Violation of template circular dependencies using template template parameters

Question

If my understanding is correct, the following classic circular relationship between pattern classes:

template <class MyB>
struct A {
    MyB *b_;
};

template <class MyA>
struct B {
    MyA *a_;
};

If we want to create an instance Awith Band Bwith A, then we cannot start with any, since we would need to write: A<B<A<B<...>>>(infinite).

I think template template options provide a solution. The following code compiles (from gccversion 4.8.2):

template <class MyB>
struct A {
    MyB *b_;
};

template <template <class> class MyA>
struct B {
    MyA<B> *a_;
};

int main() {
    using MyB = B<A>;
    using MyA = A<MyB>;
    MyA a;
    MyB b;
    a.b_ = &b; b.a_ = &a;
    return 0;
}

Did I miss the essence of the problem?

The UPDATE . I just ran into this one that offers almost the exact same solution.

Answer 1

, . , , :

• , ,

  template <class MyB>
  struct A {
      MyB *b_;
  };
  
  template <template <class> class MyA>
  struct B {
      MyA<B> *a_;
  };
  
  int main() {
      using MyB = B<A>;
      using MyA = A<MyB>;
      MyA a;
      MyB b;
      a.b_ = &b; b.a_ = &a;
      return 0;
  }
  

• , , struct/class

  template<class Common> struct A
  {
    typedef typename Common::BT B;
    B* b;
  };
  
  template<class Common> struct B
  {
    typedef typename Common::AT A;
    A* a;
  };
  
  struct Common {
    using AT = A<Common>;
    using BT = B<Common>;
  };
  
  
  int main() {
    A<Common> a;
    B<Common> b;
    return 0;
  }
  

• , ,

  template <class MyA>
  struct B;
  
  template <typename ...MyB>
  struct A {
    B<A<>> *b_;
  };
  
  template <>
  struct A<> {};
  
  template <class MyA>
  struct B {
    A<B<MyA>> *a_;
  };
  
  int main() {
  
    using BoA = B<A<>>;
    using AoBoA = A<B<A<>>>;
  
    BoA obj1;
    AoBoA obj2;
  
    obj1.a_ = &obj2;
    obj2.b_ = &obj1;
  
    return 0;
  }
  

, , CRTP - , , ( ).