C++ 'in class variable' and 'static variable' initialization

#include <iostream>
using namespace std;

class Demo
{
  public:
  int a  = 1;
    // static int b=2;  //this gives error
  static int b;
};

int Demo::b=2;  //initialize the static variable of class

int main()
{
    Demo d;
    cout << "\n value of variable a: " << d.a << endl;
    cout << "\n value of static variable b: " << Demo::b << endl;
}

OUTPUT: