RE: [boost] Re: Policy or Trait?

From: David Bergman (davidb_at_[hidden])
Date: 2004-04-02 13:17:26

Peter Dimov wrote: 

> David Bergman wrote:
> > 
> > On the meta level, a trait places a Concept in relation to other 
> > Concepts, i.e., a trait is STRUCTURAL. On the object-level, 
> a Model is 
> > related to other Models or Features.
> > 
> > trait<Type>::neighbor_type  - relates two Concepts 
> > trait<Type>::child_type - adds a Feature
> > 
> > A policy is BEHAVIORAL. And one often uses policies in a dynamic 
> > setting, adding them in runtime. We, of course, often 
> implicitly refer 
> > to the compile-time policies...
> 
> So...
> 
> In A<X>::frobnicate(), 'A' is a policy, because it's 
> behavioral, and in

No, A is not behavioral. The feature "frobnicate" of the type X is. And,
that mapped behaviors can definitely be used as a policy.
 
> template<class B> struct X
> {
>     template typename B::value_type value_type; };
> 
> 'B' is a trait, because it's structural?

No, since a trait not only has to be structural, but *also* be a (set of)
meta functions with the Concept as a range.

/David