[boost] Re: iostreams library review

From: Jonathan Turkanis (technews_at_[hidden])
Date: 2004-06-02 18:11:48

"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.

>
> > 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.

>  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.)

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.

Jonathan