mem_fun_t<Result, X> mem_fun_t<Result, X> Categories: functors, adaptors Component type: type Description Mem_fun_t is an adaptor for member functions. If X is some class with a member function Result X::f() (that is, a member function that takes no arguments and that returns a value of type Result [1]), then a mem_fun_t<Result, X> is a function object adaptor that makes it possible to call f() as if it were an ordinary function instead of a member function. Mem_fun_t<Result, X>'s constructor takes a pointer to one of X's member functions. Then, like all function objects, mem_fun_t has an operator() that allows the mem_fun_t to be invoked with ordinary function call syntax. In this case, mem_fun_t's operator() takes an argument of type X*. If F is a mem_fun_t that was constructed to use the member function X::f, and if x is a pointer of type X*, then the expression F(x) is equivalent to the expression x->f(). The difference is simply that F can be passed to STL algorithms whose arguments must be function objects. Mem_fun_t is one of a family of member function adaptors. These adaptors are useful if you want to combine generic programming with inheritance and polymorphism, since, in C++, polymorphism involves calling member functions through pointers or references. As with many other adaptors, it is usually inconvenient to use mem_fun_t's constructor directly. It is usually better to use the helper function mem_fun instead. Example struct B { virtual void print() = 0; }; struct D1 : public B { void print() { cout << "I'm a D1" << endl; } }; struct D2 : public B { void print() { cout << "I'm a D2" << endl; } }; int main() { vector<B*> V; V.push_back(new D1); V.push_back(new D2); V.push_back(new D2); V.push_back(new D1); for_each(V.begin(), V.end(), mem_fun(&B::print)); } Definition Defined in the standard header functional, and in the nonstandard backward-compatibility header function.h. Template parameters Parameter Description Default Result The member function's return type.   X The class whose member function the mem_fun_t invokes.   Model of Adaptable Unary Function Type requirements X has at least one member function that takes no arguments and that returns a value of type Result. [1] Public base classes unary_function<X*, Result> Members Member Where defined Description argument_type Adaptable Unary Function The type of the argument: X* result_type Adaptable Unary Function The type of the result: Result Result operator()(X* x) const Unary Function Function call operator. Invokes x->f(), where f is the member function that was passed to the constructor. explicit mem_fun_t(Result (X::*f)()) mem_fun_t See below. template <class Result, class X> mem_fun_t<Result, X> mem_fun(Result (X::*f)()); mem_fun_t See below. New members These members are not defined in the Adaptable Unary Function requirements, but are specific to mem_fun_t. Member Description explicit mem_fun_t(Result (X::*f)()) The constructor. Creates a mem_fun_t that calls the member function f. template <class Result, class X> mem_fun_t<Result, X> mem_fun(Result (X::*f)()); If f if of type Result (X::*) then mem_fun(f) is the same as mem_fun_t<Result, X>(f), but is more convenient. This is a global function, not a member function. Notes [1] The type Result is permitted to be void. That is, this adaptor may be used for functions that return no value. However, this presents implementation difficulties. According to the draft C++ standard, it is possible to return from a void function by writing return void instead of just return. At present, however (early 1998), very few compilers support that feature. As a substitute, then, mem_fun_t uses partial specialization to support void member functions. If your compiler has not implemented partial specialization, then you will not be able to use mem_fun_t with member functions whose return type is void. See also mem_fun_ref_t, mem_fun1_t, mem_fun1_ref_t Copyright © 1999 Silicon Graphics, Inc. All Rights Reserved. TrademarkInformation