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

I just ran across this issue, with clang++:

foo.cpp:17:8: error: must use a typedef to declare a conversion to 'void (*(int))()'

and there's a C++11 STL template which covers the identity functionality:

#include 
…
struct foo {
     void bar( ) const { }
     operator std::common_type::type( ) { return &foo::bar; }
};