Are there cases where a typedef is absolutely necessary?

Question

Consider the following excerpt from the safe bool idiom:

typedef void (Testable::*bool_type)() const;
operator bool_type() const;

Is it possibl

Answer 1

It seems that the grammar demands using a typedef in your case. A conversion-function-id must be of the form operator conversion-type-id. The conversion-type-id cannot contain parentheses. So you must use typedef when converting to a pointer-to-function type, or to a pointer-to-member-function type.