Re: static members

I have the following code , which implements the singleton pattern:

class Singleton{
private:
    static Singleton* uniqueInstance;
    //other useful instance variables here

    Singleton(){
        cout<<"Object created!"<<endl;
    }

public:
    static Singleton* getInstance(){
        if(uniqueInstance == NULL)
            uniqueInstance = new Singleton();
        return uniqueInstance;
    }
};

int main(){
    Singleton::getInstance();
    return 0;
}

But when i try to compile it , i get the following error:

dan@sea:~/school/dp$ g++ -o singleton singleton.cpp
/tmp/ccHL1rvi.o: In function `Singleton::getInstance()':
singleton.cpp:
(.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x8):
undefined reference to `Singleton::uniqueInstance'