Invalid use of incomplete type on g++

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();
    }
}开发者_如何学Go;

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"?

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();
}

See this page: What is the best way to deal with co-dependent classes in C++?

It should clear up the problem and provides a couple nice solutions.

It works when you replace Foo* foo_ptr; by the template parameter T so that you get T* foo_ptr;. In this foo_ptr does not necessarily have to be a pointer or be predefined.

template <typename T>
class Bar {
  public:
    T foo;
    void DoSomething() {
      foo.DoSomething();
    }
};

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