how to let class obs as private member of each other?

[zhujp98]

#include <iostream.h> 
class CData2;
class CData
{
private:
    int m_x;
    int m_y;
    CData2 cd2;
public:
    CData();
    CData(int a, int b);
    int getc1();
};


class CData2 
{
private:
   int m_u;
   int m_v;
   CData cd;
public:
       
	CData2(int u, int v);
    int get1();
    int get2();
	void init(CData& ad1);
};

above code have compilation error:

E:\jzhu\vc\t1\tclass.cpp(8) : error C2079: 'cd2' uses undefined class 'CData2'

How to fix this problem?
Thanks