Re: forward declaration & virtual member function return derived class

There are 2 abstract base classes:

class IB;
class IA
{
    //...
public:
    virtual IB* GetB() = 0;

}

class IB
{
    //...
public:
    virtual IA* GetA() = 0;

}

Then, this is derived classes:

class CB; //#1
class CA : public IA
{
    //...
public:
    CB* GetB(); //#2}

class CB : public IB
{
    //...
public:
    CA* GetA(); //#3

}

The question is, the declaration do not work.
In C++, the overrided function can return a class which is public
derived from the return type of the function in base class.
When the function #2 returns IB*, it is OK. #3 is OK, too.
But, when #2 returns CB*, the compiler says something like "CB is not
derived from IB".
At #1, I can not declare like this:
class CB : public IB;
The syntax is not passed.

So, what can I do?
There is cycle dependence, but I do not care of the dependence: CA and
CB is almost in a single .cpp file.