C++ templates cross definition

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,

i need to implement association between to object using templates , here objects ares AA and BB — user1368287, May 08 '12 at 10:12
Such a class design is purposeless. I don't think it is useful in any sense. — Nawaz, May 08 '12 at 10:18
This must be one of the cases where MS Dev Studio is non-compliant, because this works fine for me. — Grimm The Opiner, May 08 '12 at 10:24
On the contrary, the code you've shown use [works as is](http://ideone.com/1bsQB). The error you have likely stems from code you *haven't* shown us where the template parameter is used as if it were complete, where it isn't in the case of `A`. (Although somehow `A` is mentioned in that error -- more code you haven't shown?) — Luc Danton, May 08 '12 at 10:24

score 5 · Accepted Answer · answered May 08 '12 at 10:40

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;};

score 0 · Answer 2 · edited May 23 '17 at 10:08

0

Check out this question I asked some time back. How to forward declare a template class

Basically, if you are forward declaring a template type, you have to specify all the template parameters.

edited May 23 '17 at 10:08

Community

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answered May 08 '12 at 10:13

nakiya

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this is about template definition here the problem is with template instantiation like doing : A a; – user1368287 May 08 '12 at 10:16
I do not see where you instantiate the class. Anyway, you can't resolve the class definition error because template classes are not really classes - they are just templates for creating classes. What you are doing when you code `A` is making a new class - not an object. – nakiya May 08 '12 at 10:22
Yes A is a class thats why i wrote: [Class AA : A] ! – user1368287 May 08 '12 at 10:29

C++ templates cross definition

2 Answers2