Re: fstream
On May 30, 7:34 pm, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:
Gaijinco wrote:
Ok. I mess up with the << and >> operators using them in the wrong
places.
But, I want to have an fstream object to read something from a file,
and then write something back.
OK.
To my understanding that should be done with:
std::fstream filestr ("test.txt", std::fstream::in |
std::fstream::out);
Right?
Right. You can drop the 'std::fstream::in/out', they are the default.
Have you tried looking whether any errors are reported by the stream
itself?
So if I want to read something I would use the >> operator, like:
filestr >> x;
And to write something I should use:
filestr << x
Is there any other consideration?
The file may not have enough room for writing back.
In which case it will be extended (unless the disk is full).
More likely, the read failed. In general, any time you read to
end of file, all following operations will fail unless you
intercal a call to clear().
Also, the read and write position are common. So what he's
doing will write immediately following where he last read. If
the file originally contained "toto\n", for example, after his
operations, it would contain "totoNot toto". (At least in
practice; it's quite conceivable that on some systems,
overwriting the '\n' in a text file doesn't work quite the way
you'd expect.)
That should be considered... Beyond that, I am not sure. I
actually never used formatted I/O on an fstream. It's always
binary where I come from, or different files for writing than
for reading (and then renaming them after closing).
In practice, there's one exception that works: reading to the
end of file, then clearing the error bit, and writing
(appending) more data. Other than that, if you're going to be
seeking, you'll probably want to use binary---most of the time,
I will, in fact, use the system level interface: open, read,
write, lseek and close in my case.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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