In C++ when should we use std::any, std::variant or std::optional?

Question

I know there is a question about std::any vs std::variant, but std::optional is also about a same thing. C++ std::variant vs std::any

user_13161451 · Answer

I have a very general rule:

Every time you want to use a union use std::variant.

Every time you want to use a void* use std::any.

Every time you want to return nullptr as an indication of an error use std::optional.