Return objec of derived class whose base is singleton

Question

如何返回对象或调用基为单例的派生类的getInstance

Answer 1

You would have to use virtual static method, which is not supported in C++ (It's supported in Delphi, as far as I remember - a curiosity).

You have to decide, whether there's only one instance of both classes or of each class (eg. if you create a Derived instance by GetInstance, should you be able to create Base instance).

There is no way to solve this problem inside these classes, you have to create a class factory. Something like that (I've ommited the singleton implementation to make the idea more clear - obviously you know, how to implement one)

class SingletonFactory
{
    template<typename T>
    static T * GetInstance()
    {
        return T.GetInstance();
    }
};

class Base
{
    friend class SingletonFactory;

private:
    static Base * GetInstance()
    {
        // ...
    }

protected:
    Base()
    {
    }
};

class Derived : public Base
{
    friend class SingletonFactory;

private:
    static Derived * GetInstance()
    {
    }

protected:
    Derived()
        : Base()
    {
    }
};

// (...)

Derived * d = SingletonFactory::GetInstance<Derived>();