what are singleton classes c 2b 2b

1#include <iostream>
2
3using namespace std;
4
5class Singleton {
6   static Singleton *instance;
7   int data;
8 
9   // Private constructor so that no objects can be created.
10   Singleton() {
11      data = 0;
12   }
13
14   public:
15   static Singleton *getInstance() {
16      if (!instance)
17      instance = new Singleton;
18      return instance;
19   }
20
21   int getData() {
22      return this -> data;
23   }
24
25   void setData(int data) {
26      this -> data = data;
27   }
28};
29
30//Initialize pointer to zero so that it can be initialized in first call to getInstance
31Singleton *Singleton::instance = 0;
32
33int main(){
34   Singleton *s = s->getInstance();
35   cout << s->getData() << endl;
36   s->setData(100);
37   cout << s->getData() << endl;
38   return 0;
39}