c++ properties implementing functions

Question

Supposing a general player that will notify certain events to certain objects of classes. I create a base class with some virtual functions guaranteed to exist in the derived:

class PlayEventsReceiver{

    virtual void StartPlay()=0;
    virtual void StopPlay()=0;  
};

I would derive from PlayEventsReceiver the classes interested in such "signals"

class FooPlayer:public PlayEventsReceiver{

    void StartPlay(){...}
    void StopPlay(){...}
};

But if I wanted, rather than inheriting, implementing the signaling mechanism as a property, like this:

class BarPlayer{

    PlayEventsReceiver receiver; 

};

Is it possible to implement the pure virtual functions somehow inside the class BarPlayer in a clean way and not creating intermediate derived classes?

No because you can't instantiate a virtual class, so you have to create an intermediate derived class. — Hatted Rooster
– Hatted Rooster, Commented Aug 18, 2015 at 17:34
If you could come up with a way to "implement the pure virtual functions somehow inside the class BarPlayer" how would it be more clean than simply making BarPlayer inherit the interface and supply its own implementations? — Galik
– Galik, Commented Aug 18, 2015 at 17:40
@Galik, I was just experimenting a concept new to me, "Composition over inheritance" en.wikipedia.org/wiki/Composition_over_inheritance — tru7
– tru7, Commented Aug 19, 2015 at 9:28

Galik · Accepted Answer · 2015-08-18 17:58:31Z

1

If you don't have a derived class then the compiler has nowhere to put its functions. If you are looking for something clean then creating an explicit derived class is probably as clean as it gets.

You can't escape the fact that somewhere you need to supply the function implementations.

However, it seems you can do this anonymously by creating an unnamed class/struct. But it is not avoiding creating a derived class because an anonymous derived class is still being created:

// public interface so I use struct
struct PlayEventsReceiver {

    virtual void StartPlay() = 0;
    virtual void StopPlay() = 0;
};

class BarPlayer {

public:
    struct : PlayEventsReceiver {

        void StartPlay() override
        {
            std::cout << "Start Play\n";
        }
        void StopPlay() override
        {
            std::cout << "Stop Play\n";
        }
    } receiver;

};

int main()
{
    BarPlayer bp;

    bp.receiver.StartPlay();
}

answered Aug 18, 2015 at 17:58

Galik

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1 Comment

tru7 Over a year ago

Thanks, that's what I was looking for. If I am not missing anything, there's not a clear way to access the outer class, only passing a pointer to the container in the constructor of the inner making things a more complicated than simply inheriting the PlayEventsReceiver.

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