Question

class A
{
    private:
        A () {}

    public:
        static A* getInstance ()
        {
            return new A ();
        }
};

int main ()
{
    A.getInstance ();
    return 0;
}

results in the error stated in the title. I do realize that if I create a variable in class A and instanciate it there and return it directly, the error will vanish.

But, here I want to understand what is the meaning of this error and why can’t I use it this way.

Answer 1

You need to call the method using the scope resolution operator – :::

 A::getInstance ();

Also, if this is meant to be a singleton, it’s a very bad one. Whenever you call getInstance(), you’ll receive a new object, and you’ll run into memory leaks if you forget to delete any instances.

A singleton is usually implemented like so:

class A
{
    private:
        A () {}
        static A* instance;
    public:
        static A* getInstance ()
        {
            if ( !instance )
                instance = new A ();
            return instance;
        }
};

//implementation file
A* A::instance = NULL;