"Rob Stewart" <stewart@sig.com> wrote
From: "Andy Little" <andy@servocomm.freeserve.co.uk>
I see this:
template <typename T> class T3 { };
That is, "traits<T>" isn't part of the interface of T3 specifically, though nested types in it could be. I see traits<T> as an implementation detail injected from the scope where traits<T> is specialized.
Yes I see what you mean, a typedef is not a very good example of Use of a traits-class by another class. Perhaps more like this: template<typename T> struct adjust_traits; template <typename T> class adjust{ public: typename adjust_traits<T>::feedback_type operator() (typename adjust_traits<T>::output_type const& out ); }; class brightness; class roll_rate; template <> class adjust<brightness>; template<> class adjust<roll_rate>; ( BTW your other post in this thread I am pretty much in agreement with. One point worth making. Perhaps I should use the term *traits-class*, rather than *trait* or *traits*.I guess a trait is a member of a traits-class and traits are members of a traits-class. I have not been following that advice...but meanwhile this thread has cleared up some foggy ideas .. so it has been very helpful to me. regards Andy Little
The traits or policy discussion seems to boil down to this:
In T1 above arg_traits is not user modifiable, However
template <typename T,typename P> struct T2{ typedef P arg_traits; };
arg_traits in T2 is user-modifiable. traits<T> is a traits-class, but traits<T> could theoretically be used in T2, in which case it would be regarded as implementing a policy. To implement various policies one can define different traits-classes on the same type, however they might
then be
^^^^^^^^^^^^^^ In which case, they aren't traits classes to me.
best referred to as policy-classes. Use of traits<T> in T1 could also be regarded as a policy, but an immutable one. Based on that one could say
that
a policy is implemented, simply the Use of a traits-class or a policy-class. The term policy being useful only if the policy is modifiable.
Yes, but it is equally important, in my mind, that a traits class has a well-known, ubiquitous name. Anywhere you want a particular trait or set of traits for a type T, you always use the same traits class, xxx<T>. The name of a policy is valuable when selecting it from among the set of available policies and to clarify intent in a specialization, but in use, its alias -- the template parameter name -- is important.
-- Rob Stewart stewart@sig.com Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer; _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost