Singleton *Singleton::obj = 0; What does that line do in Singleton pattern implementation? [duplicate]

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• Why does a static data member need to be defined outside of the class? 3 answers

This code works fine as an implementation of Singleton pattern.

I want to understand Singleton *Singleton::obj = 0;.

What is that doing and why is that important?
Please explain.

#include <stdio.h>

class Singleton
{
    private:
        static Singleton *obj;
        Singleton() 
        {
            obj = NULL;
        }

        unsigned int zero = 0;

    public:
        static Singleton * getObject()
        {
            if( obj == NULL )
                obj = new Singleton();
            else
                printf( "\nMemory has already been allocated!\n" );

            return obj;
        }

        void addToZero()
        {
            zero++;
            printf( "\nzero has now become: %d\n", zero );
        }
};

Singleton *Singleton::obj = 0;

int main()
{
    Singleton *p = Singleton::getObject();
    p->addToZero();

    Singleton *p1 = Singleton::getObject();
    p1->addToZero();
}