Static variables inside a class definition

NOTE : -

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24. Static variables inside a class definition

One or more variables in a classcan be declared static. In which case, only one instance of those variables exist, shared by all instances of the class. It must be initialised outside the class declaration :

using namespace std;
#include 

class vector
{
public:

  double x;
  double y;
  static int count;

  vector (double a = 0, double b = 0)
  {
     x = a;
     y = b;
     count++;
  }

  ~vector()
  {
     count--;
  }
};

int vector::count = 0;

int main ()
{
  cout << "Number of vectors:" << endl;

  vector a;
  cout << vector::count << endl;

  vector b;
  cout << vector::count  << endl;

  vector *r, *u;

  r = new vector;
  cout << vector::count << endl;

  u = new vector;
  cout << a.count << endl;

  delete r;
  cout << vector::count << endl;

  delete u;
  cout << b.count << endl;

  return 0;
}

Output

1 2 3 4 3 2

BIBILOGRAPHY / REFERENCE : - http://www.4p8.com/eric.brasseur/cppcen.html