why do I need class forward declaration ?

[buddha87]

I am checking some codes try to understand it. I saw some class forward declarations. E.g.:

In class A in A.cpp, I saw one line code as:
class B;

Class B is defined in B.cpp while it has B.h.

So my question is if in A.cpp, I do:
#include "B.h"
Then I don't need forward declarations like "class B" anymore, correct? If yes, then when should I use class forward declaration instead of #include header file?
If no, why?

Thanks a lot.

[buddha87]

Anyone can help?

[cire]

Use forward declarations to break include cycles. If b.h must include a.h, then a.h should not include b.h. You see this pattern when two classes have circular dependencies.