//abstract Foo base class
class Foo {
public:
    virtual int getValue()=0;
    virtual void setValue(int num)=0;
};
//specific implementation of Foo functionality
class MyFoo : public Foo {
private:
    int* value;
public:
    Foo() {
        value=new int;
        *value=0;
    }
    ~Foo() {
        delete value;
    }
    virtual int getValue() {
        return *value;
    }
    virtual void setValue(int num) {
        *value = num;
    }
};
//the following function can take a pointer to a MyFoo or to any other class
//derived from Foo (even indirectly; like one derived from MyFoo!)
void setFourtyTwo(Foo* obj) {
    obj.setValue(42);
};