[boost] Re: iostreams library review

From: David Abrahams (dave_at_[hidden])
Date: 2004-06-04 10:12:32

"Jonathan Turkanis" <technews_at_[hidden]> writes:

> "David Abrahams" <dave_at_[hidden]> wrote in message
> news:ubrk18u0m.fsf_at_boost-consulting.com...
>> "Jonathan Turkanis" <technews_at_[hidden]> writes:
>>
>> > The caveat is that seeking with respect to stream positions -- 
> i.e.
>> > values of type traits_type::pos_type rather than
>> > traits_type::off_type -- only works as expected for positions
> which
>> > are equivalent to integral offsets.
>>
>> You lost me here.  What positions aren't?
>
> positions that record an offset and a code conversion state.

Oh.  Yeah, I know nothing about that stuff.

>> > Free ranging random access doesn't make much sense in multibyte
>> > streams, anyway. There's a more limited type of random access
>> > which is possible in multibyte streams, in which you can save a
>> > position and go back to it later. I've chosen not to support
>> > this, however.
>>
>> Do you mean variable-length-character streams?
>
> I mean streams of characters as they are read from a file with a
> multibyte encoding -- an encoding in which a single character may be
> represented by a sequence of bytes, possibly of varying length -- 
> before code conversion is performed.

OK.

>>  I definitely want to
>> do regular random access on streams of wchar_t, which I suppose are
>> "multibyte".
>
> Random access in wide streams is generally fine. (However, if the wide
> stream is based on an underlying multibyte stream, and the member
> function encoding() of the codecvt used to perform the conversion
> returns a value <= 0, then random-access is rather inefficient.)

:-)

> (Maybe I shouldn't have confused the issues by bringing up code
> conversion. It is an issue, however, whenever you use pubseekpos,
> since positions don't exactly correspond to offsets.)

Ugh.

>
> The offsetbuf could be defined like this (assuming a narrow stream,
> for simplicity):
>
>   class offsetbuf : public streambuf_facade<offset_filter> {
>   public:
>       offsetbuf(std::streambuf& sb, std::streamoff first,
> std::streamoff last)
>        {
>            set_next(sb);
>            open(offset_filter(first, last));
>        }
>   };
>
> Here offset_filter is the filter I described before.

If you say so.  Anyway, the library would be useful to me if it made
that possible.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com