Question

There is such code:

#include <iostream>
int main(){
  const int a = 2;
  int* ptr = (int*)&a;
  *ptr = 3;
  std::cout << &a << " " << ptr << " " << a << " " << *ptr << std::endl;
  return 0;
}

Result:

0xbf88d51c 0xbf88d51c 2 3

Why these two values differ? What does it here happen?

Answer 1

int* ptr = (int*)&a;

This is dangerous (this itself doesn’t invoke UB, though). But this,

*ptr = 3;

This invokes undefined behavior (UB), because you’re attempting to modify the const object pointing to by ptr. UB means anything could happen. Note that a is truly a const object.

§7.1.5.1/4 (C++03) says,

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.

 [Example:

 [...]

const int* ciq = new const int (3); // initialized as required
int* iq = const_cast<int*>(ciq);    // cast required
*iq = 4;                            // undefined: modifies a const object