Книга: Standard Template Library Programmer

mem_fun_ref_t

mem_fun_ref_t<Result, X>

Categories: functors, adaptors

Component type: type

Mem_fun_ref_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_ref_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_ref_t<Result, X>'s constructor takes a pointer to one of X's member functions. Then, like all function objects, mem_fun_ref_t has an operator() that allows the mem_fun_ref_t to be invoked with ordinary function call syntax. In this case, mem_fun_ref_t's operator() takes an argument of type X&.

If F is a mem_fun_ref_t that was constructed to use the member function X::f, and if x is 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_ref_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. In fact, though, mem_fun_ref_t is usually not as useful as mem_fun_t. The difference between the two is that mem_fun_t's argument is a pointer to an object while mem_fun_ref_t's argument is a reference to an object. References, unlike pointers, can't be stored in STL containers: pointers are objects in their own right, but references are merely aliases.

As with many other adaptors, it is usually inconvenient to use mem_fun_ref_t's constructor directly. It is usually better to use the helper function mem_fun_ref instead.

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<D1> V;
 V.push_back(D1());
 V.push_back(D1());
 for_each(V.begin(), V.end(), mem_fun_ref(B::print));
}

Defined in the standard header functional, and in the nonstandard backward-compatibility header function.h.

Adaptable Unary Function

• X has at least one member function that takes no arguments and that returns a value of type Result. [1]

unary_function<X, Result>

These members are not defined in the Adaptable Unary Function requirements, but are specific to mem_fun_ref_t.

[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_ref_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_ref_t with member functions whose return type is void.

mem_fun_t, mem_fun1_t, mem_fun1_ref_t

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