const_cast of a static const member

Question

The following code compile well both with GCC (4.2-4.6) and with Clang (2.1), but when I run the executable it gives me "Bus error: 10". I don't understand the reason.

#include <iostream>

struct A
{
  static int const v;
  A() { ++*const_cast<int *>(&A::v); }
};

int const A::v = 0;

int main(int argc, char * argv[])
{
  A a, b, c;
  std::cout << a.v << std::endl;

  return 0;
}

Answer 1

I think the relevant quote is:

§ 7.1.6.1 (4) from N3242:

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

The examples illustrate the point using const_cast. As James pointed out: the quote can be found in §7.1.5 in the C++03 standard.

A little elaboration: That language rule allows the compiler to use read-only memory (if it is available on the target architecture) when something is declared const. Without this rule const-ness could always be casted away without fearing any consequences and using it would only be a matter of developer discipline. The way it is you can at least tell people that they are invoking UB, which usually is a good deterrent. The const_cast itself is of minor relevance as it does not matter how you trick the compiler in letting you manipulate a const object.