// A.h

 #include "B.h"

 class A

 {

 public:

     A(B* pB):m_pB(pB) {}

     void useB();

 private:

     B* m_pB;

 };

 //A.cpp

 #include "A.h"

 void A::useB()

 {

     //do something for m_pB

 }

 //B.h

 class A;

 class B

 {

 public:

     B(A* pA):m_pA(pA) {}

     void useA();

 private:

     A* m_pA;

 };

 //B.h

 #include "A.h"

 #include "B.h"

 void B::useA()

 {

     //do something for m_pA

     m_pA = new A;

 }