The 1 to 10 scales are tricky. My rating would be 5 is use professionally or equivalent, 7 build non-trivial app and maintain legacy code, must know best practices and style guidelines, basically able to work in a large code base with other users. 9 would be contribute to the language or at least be involved in the community. But I don't know if everyone has the same interpretation of the scale, so you could be relatively competent but only rate yourself a 5 or 6.

Serious question, if the person actually had a 9 out of 10 knowledge of C++, how would he answer? Or is this just a teaser question?

I don't know C++ but I would answer pointers are addresses in physical (?) memory. They let you pass around variables by reference where the receiving function has full access and permission to edit its content. Pointers have to be dereferenced with the appropriate type before being able to read it

9 out of 10 for me means you know metaprogramming etc.

Any answer is basically fine, as long as it's clear you know what pointers are.

It's basically also a starter question to go further into memory allocation and freeing, smart pointers, etc.

The thing is, it's fine if you answer 3/10 "because it's been a while since I saw it". It's just to probe self knowledge.

I would call that a successful interview (succeeds in detecting a candidate who would likely fail at their duties). Interviews are important. But would any random programming or whiteboarding task add anything of value to this?