Coding in C++ could be a double edged sword whilst interviewing. It's hard for some interviewers to admit that they don't know C++ and they might end up surreptitiously misgrading your work. It's best to ask the interviewer's opinion on C++ before chosing it. IMO, the safest bets would be to stick to Java or Python (even Python's a fairly hairy language these days).

I once shadowed an interview once, where this happened. I felt bad for the candidate, because I wasn't allowed to speak whilst shadowing the interview, and neither did I have to submit feedback; and the candidate clearly was coding well. Although, I did reach out to the recruiter about this. And I don't know what happened after that.