Relations of classes in C++

I am making a project with many classes, which have "relations" with each other. I make that by sample below(with guariands). That sample below is working, but if I include a.h instead b.h in main.cpp, then I get "error C2079: 'B::a' uses undefined class 'A'". I guess that happens because main.cpp calls A.h, when A.h calls B.h, and B.h doesn't include A.h anymore, because it's guarded(A_INC is defined), and class B don't know what class A is. Is this correct, and if yes, how to solve this? I would like to have ability to include those two files independently(not #include "b.h" and then #include "a.h", I know this works).

Sample code:

"main.cpp"

#include "b.h"
//#include "a.h" //this without upper include causes error

int main()
{
return 0;
}


"a.h"

#ifndef A_INC
#define A_INC

#include "b.h"
class B;

class A {
B *b;
};

#endif


"b.h"


#ifndef B_INC
#define B_INC

#include "a.h"
class A;

class B {
A a;
};
#endif