Why do we need std::boolean and what is the use of it?

Question

既然我们已经有了 true 和 false 作为 C++ 中的bool类型，为什么还需要 std::boolean 以及它有什么用？

Answer 1

std::boolean used to be a part of the draft standard (eg it can be found in n4835.pdf), but in the current draft it is replaced by an exposition-only concept boolean-testable . The change happened around February.

In either incarnation it is a concept, not a type. That is, a template that says whether its argument type can be used as a boolean. bool obviously can be used this way, but not only: integral types, pointer types, and any class that defines a conversion to bool and/or overloads boolean operators like ! and && also qualifies.