[boost] [iostreams] Add char_traits to the Device concept

Subject: [boost] [iostreams] Add char_traits to the Device concept
From: thomas weidner (thomas001le_at_[hidden])
Date: 2010-03-14 12:20:39

Hi,
i came across this issue when trying to use iostreams with a custom 
character class and custom character traits. The main problem is, that 
iostreams assumes std::streampos as the position type for all devices. 
Some parts of iostreams, like detail::indirect_streambuf honor the char 
traits' custom pos_type, but this will cause build failures as the 
underlying device (even code_converter) uses std::streampos as position 
type.

My proposal is to add an associated character traits type to a Device. A 
Device type whould therefore not only provide information about the 
underlying character type, but also about the used pos_type, off_type 
and code conversion (not) done.

I would even use traits different from 
std::char_traits<Ch>/iostreams::char_traits<Ch> as default character 
traits for most devices. Most devices like for example 
file_descriptor_sink don't do any code conversion, so a pos_type which 
contains a mbstate_t is useless and leaves the user with the impression, 
that code conversion *is* done. A simple integral type like C's off_t 
should do fine. Let's call those traits binary_char_traits for now.

As an implication of this a 
iostreams::stream_buffer<file_descriptor_sink> should use the Device's 
char traits type, so should be a std::basic_streambuf<char, 
binary_char_traits<char> >. Especially it whould not be useable with 
std::iostream. This is IMO a nice and very clean result, as an iostream 
expects characters on which code conversion has been performed. You 
whould need to plug a code_converter in between.

Some thoughts about iostreams::code_converter. Perhaps we should change 
the Codecvt template parameter to a second traits type. This traits type 
will become the code_converter's associated char traits type. the 
codecvt type could be an extra template parameter or is simply 
std::codecvt<code_converter_char_type, underlying_device_char_type, 
codecvt, code_converter_traits_state_type>. this whould be the same 
mechanism as code conversion in the stdlib works (using std::codecvt 
specialization).

Comments to my thoughts are very welcome.

--- Thomas