Книга: Standard Template Library Programmer

pointer_to_unary_function

pointer_to_unary_function<Arg, Result>

Categories: functors, adaptors

Component type: type

Pointer_to_unary_function is a function object adaptor that allows a function pointer Result (*f)(Arg) to be treated as an Adaptable Unary Function. That is: if F is a pointer_to_unary_function<Arg, Result> that was initialized with an underlying function pointer f of type Result (*)(Arg), then F(x) calls the function f(x). The difference between f and F is that pointer_to_unary_function is an Adaptable Unary Function, i.e. it defines the nested typedef s argument_type and result_type.

Note that a function pointer of type Result (*)(Arg) is a perfectly good Unary Function object, and may be passed to an STL algorithm that expects an argument that is a Unary Function. The only reason for using the pointer_to_unary_function object is if you need to use an ordinary function in a context that requires an Adaptable Unary Function, e.g. as the argument of a function object adaptor.

Most of the time, you need not declare an object of type pointer_to_unary_function directly. It is almost always easier to construct one using the ptr_fun function.

The following code fragment replaces all of the numbers in a range with their absolute values, using the standard library function fabs. There is no need to use a pointer_to_unary_function adaptor in this case.

transform(first, last, first, fabs);

The following code fragment replaces all of the numbers in a range with the negative of their absolute values. In this case we are composing fabs and negate. This requires that fabs be treated as an adaptable unary function, so we do need to use a pointer_to_unary_function adaptor.

transform(first, last, first, compose1(negate<double>, ptr_fun(fabs)));

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

Adaptable Unary Function

• Arg is Assignable.

• Result is Assignable.

unary_function<Arg, Result>

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

pointer_to_binary_function, ptr_fun, Adaptable Unary Function

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