(2) | (c++11, but defect) |
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.
The overloads (2) are reported as defect. The resolution, which has recently been voted "Tentatively Ready" proposes to remove all the overloads (2). This will break some code, see Example 3.
Contents |
[edit] Parameters
| pm | - | pointer to member that will be wrapped |
[edit] Return value
std::mem_fn returns an call wrapper of unspecified type that has the following members:
std::mem_fn Return type
Member types
| type | definition |
result_type
|
the return type of pm if pm is a pointer to member function, not defined for pointer to member object |
argument_type
|
T*, possibly cv-qualified, if pm is a pointer to member function taking no arguments
|
first_argument_type
|
T* if pm is a pointer to member function taking one argument
|
second_argument_type
|
T1 if pm is a pointer to member function taking one argument of type T1
|
Member function
| operator() |
invokes the target on a specified object, with optional parameters (public member function) |
[edit] Exceptions
None.
[edit] Example 1
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
[edit] Example 2
Pass a member function to std::transform to create a sequence of numbers:
#include <iostream> #include <functional> #include <iterator> #include <memory> #include <string> #include <vector> #include <algorithm> int main() { std::vector<std::string> words = {"This", "is", "a", "test"}; std::vector<std::unique_ptr<std::string>> words2; words2.emplace_back(new std::string("another")); words2.emplace_back(new std::string("test")); std::vector<std::size_t> lengths; std::transform(words.begin(), words.end(), std::back_inserter(lengths), std::mem_fn(&std::string::size)); // uses references to strings std::transform(words2.begin(), words2.end(), std::back_inserter(lengths), std::mem_fn(&std::string::size)); // uses unique_ptr to strings std::cout << "The string lengths are "; for(auto n : lengths) std::cout << n << ' '; std::cout << '\n'; }
Output:
The string lengths are 4 2 1 4 7 4
[edit] Example 3
#include <iostream> #include <functional> struct X { int x; int& easy() {return x;} int& get() {return x;} const int& get() const {return x;} }; int main(void) { auto a = std::mem_fn (&X::easy); // no problem at all // auto b = std::mem_fn<int& >(&X::get ); // no longer works with new specification auto c = std::mem_fn<int&()>(&X::get ); // works with both old and new specification auto d = [] (X& x) {return x.get();}; // another approach to overload resolution X x = {33}; std::cout << "a() = " << a(x) << '\n'; std::cout << "c() = " << c(x) << '\n'; std::cout << "d() = " << d(x) << '\n'; }
Output:
a() = 33 c() = 33 d() = 33
[edit] 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) |