Re: Traits in STL

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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 12:08:03 +1200
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On 07/17/11 11:54 AM, Narinder wrote:
> On Jul 17, 12:46 am, Ian Collins<ian-n...@hotmail.com>  wrote:
>> 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!
>
> hmmm ... there something quite disturbing about that. Surely the
> implementation of the compilers should depend on the standard not the
> other way round ...

Imagine where we'd be if the standard library required export....

-- 
Ian Collins

Thread

Traits in STL Narinder <narinder.claire@gmail.com> - 2011-07-16 16:36 -0700
  Re: Traits in STL Ian Collins <ian-news@hotmail.com> - 2011-07-17 11:46 +1200
    Re: Traits in STL Narinder <narinder.claire@gmail.com> - 2011-07-16 16:54 -0700
      Re: Traits in STL Ian Collins <ian-news@hotmail.com> - 2011-07-17 12:08 +1200
        Re: Traits in STL Narinder <narinder.claire@gmail.com> - 2011-07-16 17:13 -0700
          Re: Traits in STL Ian Collins <ian-news@hotmail.com> - 2011-07-17 14:09 +1200
  Re: Traits in STL Qi <no@no.com> - 2011-07-17 09:47 +0800

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