Invalid use of incomplete type on g++

Question

I have two classes that depend on each other:

class Foo; //forward declaration

template <typename T>
class Bar {
  public:
    Foo* foo_ptr;
    void DoSomething() {
      foo_ptr->DoSomething();
    }
};

class Foo {
  public:
    Bar<Foo>* bar_ptr;
    void DoSomething() {
      bar_ptr->DoSomething();
    }
};

When I compile it in g++, it was giving error of "Invalid use of incomplete type", but it was compiled nicely in MSVC 10. Is it possible to solve this problem while keeping the declaration and definition in one header file? (no cpp files) If this is not allowed in the standard, so is this one of the MSVC "bug" or "feature"?

Answer 1

Yes, just move the method definitions out of the class definition:

class Foo; //forward declaration

template <typename T>
class Bar {
  public:
    Foo* foo_ptr;
    void DoSomething();
};

class Foo {
  public:
    Bar<Foo>* bar_ptr;
    void DoSomething() {
      bar_ptr->DoSomething();
    }
};

// Don't forget to make the function inline or you'll end up
// with multiple definitions
template <typename T>
inline void Bar<T>::DoSomething() {
  foo_ptr->DoSomething();
}