I am somewhat bemused as to why "can you delete this?" is considered to be an interesting question. Anyone who has written COM code in C++ knows that the very first thing you learn on day one of basic COM programming is how to use "delete this" correctly. I suppose it might be useful to determine whether, say, someone is lying on their resume about having written COM programs, but as a general knowledge C++ question, if someone couldn't answer it immediately then they are not going to be a good fit for my team.
Anyway, if your goal is to come up with a question that measures C++ skills, then picking one question is the wrong way to go. Turn it around. The right question to ask is:
On a scale from one to ten, how good a C++ programmer are you?
This is not the question that gets you the answer you want. Everyone says "eight" regardless. The question that gets you the answer you want is:
OK, so you're an eight. What is a problem area that you think a seven would have a difficult time working with?
And boom, now you've got them. If the candidate thinks that "something to do with recursion" or "when to use a virtual destructor" is the sort of thing that a seven has a hard time with, then you know that they know a little bit about recursion or destructors or whatever, and that their knowledge does not go much farther than that.
That should give you much better calibration than coming up with some trivia question. If you were to force me to come up with a good single question about a fact about C++, I'd probably ask something like "how would you design the semantic analyzer and code generator for the portion of a C++ compiler that deals with virtual methods called in a base class destructor?" You should ask questions that have to do with real stuff that you work on, and that the candidate is likely to work on. That's a problem I had to work on once, and I think it would give a pretty good insight into how a person designs semantic analyzers and code generators, as well as their knowledge of C++.