Re: [boost] Re: Policy or Trait?

From: Rob Stewart (stewart_at_[hidden])
Date: 2004-04-02 11:17:09

From: "Gennadiy Rozental" <gennadiy.rozental_at_[hidden]>
> "Mark Rodgers" <mark.rodgers_at_[hidden]> wrote in message
> news:002601c41836$33148560$0100a8c0_at_cadenza.co.nz...
> >
> > 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.

std::char_traits is a traits class.  It also happens that
std::basic_string accepts a policy for controlling how it does
various things.  For expediency, std::char_traits provides the
interface expected of std::basic_string's policy class.  Thus,
std::char_traits is used as the default policy class for
std::basic_string.  That doesn't stop std::char_traits from being
a traits class.  You can still write code that references --
without making it a template parameter --
std::char_traits<T>::foo.

> > 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.

There's nothing wrong with that.  It is std::basic_string's
interface that provides for the use of a policy class.  That
std::char_traits happens to be the most appropriate -- it rarely
isn't -- implementation of that policy is beside the point.  You
can argue that std::basic_string shouldn't have such a policy
parameter, and you'd probably be right, but that's not
std::char_traits' fault.

-- 
Rob Stewart                           stewart_at_[hidden]
Software Engineer                     http://www.sig.com
Susquehanna International Group, LLP  using std::disclaimer;