Re: [boost] Policy or Trait?

From: Mark Rodgers (mark.rodgers_at_[hidden])
Date: 2004-04-01 17:10:52

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.  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;

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.

Mark