//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;
    }
};
//another
class SimplerFoo : public Foo {
private:
    int value;
public:
    Foo() {
        value=0;
    }
    //no destructor needed
    virtual int getValue() {
        return value;
    }
    virtual void setValue(int num) {
        value = num;
    }
};
//the following function can actually take a pointer to a MyFoo or a
//SimplerFoo or any other class derived from Foo (even indirectly; like
//one derived from MyFoo!)
void setFourtyTwo(Foo* obj) {
    //because it's declared as virtual, this will not call Foo::setValue
    //(which in this case is pure virtual so it would even crash) but
    //instead the setValue from the real class
    obj->setValue(42);
};
int main() {
    MyFoo one;
    SimplerFoo two;
    setFourtyTwo(&one);
    setFourtyTwo(&two);
};