Singleton design pattern in C++ with real life example

The main purpose of a singleton design pattern is that  it will ensure a class should have only one instance of it and allow global access to it

When we use a singleton design pattern

Suppose you have a network and many users are using it to print their documents

As you know, a printer is a peripheral device that is slower than computer operations so, it will take little time to print each document

A printer can not print all the documents at a time

For this reason, a printer will have a spooler program that controls the concurrent access to a shared resource

similarly, every network   system should have one printer spooler

#include <iostream>
using namespace std;
class Singleton {
    private:
   static Singleton *instance;
   int data;
   protected:
    Singleton() {
      data = 0;
   }
   public:
   static Singleton *getInstance() {
      if (instance==0){
      instance = new Singleton;
      }
      return instance;
   }
   int getData() {
      return this -> data;
  }
   void setData(int data) {
      this -> data = data;
   }
};

Singleton *Singleton::instance = 0;
int main(){
   Singleton *s = s->getInstance();
   Singleton *s1 = s1->getInstance();
   Singleton *s2 = s2->getInstance();
   cout << s->getData() << endl;
   s->setData(100);
    s1->setData(200);
    s2->setData(300);
   cout << s->getData() << endl;
    cout << s1->getData() << endl;
    cout << s2->getData() << endl;
   return 0;
}

Output

0
300
300
300

Line 5: a class called singleton is created

Line 14: it will get the instance of class singleton

Line 15:check the instance of the singleton class is created or not suppose if it is not created then it will create a new object of the singleton class

Line 19: if the instance is already created then it will return the instance of the singleton class

Line 34-36: three instances (s,s1,s2) is created of class singleton

Line 38-40: set the data values as 100 for s 200 for s1 and 300 for s2

Observe the output, you might notice that there are  three  different values were set for s,s1,s2 but the output is the same

Why?

because the singleton class can create only one instance