[Boost-users] Re: Functors and function_traits<>

From: David Abrahams (dave_at_[hidden])
Date: 2004-12-28 10:27:46

Hossein Haeri wrote:
> Dave,
> 
>> > AFAIK, std::iterator_traits<> is one of the
>> > counterparts of function_traits<>, not?
>> > 
>> > A sort of symmetry which iterator_traits<> has got
>> is
>> > that it can work for Iterators as well as plain
>> > pointers. 
>> 
>> That's the wrong way to think about it.  It just
>> works for iterators.
>> Plain pointers just happen to be iterators, by the
>> *standard's
>> definition* of "iterator."
> 
> Smart Alec... ;) Alright I could be more specific
> speaking like this: 
> 
> "... it can work for Iterators which happen to be
> classes as well as plain pointers."
> 
> Excused me? :)

I'm not just being pedantic.  It's hard to explain why function_traits
is the way it is without making that distinction.

>> > This seems to be not the case for
>> function_traits<>.
>> > That is, you can pass a function type to it, but
>> you
>> > can't do that for a functor. Is that right?
>> 
>> That's because function object types (what you're
>> calling functors)
> 
> It's not me, AFAIK. That's another common name of
> them. Isn't that?

Yes, but it's frowned upon by many because it's a completely invented
meaning for a term that already had a well-established meaning in
mathematics and computing.  If you read Stroustrup or almost any other
author who's also on the committee you'll see the term "function object"
and not "functor."

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com