From: "Andy Little" <andy@servocomm.freeserve.co.uk>
But the root of the problem/thread is exactly what is a trait , or traits and what is a policy. ie to try to come up with an ambiguous specification of the word trait ,traits and policy in a C++ context.
I'd prefer unambiguous definitions or specifications, if you please. ;-)
A trait for an entity is used to describe a feature of the entity, usually by parameterising the entity or some type involved in the entity via a traits mechanism.
I'm sure you meant the right thing, but you've stated what parameterizes what backwards.
(The traits mechanism associates an entity (usually a type) with a particular family of traits ).
I've always thought of it the other way around, but the effect is the same. That is, I've always thought of it as associated the set of traits with the entity.
A policy customises (part of) an entity, usually by providing an (alternative) implementation of (some) functionality of the entity.
I don't agree with how you're characterizing this. A policy provides the implementation -- informational, behavioral, or both -- and, by substitution, can affect the entity.
non UDT examples:
sizeof // has characteristics of a trait. Useful but cant change it
Right.
atexit // has characteristics of a policy. Usually has a default functionality but can customise it
Since you can't change it, it isn't a policy. You can either take advantage of the functionality or not. -- Rob Stewart stewart@sig.com Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer;