SingletonClass.cpp

#include <iostream>
using namespace std;
class Singleton
{
    private:
        Singleton() {};
        Singleton(Singleton& ) = delete;
        Singleton operator=(Singleton&) = delete;

    public:
        static Singleton * instance;
        static Singleton * getInstance();

        void foo() { cout << "test object" << endl;}
};

Singleton * Singleton::instance = nullptr;
//Singleton * getInstance()
Singleton * Singleton::getInstance()
        {
            if (Singleton::instance ==  nullptr)
            {
                Singleton::instance = new Singleton();
            }
            return Singleton::instance;
        }
int main()
{
  // Singleton * s = new Singleton(); // error
  Singleton* ss = Singleton::getInstance();
  Singleton* aa = Singleton::getInstance();
  cout << ss << endl;
  cout << aa << endl;

  return 0;
}