关于C++ 单例 编译器 报错问题的解决

class Singleton{

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

private:
    static Singleton * instance;
    Singleton(){};

};
Singleton* Singleton::instance = NULL;

int main(){
Singleton * instance = NULL;
instance = Singleton::getInstance();
    return 0;
}