class NumberBox {
int number;
public:
NumberBox(int i);
int get();
};
class BoxContainer {
NumberBox nbox;
public:
BoxContainer(NumberBox& nb);
void say();
};
BoxContainer::BoxContainer(NumberBox& nb) : nbox(nb) { ... }
class NumberBox {
...
public:
NumberBox();
NumberBox(int i);
...
};