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From: Ian Collins <ian-news@hotmail.com>
Newsgroups: comp.lang.c++
Subject: Re: Traits in STL
Date: Sun, 17 Jul 2011 11:46:55 +1200
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On 07/17/11 11:36 AM, Narinder wrote:
> Hello
>
> I am somewhat curious about the reason for the way types in the STL
> are dependant on trait classes. For example the template class
> basic_string is defined as :
>
> template<class charT,
>           class traits = char_traits<charT>,
> 		 class Allocator = allocator<charT>  >
> 		>
> class basic_string;
>
> but not as:
>
> template<class charT,
>           template<class>  class traits_t = char_traits,
> 		 template<class>  class Allocator_t = allocator>
> 		>
> class basic_string
> {
> 	typedef traits_t<charT>  traits;
> 	typedef Allocator_t<charT>  Allocator;
> ....
>
>
> You would think (or perhaps not) that the second approach is less
> error prone since it would be less likely that your traits would be
> inconsistent with the char type. The only reason I can think of is
> that you may have a traits class specifically for char which wouldn't
> make sense for any other types and thus wouldn't need to be a
> template. In that case you could always write it as template class and
> use traits to activate it only for char types.

Another reason could be a lot of compilers didn't support template 
template parameters when the standard was published (as an aside, I 
don't think template template parameters are used anywhere in the 
current standard library).  Not being able to support std:;string would 
have been a bit of a problem!

-- 
Ian Collins