3 ms·

It's not just the function that becomes undefined, it's the entire program. There's literally nothing that the compiler is supposed to do following undefined be

• noselasd 12y ago
  Is the undefined behavior applicable at runtime or at compile time ? i.e. Is it undefined behavior if there are code paths that might access an array out of bounds, but at runtime it actually never would ?
  • PeterisP 12y ago
    That's the exact thing that the compiler is doing - if there are code paths that might access an array out of bounds, then the compiler is assuming that in runtime it actually never would happen. With this assumption you might even deduce that int f(int x) { if (x != 42) undefined_behavior; return x; } is the equivalent of int f(int x) { return 42; } since (according to the assumption) in runtime it would always be called in a way that doesn't reach the undefined behavior, i.e., with x=42. And, of course, the many similar assumptions about array boundaries, pointer nullabilities, numbers not reaching overflow, etc.