Question

So I have a simple singleton template base class (minified for question purposes):

template <typename T>
class Singleton {
public:
    static T& instance() {
        static T me;
        return me;
    }
};

Now I want to have a “Base class that’s a singleton”:

class Base : public Singleton<Base> {
public:
    void print() { std::cout << &instance() << std::endl; }
}

What I want now is deriving children classes from this base class, which are their own singletons:

class A : public Base {
    // ...
}

class B : public Base {
    // ...
}

Of course if I do this

A::instance().print();
B::instance().print();

I get the same address in both cases. Is there a way to accomplish this?

In case you’re wondering what for: I want to program a ResourceManager base class which is being inherited by a class ImageManager, a class ‘AudioManager’, etc. And I’d like them not to share actually the same instance, but still having only one instance per manager…

Answer 1

You will have to make Base a template and pass that to Singleton, then inherit from Base<A> if you wish to achieve this. Or, if you want Base to be a Singleton and A/B to be a Singleton, then they will have to inherit from Singleton<A>, Singleton<B> in addition to Base.

But dear lord, man, Singletons are a curse on you and your children for a hundred generations. Why would you do that to yourself?