类成员函数声明为另外一个类的友元

class Class2;

class Class1{
private:
    int m;
public:
    void display( Class2 &cl2 );
    Class1():m(1){}
    friend class Class2;
    void print(){cout<<"m="< < };

class Class2{
private:
    int m;
public:
    friend void Class1::display( Class2 &cl2 );
    Class2():m(2){}
    void print( Class1 &cl1 ){cout<<"m="<< cl1.m << endl;}
};

void Class1::
    display( Class2 &cl2 ){
        cout << cl2.m << endl;
}