Collection of Interfaces depends on each other

Question

Let I've two interfaces:

class foo{
public:
    virtual void fooo(bar * b)=0;
}

class bar{
public:
    virtual void barr(foo * f)=0;
}

But this code snippet is ill-formed. How can I write this interfaces correctly?

Answer 1

How can I write this interfaces correctly?

You can always forward declare classes and use pointers or references in the code that sees the forward declaration.
As soon any code needs to refer class members, the full class declaration must be seen.

class bar; // << Note the forward declaration
class foo{
public:
    virtual void fooo(bar * b)=0;
}

class bar{
public:
    virtual void barr(foo * f)=0;
}

Answer 2

Just forward declare bar;

class bar;

class foo{
public:
    virtual void fooo(bar * b)=0;
}

class bar{
public:
    virtual void barr(foo * f)=0;
}