[boost] Re: Policy or Trait?

From: David Abrahams (dave_at_[hidden])
Date: 2004-04-01 16:09:53

Rob Stewart <stewart_at_[hidden]> 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