Is undefined behavior only an issue if you are deploying on several platforms?

Question

Most of the conversations around undefined behavior (UB) talk about how there are some platforms that can do this, or some compilers do that.

What i

Answer 1

No, that's not safe. First of all, you will have to fix everything, not only the compiler version. I do not have particular examples, but I guess that a different (upgraded) OS, or even an upgraded processor might change UB results.

Moreover, even having a different data input to your program can change UB behavior. For example, an out-of-bound array access (at least without optimizations) usually depend on whatever is in the memory after the array. UPD: see a great answer by Yakk for more discussion on this.

And a bigger problem is optimization and other compiler flags. UB may manifest itself in a different ways depending on optimization flags, and it's quite difficult to imagine somebody to use always the same optimization flags (at least you'll use different flags for debug and release).

UPD: just noticed that you did never mention fixing a compiler version, you only mentioned fixing a compiler itself. Then everything is even more unsafe: new compiler versions might definitely change UB behavior. From this series of blog posts:

The important and scary thing to realize is that just about any optimization based on undefined behavior can start being triggered on buggy code at any time in the future. Inlining, loop unrolling, memory promotion and other optimizations will keep getting better, and a significant part of their reason for existing is to expose secondary optimizations like the ones above.