[boost] result_of<F()>::type is void ... why?

From: Eric Niebler (eric_at_[hidden])
Date: 2007-05-20 22:44:44

I've noticed something odd about the specification of tr1::result_of. In 
result_of<F()>::type, if F is a class type that doesn't have a nested 
result_type typedef, the result is "void". This is only true for F(); 
that is, when there are no arguments. That leads to the following 
strangeness:

   struct foo {
     typedef int result_type;
   };

   struct bar {
     template<class Sig> struct result {
       typedef int type;
     };
   };

   result_of<foo()>::type == int      // 1
   result_of<bar()>::type == void     // 2 huh?

   result_of<foo(int)>::type == int   // 3
   result_of<bar(int)>::type == int   // 4

This is the way it's specified in TR1, and boost::result_of implements 
this behavior faithfully, but I just can't understand why (2) is void 
instead of int. Can somebody explain it to me?

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com