Emulating GCC's __builtin_unreachable?

Question

I get a whole lot of warnings about switches that only partially covers the range of an enumeration switched over. Therefor, I would like to have a \"default\" for

Answer 1

template class Unreachable_At_Line {}; 
#define __builtin_unreachable() throw Unreachable_At_Line<__LINE__>()

Edit:

Since you want to have unreachable code to be omitted by compiler, below is the simplest way.

#define __builtin_unreachable() { struct X {X& operator=(const X&); } x; x=x; }

Compiler optimizes away x = x; instruction especially when it's unreachable. Here is the usage:

int foo (int i)
{
  switch(i)
  {
  case 0:  return 0;
  case 1:  return 1;
  default: return -1;
  }
  __builtin_unreachable();  // never executed; so compiler optimizes away
}

If you put __builtin_unreachable() in the beginning of foo() then compiler generates a linker error for unimplemented operator =. I ran these tests in gcc 3.4.6 (64-bit).