From: "Hurd, Matthew" <hurdm@sig.com>
From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org]
On Behalf Of David Abrahams
Rob Stewart <stewart@sig.com> writes:
Can a method call be part of a trait class? I don't think traits should
In my book, certainly.
have anything to do with enacting behaviour (they may describe it).
I don't agree.
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 trait-like.
I only think of policies as template parameters that inject types, values, and functions to control the template. I think of traits as standalone entities known by their names. (That's not to say the latter can't be passed to a template to be used as the former.)
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.
That's where we differ. In your example, the traits class is a policy class precisely because it is passed as a template parameter.
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.
A traits class is always used within the implementation by its well-known name.
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...
I don't see how being able to chain policies requires a new name. -- Rob Stewart stewart@sig.com Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer;