How to write a singleton class in C++?

Singleton design pattern is a software design principle that is used to restrict the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. For example, if you are using a logger, that writes logs to a file, you can use a singleton class to create such a logger. You can create a singleton class using the following code.

Example

#include <iostream>
using namespace std;
class Singleton {
   static Singleton *instance;
   int data;
   // Private constructor so that no objects can be created.
   Singleton() {
      data = 0;
   }
   public:
      static Singleton *getInstance() {
         if (!instance)
         instance = new Singleton;
         return instance;
      }
   int getData() {
      return this -> data;
   }
   void setData(int data) {
      this -> data = data;
   }
};
//Initialize pointer to zero so that it can be initialized in first call to getInstance
Singleton *Singleton::instance = 0;
int main(){
   Singleton *s = s->getInstance();
   cout << s->getData() << endl;
   s->setData(100);
   cout << s->getData() << endl;
   return 0;
}

Output

0
100