C++ const_cast does not remove the const-ness of variable

Question

I tried executing the following program.

#include<iostream>
using namespace std;
int main()
{
const int a = 0;
cout << &a <<endl;
int* ptr = const_cast<int*>(&a);
*ptr = 2;
cout << ptr <<endl;
cout << *ptr <<endl;
cout << a <<endl;
return 0;
}

The output was:

   0xbf92ebd8
   0xbf92ebd8
   2
   0

From what I know, const_cast makes a mutable, so that, it can be changed. The change is reflected when I display *ptr, but the change is not reflected in a. Can anyone explain ?

EDIT: Thanks for all the answers. I get it that I am observing an undefined behavior. And, I was initially confusing between mutable and *const_cast*. But then, could you tell me for what all scenarios, do we use const_cast?

Answer 1

From C++03 5.2.1 const_cast /7:

[Note: Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member resulting from a const_cast that casts away a const-qualifier may produce undefined behavior (7.1.5.1). ]

In 7.1.5.1 The cv-qualifiers /4:

Except that any class member declared mutable (7.1.1) can be modified, any attempt to modify a const object during its lifetime (3.8) results in undefined behavior.

And, in the terms and definitions section:

[Note: permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

Minor changes have been made in the latest standard but the general idea still holds. Bottom line, don't do that.