[boost] Re: Policy or Trait?

From: David Abrahams (dave_at_[hidden])
Date: 2004-04-01 17:43:20

"Rozental, Gennadiy" <gennadiy.rozental_at_[hidden]> writes:

>> > both for that type.  A trait class is never passed as a template 
>> > parameter; it's name is ubiquitous.
>> 
>> Whoops, can't agree there.  The boost type traits are passed 
>> as template parameters all the time:
>> 
>>      mpl::apply_if<
>>          is_pointer<X>, remove_pointer<X>, add_reference<X>
>>      >
>
> What he meant, I think, is "never passed as a template parameter together
> with primary type"

I don't know what "primary type" means here.  

>> I think your definitions are mostly on target, but I think 
>> where you go wrong is that traits/policies has less to do 
>> with how a template is defined than how it's used.
>
> Could you please rephrase. It's a bit unclear.

Technically one can never say "class template foo is a traits
template" or "class template foo is a policy" without examining how
it's used.  There's no fundamental reason that the same class template
can't be used in both ways, so for any given class template, there may
be no either/or answer.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com