From: "Andy Little" <andy@servocomm.freeserve.co.uk>
"David Abrahams" <dave@boost-consulting.com> wrote
I simply reject the idea that a trait specialization is part of the definition of the type(s) it applies to.
hmm ..From the latest qualifications of your objections above I now assume you are seeing this.
struct traits<T> { ... }; Where traits is the subject, traits<T> is Not part of T's definition.
But we have been discussing numeric::converter and std::basic_string and differences between traits and policies... I have been seeing this :
template <typename T> struct T1{ typedef traits<T> arg_traits; }; Where T1 is the subject, traits<T> Is part of T1's definition. Also see standard 21-2 relating to strings. " <strings> ..need a set of related types... to complete the definition of their semantics".
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.
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;