From: Robert Bell <belvis@imageworks.com>
Andy Little wrote:
"David Abrahams" <dave@boost-consulting.com> wrote in message news:uptaqlaqz.fsf@boost-consulting.com...
Surely a trait is *part of the *definition* * of an *entity* in a wider sense. (perhaps *definition* has wrong connotations)
Perhaps. It seems that the crucial distinction is that an entity's definition is complete without the trait; traits are not part of the definition of an entity, they add on to what the program can know (or do) with the entity.
I was bothered by it, too, but I didn't reply before now. You're right. The traits augment a type with additional information needed in a context beyond the context in which it was created.
Using phrases like "a trait is part of the definition of an entity" confuses the issue because of the use of the word "definition".
Yeah. I agree.
For myself, I'm leaning towards the "xxx is a traits or a policy depending on how it's used" viewpoint. I find the
template<typename T, typename traits_or_policy = traits<T> > class Foo { };
example pretty compelling. Whether or not this is an example of good design is a red herring; because this idiom can be misused does not affect the point that whether traits_or_policy is used as a traits or policy class depends on how it's used by the client of Foo.
Whoa! I think you've taken the notion past what we've been discussing. "traits" in your example isn't a traits class or policy class depending upon how its used. If the name is to be believed, "traits" is a traits class because it associates stuff with T. However, within Foo, the template parameter traits_or_policy is a policy class. Let me illustrate: template<typename T, typename policy = traits<T> > class Foo { Foo() { policy::foo(traits<T>::bar); } }; traits<T> is both a traits and a policy class within Foo. The ctor unconditionally uses traits<T>::bar to get something associated with T. That isn't affected by the policy class given as the second template parameter. By contrast, the ctor uses the policy class when it calls policy::foo(). That the second template argument happens to be traits<T> is immaterial. It could be another class, thus affecting the ctor's behavior. (A traits class isn't restricted from having static member functions and a policy class isn't restricted from having static data members, I just haven't shown them in this example.) -- Rob Stewart stewart@sig.com Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer;