Дружественные классы

#include 
using namespace std;
class B;
class A{
double x;
public:
A(double z){x=z;}
double summa(B b);
double product(B b);
}a(3.5);
class B{
double y;
public:
B(double z){y=z;}
friend class A;
}b(2.3);
int main(){
cout << "Total is " << a.summa(b) << endl;
cout << "Product is " << a.product(b) << endl;
return 0;
}
double A::summa(B b){
return x+b.y;}
double A::product(B b){
return x*b.y;}