[zajio1am]

> UB is meant to add value. It’s possible to write a language without it, so why do we have any UB at all? We do because of portability and because it gives flexibility to compilers writers.

Implementation-defined behavior is here for portability for valid code. Undefined behavior is here so that compilers have leeway with handling invalid conditions (like null pointer dereference, out-of-bounds access, integer overflows, division by zero ...).

What does it mean that a language does not have UBs? There are several cases how to handle invalid conditions:

1) eliminate them at compile time - this is optimal, but currently practical just for some classes of errors.

2) have consistent, well-defined behavior for them - platforms may have vastly different way how to handle invalid conditions

3) have consistent, implementation-defined behavior for them - usable for some classes of errors (integer overflow, division by zero), but for others it would add extensive runtime overhead.

4) have inconsistent behavior (UB) - C way