Why is Mutable keyword used [duplicate]

Question

Possible Duplicate:
C++ 'mutable' keyword

class student {

   mutable int rno;

   public:
     student(int r) {
         rno = r;
     }
     void getdata() const {
         rno = 90;
     } 
}; 

Answer 1

It allows you to write (i.e. "mutate") to the rno member through a student member function even if used with a const object of type student.

class A {
   mutable int x;
   int y;

   public:
     void f1() {
       // "this" has type `A*`
       x = 1; // okay
       y = 1; // okay
     }
     void f2() const {
       // "this" has type `A const*`
       x = 1; // okay
       y = 1; // illegal, because f2 is const
     }
};

Answer 2

The mutable keyword is used so that a const object can change fields of itself. In your example alone, if you were to remove the mutable qualifier, then you would get a compiler error on the line

rno = 90;

Because an object that is declared const cannot (by default) modify it's instance variables.

The only other workaround besides mutable, is to do a const_cast of this, which is very hacky indeed.

It also comes in handy when dealing with std::maps, which cannot be accessed using the index ing operator [] if they are const.

Answer 3

In your particular case, it's used to lie and deceive.

student s(10);

You want data? Sure, just call getdata().

s.getdata();

You thought you'd get data, but I actually changed s.rno to 90. HA! And you thought it was safe, getdata being const and all...