Re: [Alephmodular-devel] CFileDesc landed

[Br'fin <br...@ma...>]

On Wednesday, February 12, 2003, at 06:45  PM, Br'fin wrote:
>>> Ah, I didn't realize that that processing was a feature of AlephOne 
>>> specifically. I'll have to look into it. Does A1 deal with a 
>>> MacBinary of a map file too? Or does that still need to be 
>>> explicitly split out into two files?
>>
>> It looks like (though I have not tested this myself) that it should 
>> indeed be able to read both forks from the single MacBinary file, no 
>> need to split.
[snip]
> That's just cute. Insanely cute :)
>
> Hsm. I'm not sure how on earth I would do that with iostreams. I mean, 
> I know how I might do it, but I honestly have no idea as to whether I 
> can do it that way, I don't know which kinds of methods I can override 
> safely. Durn C++.
>

Pester some folks on the newsgroups, and I think I understand what I 
would need to do on my end now.

-Jeremy Parsons

Newsgroups: comp.lang.c++
Subject: Re: Limiting an ifstream to part of a file?
Simon Turner wrote:

Br'fin wrote:
 >  "White Wolf" wrote:
 >
 > > "Br'fin" wrote:
 > > > I'm trying to find a safe way to open an ifstream or equivalent
 > > > subclass, but set it up to that it is limited to only a portion 
of a
 > > > file.
 > > >
<snip>
 > >
 > > IMHO you will need to make a custom stream buffer to achieve that.
 > >
 > > WW
 >
 > Offhand, subclassing filebuf looks straightforward. As would
 > interfering with its seek and other methods to treat the artificial
 > bounds limits as actual ones.
 >
 > But I'm having trouble understanding how I would use this subclass
 > with an ifstream.

You can create an ifstream to use with your custom streambuf by
passing a pointer to the constructor, like:

   my_restricted_filebuf mf( "file.ext" );
   std::ifstream restricted_file( &mf );

If you have an existing ifstream you want to redirect to your custom
streambuf, you can use:

   my_file.rdbuf( &mf );

 >
 > It seems like I would need to ditch ifstream and use the
 > basic_ifstream code as a sample or nearly verbatim for writing my
 > own istream subclass

Fortunately not, although you need to take care of the streambuf's
lifetime if you do it this way.

 > :/

PS.I just looked this up in my copy of "The C++ Standard Library", by
Nicolai Josuttis. If you're doing a lot of tricky iostreams work, it
might be time to go shopping.
;-D

 >
 > -Jeremy Parsons