[boost] Re: Policy or Trait?

From: David Abrahams (dave_at_[hidden])
Date: 2004-04-02 14:37:48

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

>> > And I think the same. IMO any example of trait/policy mixure in the
>> > same class would be an example of bad design.
>>
>> I don't know; it seems to me that the
>>
>>   template <class T, class Policy = xxx_traits<T> >
>>   class Foo;
>>
>> idiom might not be a bad one for some uses if xxx_traits is only
>> supplying associated types.  It allows customization of the meaning of
>> Foo<X> for any X by specializing xxx_traits<X>, while also allowing
>> users to explicitly choose Foo<X, Y> when they don't like the default
>> for X.
>
> And you get basic_string design as a result.

My problems with basic_string arise from other choices.

> If user do not like default value for the trait. 

It's not a default value of a trait.  It's the default value of the
Policy.  The default value of xxx_traits<T>::whatever is given by the
primary xxx_traits template.

> It's value should be set *externally* to the component
> definition. 

Why?

> That is the major point I am trying to prove in regards
> to numeric_cast design and in thic generic discussion.

I don't know enough about how this relates to numeric_cast to comment.
-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com