"David Abrahams" <dave@boost-consulting.com> wrote in message news:uzn9vnwtz.fsf@boost-consulting.com...
"Rozental, Gennadiy" <gennadiy.rozental@thomson.com> writes:
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> >
What he meant, I think, is "never passed as a template parameter together with primary type"
I don't know what "primary type" means here.
X in example above. Actually you may say that even my rephrasing isn't correct. One could always says mpl::apply_if<is_pointer<X>, X, "something else" > Let me try this way: For any type "T" listed in template definition no class that specify trait of the type "T" is used in a same template parameter list.
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.
Could you please rephrase. It's a bit unclear.
Technically one can never say "class template foo is a traits template" or "class template foo is a policy" without examining how it's used. There's no fundamental reason that the same class template can't be used in both ways, so for any given class template, there may be no either/or answer.
Well, technically you maybe right, though it's quite difficult to imagine an example of that. But still trait is entity that *define* compile time mapping of type to some kind of value, which maybe numaric value, another type or any other more complex thing (including something that may defines behavior). You may use it as a template parameter (not quite as a policy though in majority of the cases). Like in example above. Here we are using type trait as a selector. But is_pointer is still a trait class. Would you advokate a design that supply is_pointer along with type it applied to as a template parameters? template<typename T, typename TIsPointer> struct my_container {...}; template<typename T> struct C { my_container<T,is_pointer<T> > storage; }; So, could we say that is_pointer defines a trait? Regards, Gennadiy.