Книга: Standard Template Library Programmer

mem_fun1_ref_t

mem_fun1_ref_t<Result, X, Arg>

Categories: functors, adaptors

Component type: type

Mem_fun1_ref_t is an adaptor for member functions. If X is some class with a member function Result X::f(Arg) (that is, a member function that takes one argument of type Arg and that returns a value of type Result [1]), then a mem_fun1_ref_t<Result, X, Arg> 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_fun1_ref_t<Result, X, Arg>'s constructor takes a pointer to one of X's member functions. Then, like all function objects, mem_fun1_ref_t has an operator() that allows the mem_fun1_ref_t to be invoked with ordinary function call syntax. In this case, mem_fun1_ref_t's operator() takes two arguments; the first is of type X and the second is of type Arg.

If F is a mem_fun1_ref_t that was constructed to use the member function X::f, and if x is an object of type X and a is a value of type Arg , then the expression F(x, a) is equivalent to the expression x.f(a). The difference is simply that F can be passed to STL algorithms whose arguments must be function objects.

Mem_fun1_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_fun1_ref_t is usually not as useful as mem_fun1_t. The difference between the two is that mem_fun1_t's first argument is a pointer to an object while mem_fun1_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_fun1_ref_t's constructor directly. It is usually better to use the helper function mem_fun_ref [2] instead.

Given a vector of vectors, extract one element from each vector.

int main() {
 int A1[5] = {1, 2, 3, 4, 5};
 int A2[5] = {1, 1, 2, 3, 5};
 int A3[5] = {1, 4, 1, 5, 9};
 vector<vector<int> > V;
 V.push_back(vector<int>(A1, A1 + 5));
 V.push_back(vector<int>(A2, A2 + 5));
 V.push_back(vector<int>(A3, A3 + 5));
 int indices[3] = {0, 2, 4};
 int& (vector<int>::*extract)(vector<int>::size_type);
 extract = vector<int>::operator[];
 transform (V.begin(), V.end(), indices, ostream_iterator<int>(cout, "n"), mem_fun_ref(extract));
}

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

Adaptable Binary Function

X has at least one member function that takes a single argument of type Arg and that returns a value of type Result. [1]

binary_function<X, Arg, Result>

These members are not defined in the Adaptable Binary Function requirements, but are specific to mem_fun1_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_fun1_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_fun1_ref_t with member functions whose return type is void.

[2] This helper function was called mem_fun1_ref in drafts of the C++ standard, but it is called mem_fun_ref in the final standard. This implementation provides both versions for backward compatibility, but mem_fun1_ref will be removed in a future release.

mem_fun_t, mem_fun_ref_t, mem_fun1_t

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