Question

I want to know if is it possible in C++ to do that :

 template <typename T> class A { T link;};
 template <typename U> class B { U link;};

 class AA : A<BB> {};
 class BB : B<AA> {};

because it generates the error :

 error: ‘BB’ was not declared in this scope
 error: template argument 1 is invalid

i have tryed to use anticipate declaration :

 class AA;
 class BB;

 class AA : A<BB> {};
 class BB : B<AA> {};

but it didn’t work :

 In instantiation of ‘A<AA>’:
 error: ‘A<T>::s’ has incomplete type
 error: forward declaration of ‘struct AA’

thank you for your help,

Answer 1

Your problem isn’t the templates, it’s the infinite nesting (and yes, technically from using incomplete types to define members). Remove the templates and you’ll get the same issue:

struct A;
struct B;

struct A { B x; };

struct B { A y; };

Conceptually, this can’t work. Because, really, what you’d get here is an A that contains a B that contains an A that contains a B … to infinity. Turtles all the way down.

What does work, however, is using pointer members instead. Those work with incomplete types, and consequently the code works – and even with templates.

template <typename T> class A { T* link;};
template <typename U> class B { U* link;};