Singleton design pattern template

singletonPattern.cpp

//  A Template for implementing singleton design pattern.

class Singleton
{
public:
    int getX(){ return var_x;}
    int getY(){ return var_y;}
    void setX(int x){ var_x = x;}
    void setY(int y){ var_y = y;}
    static Singleton *instance(){
        if(!i_instance)
            i_instance = new Singleton;
        return i_instance;
    }

private:
    int var_x;
    int var_y;
    static  Singleton *i_instance;
    Singleton(){}

};

void foo(int yi){
    Singleton::instance()->setY(yi);
    cout << "global y value ptr is" << Singleton::instance()->getY() << endl;
}

void bar(int xi){
    Singleton::instance()->setX(xi);
    cout << "global x value ptr is" << Singleton::instance()->getX() << endl;
}

int main(){
    cout << "global x value ptr is" << Singleton::instance()->getX() << endl;

    Singleton::instance()->setX(5);
    cout << "global x value ptr is" << Singleton::instance()->getX() << endl;

    foo(-23);

    bar(20);
}