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Re: Deriving from ifstream

On 11 avr, 17:44, "mc" <mc_r...@yahoo. comwrote:
Let me add some reasons to some of the whys. First off, what
I provided is just a small snapshot of a very large file which
parses an old data storage file. I did not include the
documentation with this post as it's north of hundreds of
pages. So the fact the the float is read as an int is very
well detailed in said docs. But that's beyond the point.
OK. If the actual format is well documented, that's half the
battle won already.

Note, however, that reading a float as an int is still very
implementation dependent, since the actual internal format of a
float varies between machines. The documentation should specify
the format of the float in the file, either by reference to some
known format (e.g. IEEE), or by explicitely specifying how the
data is laid out (high bit sign, then 11 bits exposant, in
excess 1023, then mantissa, base 2...---most people just use an
established format).
Then, as I mentioned unclearly, the heart of the problem for
me lies in the constructor: how can I create an le_ifstream
off of an existing ifstream??
In a certain sense, you can't. iostream's aren't copiable.
The usual solution binary I/O is to create a new stream type,
iMyFormatStream , which derives from std::ios (for the error
handling), and define the usual >and << operators on it (but
in a way that they do binary I/O, in your format, and not text
I/O). The basic abstraction of an istream or an ostream is
text, and deriving from it for binary I/O is a potential source
of errors.

All that you use from an existing, open stream, would be the
streambuf. And even then, if the file is pure binary, it's
generally preferable to provide your own derived classes, along
the model of ifstream, and take care of the open yourself. That
way, you can ensure that the filebuf is opened in binary mode,
and that it is imbued with the "C" locale. (Another thing you
have to watch out for---some locales might do code translation.)
The ifstream points to a file opened in binary mode that
actually contains many files stuck one after the other. Each
time a file of a different type is encountered, it's parsed
using an ifstream cast/converted into the relevant derived
class, e.g. le_ifstream. How I read the data is irrelevant.
As long as it is correct. The code you posted had several
serious errors, which would cause it to fail in some specific

James Kanze (GABI Software) email:ja******* **@gmail.com
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Jun 27 '08 #1
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