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From: Hurd, Matthew (hurdm_at_[hidden])
Date: 2004-04-01 18:47:07

From: boost-bounces_at_[hidden]
> On Behalf Of David Abrahams
> > Rob Stewart <stewart_at_[hidden]> writes:
> 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.

Can a method call be part of a trait class? I don't think traits should
have anything to do with enacting behaviour (they may describe it).

If it is not clear and I see a method I think of it as policy-like. A
collection of types and static constants and I think of it as

I think it gets confusing with the overloading of the English word
policy. A policy on how to do something could be passed by value as a
polymorphic object or as a simple Boolean flag for that matter... which
is not the common intended meaning that MCD brought into common focus.

I'll try to add something...

Traits are used to get information at compile time. Policies are used
to modify behaviour. Traits could be used by a client to modify
behaviour which makes them feel like a policy class, but they aren't in
what we normally think of as a policy.

If a trait is used to direct the modification of behaviour, is it a
policy from the client classes point of view I guess... which points to
the terminology confusion.

Policies can be chained, multiply inherited from or just be ordinary
template classes with static methods, maybe such different styles should
have different names... a chain aspect, a base aspect (MCD-like), a
static aspect...

You could then say aspects and traits could be used as policies

A policy is then something that maybe used to modify behaviour. A trait
carries information for use such as for behaviour modification. An
aspect extends the definition of trait to also encapsulate behaviour.

An aspect and a trait may be used as a policy.

Does that work?

It would be nice if something new like "aspect class" or some such could
be defined and used commonly to avoid the mess, but I suspect it is too


Matt Hurd

Susquehanna Pacific P/L

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