C / C++ / MFC

//File A.h:

#include <B.h>

class A
{
public:

    A( );

    void DoStuff( );

private:

    B m_B;
};


//File B.h:

//Forward declare class A
class A;

class B
{
public:

    B( A* pA );

    void DoOtherStuff( );

private:

    A*  m_pA;
};

//File B.cpp:
#include <B.h>
#include <A.h>

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

}

void B::DoOtherStuff( )
{
    //Here we can use m_pA because it points to a valid object
}

//File A.cpp:

#include <A.h>

A::A( )
    : m_B( this )
    //will probably generate a compiler warning
    //but we can safely ignore that.  Just remember
    //that B cannot use the pointer it gets in the 
    //constructor because A hasn't been fully 
    //constructed yet!
{

}

void A::DoStuff( )
{
    m_B.DoOtherStuff( );
}