Книга: Standard Template Library Programmer

mem_fun1_t

mem_fun1_t<Result, X, Arg>

Categories: functors, adaptors

Component type: type

Mem_fun1_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_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_t<Result, X, Arg>'s constructor takes a pointer to one of X's member functions. Then, like all function objects, mem_fun1_t has an operator() that allows the mem_fun1_t to be invoked with ordinary function call syntax. In this case, mem_fun1_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_t that was constructed to use the member function X::f, and if x is a pointer 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_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_fun1_t's constructor directly. It is usually better to use the helper function mem_fun [2] instead.

struct Operation {
 virtual double eval(double) = 0;
};
struct Square : public Operation {
 double eval(double x) { return x * x; }
};
struct Negate : public Operation {
 double eval(double x) { return –x; }
};
int main() {
 vector<Operation*> operations;
 vector<double> operands;
 operations.push_back(new Square);
 operations.push_back(new Square);
 operations.push_back(new Negate);
 operations.push_back(new Negate);
 operations.push_back(new Square);
 operands.push_back(1);
 operands.push_back(2);
 operands.push_back(3);
 operands.push_back(4);
 operands.push_back(5);
 transform(operations.begin(), operations.end(), operands.begin(), ostream_iterator<double>(cout, "n"), mem_fun(Operation::eval));
}

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_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_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_t with member functions whose return type is void.

[2] This helper function was called mem_fun1 in drafts of the C++ standard, but it is called mem_fun in the final standard. This implementation provides both versions for backward compatibility, but mem_fun1 will be removed in a future release.

mem_fun_t, mem_fun_ref_t, mem_fun1_ref_t

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