I want to open a file for both reading and writing, but when I do
"ios::in|ios::o ut" with an fstream, it creates the file if it doesn't
already exist. I don't like that last part. C's fopen has the "r+"
mode, where the file will open for reading "+ also writing", but it
will not create a new file if one isn't already there; it fails with
an error. POSIX also has this behavior with it's 'open' call, you just
leave the 'create' bit out, and you can open for read/write without
creating.
But I can't figure out how to do this with iostreams. The only
ios::openmode values I know of are: app, ate, binary, in, out,
trunc... and none of them have to do with create-if-doesn't-exist. So
far as I can tell, with iostreams, that is implicit in ios::out,
whether you want it or not.
Is there a way, or do I have to open the file for reading to check
that it exists, then close it and reopen it for read/write?
Thanks in advance for any advice.
--
Dave O'Hearn
Jul 22 '05
10  6723 
"Chuck McDevitt" <Ch************ @c-o-m-c-a-s-t.net> wrote: You are obviously on Windows. As far as I know, it's implementation
defined if the file gets created or not.
In the Windows implementation it does, on most Unix systems it does not.
Windows has a ios::_Nocreate you can set.
I found out a little more about this. So far as I can tell, you are
right about it being implementation defined to create the file or not.
But it seems that all (or most) classic iostreams implementations
create the file and have some form of ios::nocreate to suppress it,
while all standard iostreams implementations will not create the file
when you open for in|out mode.
This is somewhat more satisfying for me to rely on... "All new
compilers do it one way," was sketchy, but "Really every standard
iostreams implementation does it this way, and really every outdated
classic iostreams implementation does it the other way," makes me feel
comfortable relying on the behavior of the file not being created, and
just saying my application requires standard, not classic, iostreams.
Still I can't find any text in the actual Standard that promises it,
unfortunately.
--
Dave O'Hearn This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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