I have some template class
template <typename T> class Base { };
I trying to inherit Derived class from Base and use Internal class that is nested of Derived as a generic argument of Base.
class Derived : Base<Internal> {
public: class Internal { };
}
But compiler doesn't see Internal in Base.
Is it possible to solve my problem with keeping Internal class as nested of Derived? And if it is possible - How?
I don't think that this can work. In order to define the class Derived you need to know the base class ("it must be a complete type") first; to instantiate the template Base into a class you need to have a type. Inner however is only a type "inside" of Derived; it's part of it's definition.
Definition of Derived ---needs---> Base<Derived::Inner>
^ |
|----------needs----------------|
Even an indirection does not help:
template<typename T>
class Base {};
template<template<class> class X, typename T>
struct UseInner : public X<typename T::Inner> {};
class Derived : UseInner<Base, Derived>
{
public: class Inner {};
};
In contrast to this, the CRTP works because there the template parameter can be an incomplete type ("no need to access internals").
Wrapping in a template doesn't work:
template<typename T>
class Base {};
template<template<class>class X>
struct Wrap
{
struct Derived : X<typename Derived::Inner> {
struct Inner {};
};
};
using RealDerived = typename Wrap<Base>::Derived;