mutable keyword in C++

This keyword is only used in C++.
The mutable keyword can be used for class member variables. Mutable variables are allowed to change from within const member functions of the class.
```
#include <iostream>
using namespace std;

class A {
public:  
  mutable int v;
  void change (int i) const { v = i; }
};

int main() {
  A a;
  a.change(1);
  cout << a.v << endl;
}
```