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

Asked Apr 02 '19 at 08:05

Active Apr 02 '19 at 08:05

Viewed 43 times

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();
}

asked Apr 02 '19 at 08:05

Aquarius_Girl

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It's the single instance... – user1810087 Apr 02 '19 at 08:07
But we are not using that singleton instance in `int main` for accessing anything. @user1810087 – Aquarius_Girl Apr 02 '19 at 08:08
Actually, you do. `Singleton::getObject();` returns `static Singleton *obj;`, which is initialized in the line you asked extra explanation for. – xEric_xD Apr 02 '19 at 08:10
Of course you do, twice: `Singleton::getObject();` returns the object `return obj;`. – user1810087 Apr 02 '19 at 08:10
It's simply defining the static member variable. – zar Apr 02 '19 at 14:35

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

0 Answers0