Question

I’m having trouble understanding why the code below doesn’t compile — could someone please explain?

How do I access a typedef in a derived class from the base class?

template<class Derived>
struct Test
{
    template<class T>
    typename Derived::value_type foo(T);
};

struct Derived : public Test<Derived>
{
    typedef int value_type;
};

Answer 1

At the time of declaring Derived, Derived is not yet a complete type — you’ve only just started declaring it! Hence in the specialization Test<Derived>, the template argument is an incomplete type, and thus you mustn’t refer to a nested name such as Derived::value_type — that’s circular logic.

You could decycle the problem by making the return type a separate argument:

template <typename T, typename R> struct Test
{
    template <typename U> R foo(U);
};

template <typename R>
struct BasicDerived : public Test<BasicDerived, R>
{
    typedef R value_type;
};

typedef BasicDerived<int> Derived;