sdm@cs.brown.edu (Scott Meyers) (04/09/91)
Regarding direct access in input streams, the istream man page says:
insp=&ins.seekg(off,dir)
Repositions ins.rdbuf()'s get pointer. See
sbuf.pub(3C++) for a discussion of positioning.
insp=&ins.seekg(pos)
Repositions ins.rdbuf()'s get pointer. See
sbuf.pub(3C++) for a discussion of positioning.
pos=ins.tellg()
The current position of ios.rdbuf()'s get pointer.
See sbuf.pub(3C++) for a discussion of position-
ing.
Okay, this is from the sbuf.pub man page:
pos=sb->seekpos(pos, mode)
Repositions the streambuf get and/or put pointer to
pos. mode specifies which pointers are affected as for
seekoff(). Returns pos (the argument) or EOF if the
class does not support repositioning or an error
occurs. In general a streampos should be treated as a
"magic cookie" and no arithmetic should be performed on
it.
So how do I implement a binary search using iostream I/O? I need to be
able to get two stream positions (corresponding to the top and bottom of
the region I'm about to bisect) that I can perform arithmetic on. For
example, during the first iteration of the search, I want to go to the top
of the file and get a streampos, call it TOP, then to the BOTTOM of the
file to get another streampos, call it BOTTOM, then I want to jump to the
position in the file corresponding to (BOTTOM-TOP)/2. However, the man
page says that streampos objects don't have to obey the laws of arithmetic.
What do I do?
Thanks,
Scott
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What do you say to a convicted felon in Providence? "Hello, Mr. Mayor."jss@gold.kpc.com (Jerry Schwarz) (04/10/91)
In article <71329@brunix.UUCP> sdm@cs.brown.edu (Scott Meyers) writes:
So how do I implement a binary search using iostream I/O? I need to be
able to get two stream positions (corresponding to the top and bottom of
the region I'm about to bisect) that I can perform arithmetic on. For
example, during the first iteration of the search, I want to go to the top
of the file and get a streampos, call it TOP, then to the BOTTOM of the
file to get another streampos, call it BOTTOM, then I want to jump to the
position in the file corresponding to (BOTTOM-TOP)/2. However, the man
page says that streampos objects don't have to obey the laws of arithmetic.
What do I do?
The iostream interface doesn't allow you do to this because it can't
be done portably. On operating systems with record oriented file
systems there is just no way of doing arbitrary arithmetic on file
positions. If you aren't worried about portability to such systems
and the system you're on defines |streampos| as an integral type (as
some do) then you can do arithmetic to your hearts content.
Jerry Schwrzmjv@objects.mv.com (Michael J. Vilot) (04/10/91)
>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