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From: Gennadiy Rozental (gennadiy.rozental_at_[hidden])
Date: 2004-04-01 20:00:38

"Mark Rodgers" <mark.rodgers_at_[hidden]> wrote in message
> 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
> 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.

That's is namely the problem I have with basic_string design. Trais class is
used as policy. Actually it's even worse - char_traits contain partially
functionality that may belong to some kind of policy (for example comparison
method). IMO it should be separated.

> It is quite reasonable for specialisations of traits
> templates to be used as arguments to policy parameters, which is exactly
> what happens when we write
> std::basic_string< char,std::char_traits<char> >
> 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;

Exactly, so we could use complete different class that will define different
trait values. And this is not good IMO.

> so the "traits" parameter is indeed misnamed.
> 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.

IMO it should've been.

> Mark


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