On Jun 30, 9:26 am, "Alf P. Steinbach" <al...@start.no> wrote:
> * nko...@gmail.com:

> > When outputting or inputting to a file is there some easy
> > way to move to and overwrite a specific line?

> No, not if you're talking about an ordinary text file.

If you know the length of the line, there is. You have to save
the position prior to reading the line.

> That hasn't anything to do with C++, it has to do with the
> structure of an ordinary text file.

Not really. The C++ standard says very clearly what you can and
cannot do with regards to seeking in a text or a binary file.
(Actually, I think I'm lying about "clearly". The C++ standard
defines pretty much everything concerning IO by reference to the
C standard, which means that you usually have to look in two
different standards. And even when it doesn't, "clear" and "the
C++ Standard" seems to be a oxymoron.)

> > With seekg it seems you can only move positions without
> > knowing whether you've moved to a newline or not.

> Yes.

The one argument form of seekg just calls seekpos on the
streambuf. If the streambuf is in fact a filebuf, "If sp has
not been obtained by a previous successful call to one of the
positioning functions (seekoff or seekpos) on the same file the
effects are undefined." The two argument form calls
streambuf::seekoff, which for a filebuf is defined "as if" you
called fseek, which (in the C standard) says: "For a text
stream, either offset shall be zero, or offset shall be a value
returned by an earlier successful call to the ftell function on
a stream associated with the same file and whence shall be
SEEK_SET."

If you want to move around in an arbitrary fashion in a text
file, seeking according to the line number, you have to first
read the entire file (using getline, for example), saving the
results of ftell in an array.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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