A shared resource for an object

Question

Say I have this class called Dog. Every dog has a different name but the same barking voice (which is loaded from a resource file).

class Dog {
public:
    Dog(const string &name) : _name(name) {
        _barkingVoice.load();
    }
    ~Dog() {
        _barkingVoice.free();
    }

    string getName() const { return _name; }
    void bark() { _barkingVoice.play(); }

private:
    string _name;
    VoiceResource _barkingVoice;
};

I want to call _barkingVoice.load() only if the instance of Dog is the first one, and _barkingVoice.free() only if there are no more instances of Dog.

The obvious solution is to set _barkingVoice as static and keep a reference counter of Dog as a data member.

My question is if there's an easier way to do this. Maybe an std implementation or something like that.

Answer 1

Make a reusable class to encapsulate the reference counting:

template<class ResourceType, class OwnerType>
class RefCounted {
public:
    RefCounted()          { if (++_refCount == 1) _resource.load(); }
    virtual ~RefCounted() { if (--_refCount == 0) _resource.free(); }
    ResourceType& operator*()  { return  _resource; }
    ResourceType* operator->() { return &_resource; }
private:
    static unsigned _refCount;
    static ResourceType _resource;
};

template<class T, class U> unsigned RefCounted<T, U>::_refCount = 0;
template<class T, class U> T RefCounted<T, U>::_resource;

class Dog {
public:
    Dog(const string &name) : _name(name) { }

    string getName() const { return _name; }
    void bark() { _barkingVoice->play(); }

private:
    string _name;
    RefCounted<VoiceResource, Dog> _barkingVoice;
};

Every template instantiation will have their own _refCount and _resource.

The second template parameter is to handle cases where you instantiate RefCounted with the same ResourceType but want to have separate reference counting for those instantiations. E.g. if you add a Cat class and want it to have its own Refcounted<VoiceResource>:

class Cat {
    // ...
private:
    RefCounted<VoiceResource, Cat> _meowingVoice;
};