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

Question

Say I have a class foo with an object of class bar as a member

class foo
{
    bar m_bar;
};

Now suppose bar needs to keep track of the foo

Answer 1

I suppose one way is to make abstract interface classes with no members. Then subclass foo and bar from those interface classes.

e.g.:

class FooInterface
{
   // methods here
}

class BarInterface
{
   // methods here
}

class Foo : public FooInterface
{
   BarInterface *pBar;
}

class Bar : public BarInterface
{
   FooInterface *pFoo;
}

This breaks the cycle of dependencies (unless the interfaces themselves depend on each other, and if that's the case then they should probably be declared in the same file)