C ++ Singleton no se puede vincular en Mac OS

I am trying to create a very classic singleton on C++, Mac OS using Xcode I create the Class MySingleton as follow:

class MySingleton{
private:
    int val;
    static MySingleton *instance;
    MySingleton(){
        val = 0;
    }
public:
    int getVal(){
        return val;
    }
    void setVal(int iVal){
        val = iVal;
    }
    static MySingleton* getInstance(){
        if(instance == NULL)
            instance = new MySingleton();
        return instance;
    }

};

Linker is complaining about static MySingleton* getInstance() Follow the Linker message:

Undefined symbols for architecture x86_64: "MySingleton::instance", referenced from: MySingleton::getInstance() in main.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)

Could somebody help on this? Perhaps I need to set something on Xcode, which by the way is version 4.2.1, and I am not able in doing this. Thanks.