Rob Stewart <stewart@sig.com> writes:
Recent messages in the thread, "Numeric conversions lib comments," involve questions regarding what is a trait and what is a policy. I have my own ideas colored by MCD and other sources, but try as I might, I couldn't find any authoritative or explicit defintion of the terms. Considering how much they are bandied about here on Boost, perhaps it would be good to agree upon Boost definitions of the terms. That way, there is less room for confusion when those terms are used.
My view on "traits" is here: http://tinyurl.com/25db2
To get the ball rolling, here are my definitions. They provide clear, non-overlapping concepts, which I think are important characteristics.
Trait - A class template parameterized on a single type that associates information with that type. A traits class, therefore, provides an external, named grouping of metainformation, behavior, or both for that type. A trait class is never passed as a template parameter; it's name is ubiquitous.
Whoops, can't agree there. The boost type traits are passed as template parameters all the time: mpl::apply_if< is_pointer<X>, remove_pointer<X>, add_reference<X> >
A traits class never has state.
Policy - A class template passed to other templates for the purpose of providing a named grouping of metainformation,
I think a Policy might be a non-templated class.
behavior, or both to those templates. Because it is a template parameter, different policy classes can be used as desired. In many cases, a policy class is used as a base class.
What do you think? I know that my definitions mean that std::char_traits is a policy class, despite its name, and they probably deviate from some other ad hoc definitions. The question is whether these provide sound, distinct, and defensible concepts that could be codified for future use at Boost (and elsewhere).
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. -- Dave Abrahams Boost Consulting www.boost-consulting.com