How to use interface in a base-class?

Question

considering following code:

#include <stdio.h>
struct ITimer {
    virtual void createTimer() = 0;
};
class A : public ITimer
{
    public:
        void showA() {
            printf("showA\n");
            createTimer();
        }
};

class B : public ITimer
{
    public:
        void showB() {
            printf("showB\n");
        }
        void createTimer() {
            printf("createTimer");
        }
};

class C: public A, public B
{
    public:
        void test() {
            showA();
            showB();
        }
};

int main()
{
    C c;
    c.test();
    return 0;
}

I need to use interface ITimer in class A, but the method is implemented in class B. So I inherited the interface in A, but the compiler is not happy with it:

test.cc
test.cc(38) : error C2259: 'C' : cannot instantiate abstract class
        due to following members:
        'void ITimer::createTimer(void)' : is abstract
        test.cc(5) : see declaration of 'ITimer::createTimer'

How could I use the interface in baseclass A while its method is implemented in class B.

Thanks.

@Nawaz: `I need to use interface ITimer in class A, but the method is implemented in class B. So I inherited the interface in A, ` — Jichao, Sep 26 '13 at 07:32

score 4 · Accepted Answer · edited May 23 '17 at 12:05

Inheritance is the base-class of all evil.

A nor B are ITimers

A doesn't even implement the pure virtual, so it cannot be instantiated. Therefore, inheriting from A makes C abstract too (cannot be instantiated).

You don't want to use inheritance here. See

Liskov Substitution Principle ("is-a" rule)

In this case, the dreaded diamond-of-death hierarchy could be fixed by adding virtual:

class A : public virtual ITimer
//...
class B : public virtual ITimer

See it Live on IdeOne. I don't recommend this, though. Consider fixing the design.

How to use interface in a base-class?

1 Answers1