How to make friend classes if they in different files .h

Question

i have two classes a.h and b.h . and without .cpp files. because a code is in .h files . And i cant understand how to make them a friends.
in function:
public:
void damage(a &obj){
obj.hp-=dmg;
}
Compile said me " a is not define"

Answer 1

Did you include your a.h in b.h?
#include "a.h"
Better practise to devide class declaration (a.h) and class description (a.cpp).

Answer 2

yes, i agree with you. but how to do friendly classes when i make descriptiin and declaration in same file.
when i #include a.h in b.h and #include b.h in a.h , compile said me that included class define self.

Answer 3

Try to use forward declaration.
class A;
class B { ... };
class A { ... };

Answer 4

yes it work when write all in one file .cpp.
is this possible by using only .h files for classes?