Cannot use typedefs inherited from std::unary_function [duplicate]

Question

prog.cpp:9:13: error: ‘result_type’ does not name a type
prog.cpp:9:13: note: (perhaps ‘typename std::unary_function<_Arg, _Result>::result_type’ was intended)

Compiler: http://ideone.com/vttG8W

Why cannot I use result_type directly?

#include <functional>

using namespace std;

template <typename ARGUEMENT, typename RESULT>
class FunctorBase : public std::unary_function<ARGUEMENT, RESULT>
{
public:
    virtual result_type operator () (argument_type) = 0;
        FunctorBase() {}
        virtual ~FunctorBase() {}
};

int main()
{
    FunctorBase<int&, void>();
}

Answer 1

Because it is an unqualified name.

When you use unqualified names in a class template, you have to tell the compiler that it should not immediately search for global names during the first phase of the two-phase name lookup, but rather wait until instantiation (because the name could come from a base class, as is the case here).

In this case, you could do this (and make result_type a dependent, qualified name):

typedef typename std::unary_function<ARGUEMENT, RESULT>::result_type result_type;

Notice that the same applies to argument_type.

typedef typename std::unary_function<ARGUEMENT, RESULT>::argument_type argument_type;

Armed with those two typedefs, the original member function declaration will now compile:

virtual result_type operator () (argument_type) = 0;

Answer 2

Because result_type and argument_type depend on the template parameters. Use:

virtual typename std::unary_function<ARGUEMENT, RESULT>::result_type
  operator () (typename std::unary_function<ARGUEMENT, RESULT>::argument_type) = 0;

or, if you need it in more places, add

using typename std::unary_function<ARGUEMENT, RESULT>::result_type;
using typename std::unary_function<ARGUEMENT, RESULT>::argument_type;

in the beginning of your class.