Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!sdd.hp.com!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!unixhub!ditka!zinn!nuucp
From: mjv@objects.mv.com (Michael J. Vilot)
Newsgroups: comp.lang.c++
Subject: RE: Binary Search Using seekg/tellg
Message-ID: <1273@zinn.MV.COM>
Date: 10 Apr 91 11:42:20 GMT
Sender: news@ditka.chicago.com (Pulitzer at ditka)
Lines: 32

>From brunix!sdm
>Newsgroups: comp.lang.c++
>Message-ID: <71329@brunix.UUCP>
>Organization: Brown University Department of Computer Science
>
>So how do I implement a binary search using iostream I/O? ... the 
man
>page says that streampos objects don't have to obey the laws of 
> arithmetic.
>What do I do?

Scott,

The semantics of tellg() and seekg() are more analogous to the ANSI C 
library functions fgetpos() and fsetpos() than to ftell() and 
fseek(), and the type 'streampos' is akin to 'fpos_t'.

The wording of the ANSI C standard is very careful to permit 
implementation of its library functions on non-Unix systems.  We've 
received comments on the version of iostreams proposed for the C++ 
standard that we'd better be just as careful in our semantics.

To solve your specific problem, I'd suggest that you derive a new 
kind of streambuf that knows how to do seeking arithmetic on your 
system, and use that.  You can probably implement it in a way that 
works on all UNIX systems, and localizes your porting efforts if you 
have to move onto a non-UNIX platform (best case would result in a 
sibling class for the new platform).

Hope this helps,

Mike