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From: ruben safir <ruben@mrbrklyn.com>
Newsgroups: comp.lang.c++
Subject: Re: base_ostream template arguments
Date: Mon, 13 Jun 2011 06:38:19 +0000 (UTC)
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On Sun, 12 Jun 2011 22:18:03 -0700, Öö Tiib wrote:

> On Jun 13, 7:40 am, ruben safir <ru...@mrbrklyn.com> wrote:
>> The base_ostream template is defined as follows (according to
>> Struostrup)
>>
>> template<class Ch, class Tr = char_traits<Ch> > class
>> std::basic_ostream : virtual public basic_ios<Ch, Tr > { public:
>>         virtual ~basic_ostream();
>> //...
>>
>> };
>>
>> since Tr is always defined as a function of Ch, why was it necessary
>> for two template parameters?
> 
> Tr is not always defined as char_traits<Ch>, it is default defined as
> char_traits<Ch>. You can put there your own custom-made traits as second
> template argument.


It's very interesting.  The very next template is for basic_ios which 
Strousup defines as this


template <class Ch, class TR = char_traits<Ch> >
public:
	typedef Ch char_type;
	typedef Tr traits_type;
	typedef typename Tr::int_type int_type;
	typedef typename Tr::pos_type pos_type;
	typedef typename Tr::off_type off_type;
};


It also defaults to the same type.  But what i find interesting and new
is the last 3 typedef's

	typedef typename Tr::int_type int_type;
	typedef typename Tr::pos_type pos_type;
	typedef typename Tr::off_type off_type;

There is no explanation to how any of these would be used and I've never
seen the keyword typename used outside of <>.  How does that work?

Ruben