Is there a way to forward declare covariance?

Question

Suppose I have these abstract classes Foo and Bar:

class Foo;
class Bar;

class Foo
{
public:
  virtual Bar* bar() = 0;
};

class Bar
{         


        

Answer 1

How about this.

template 
class Foo
{
public:
    virtual BarType* bar() = 0;
};

template 
class Bar
{
public:
    virtual FooType* foo() = 0;
};

class ConcreteBar;
class ConcreteFoo : public Foo
{
public:
    ConcreteBar* bar();
};

class ConcreteBar : public Bar
{
public:
    ConcreteFoo* foo();
};