class variable in C++

A static variable inside a class should be initialized explicitly using the class name and scope resolution operator outside the class.
```
#include<iostream>
using namespace std;

class C {
public:
    static int si;
};
int C::si = 0; 

int main() {
    C c;
    c.si = 3;

    cout << c.si<<" "<<C::si;   // 3 3
}
```