From: "Mark Rodgers" <mark.rodgers@cadenza.co.nz>
Well I think in terms of code, and it really is quite simple IMHO:
Traits look like this:
template< typename T > class SomeTraits { // Stuff that tells us about T };
and Policies look like this
template< typename Policy > class SomeThing { // Delegates some behaviour to Policy };
IOW, traits are class templates and policies are template parameters so there really can't be any confusion between the two. So in the case
I think that misses an important traits trait 8^) that I didn't communicate well: for a given namespace/library, there is only one traits class for a given set of attributes. You can create other traits that collect different information, collect the same information with a different interface, but for a given interface and set of information, there's only one traits class.
std::basic_string
template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string {
"char_traits" is a traits class template but "traits" is actually a policy template parameter. It is quite reasonable for specialisations of traits templates to be used as arguments to policy parameters, which is exactly
That's reasonable only if the traits class provides the semantics expected of the policy which is, obviously, the case in your basic_string example.
what happens when we write
std::basic_string< char,std::char_traits<char> >
I agree.
But I don't think there is anything in basic_string that requires the template argument to be a specialisation of a class template. I think it would be quite legitimate to write
class MyFunkyCharBehaviour { /*...*/ }; typedef std::basic_string<char,MyFunkyCharBehaviour> MyFunkyString;
so the "traits" parameter is indeed misnamed.
Right.
However std::char_traits is most definitely a traits template and doesn't *have* to be used as a policy. In fact std::basic_string could have been written to exclusively use char_traits *instead* of a policy.
Right. -- Rob Stewart stewart@sig.com Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer;