Question

I’ve got two objects that need to point to one another… the only problem is that since they are declared in a particular order, one or the other doesn’t know about the other object existing. For example:

...
#define foobar_h

class Foo {
  Bar* b;
};

class Bar {
  Foo* f;
};

...

How can I declare these classes so that they’ll be happy referencing one another?

Answer 1

You do a pre-declaration:

class Bar;

class Foo {
  Bar* b;
};

class Bar {
  Foo* f;
};

This makes the compiler know that the type Bar is a class, so it can correctly figure out how to represent a pointer to it.

Of course, this only works when the exact layout and size of values of type Bar are not needed, if you had tried to embed a value with:

class Foo {
  Bar my_bar;
};

You had not been able to succeed, but since you used a pointer it’s okay. A reference had also worked, since references don’t embed the entire object.