Question

Does anyone knows how you can trick the C++ compiler to compile something like that (with the condition that TheObservedObject remains inside MyClass ):

template< typename Type >
class Observabile
{
public:
   typename typedef Type::TheObservedObject TheObject;

   void Observe( TheObject& obj ) {}
};

class MyClass : public Observabile< MyClass >
{
public:
   class TheObservedObject
   {
   };
}

Answer 1

Sadly, that is not directly possible, as MyClass, at the point of instantiation of Observable, is not yet complete and as such you can’t access any typedefs. You can work around this by adding a small wrapper:

template< typename Type, typename Wrapper >
class Observable
{
public:
   typename typedef Wrapper::TheObservedObject TheObject;

   void Observe( TheObject& obj ) {}
};

struct MyClassWrapper{
   class TheObservedObject
   {
   };
};

class MyClass : public Observable< MyClass, MyClassWrapper>
{
public:
    typedef MyClassWrapper::TheObservedObject TheObservedObject;
};

Or generelly just put TheObservedObject outside of MyClass(Wrapper).