Behaviour that merely differs based on implementation is either unspecified behaviour or implementation-defined behaviour. That and undefined behaviour are different things in the C++ standard.
I think for implementation-defined behavior the code has to do something sensible, but the standard doesn't specify what; the distinction for undefined behavior is that it could be erroneous (meaning it triggers an exception or just goes completely bonkers) but it could also do something sensible and expected, again depending on environment.
