Re: problem with seekg
On Mar 28, 9:21 pm, "Julian" <jul...@nospamtamu.edu> wrote:
I am having problems with a function that I have been using in my program=
to
read sentences from a 'command file' and parse them into commands. the
surprising thing is that the program works fine on some computers and not=
so
fine on others. I tried debugging and cannot make any sense of it. I
narrowed it down to the seekg function and made this simple program which
(from what I understand) does not seem to be working as expected in all t=
he
computers I have tried so far.
please let me know if I have got something wrong...
#include <iostream>
#include <fstream>
Officially, you also need to include <istream>.
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
This line doesn't compile on my systems. What's _TCHAR? (For
that matter, what's _tmain? I would have expected main here,
and in fact, must use main if I don't want an error at link
time.)
{
ifstream is("test.txt");
char test[256];
is >> test;
cout << test << endl;
is.seekg(-2,ios::cur);
The above line is undefined behavior. In a file opened in text
mode (as yours is), you are only allowed to seek to the
beginning, to the current position, or to a position returned
from a previous call to is.tell.
is >> test;
cout << test << endl;
return 0;
}
here are the different versions of test.txt and their corresponding outpu=
ts
in all the cases I expected that the output would be
createModel
el
About the only way to reliably get this effect is to read
character by character, doing an is.tellg() after each
character.
what am I doing wrong ?
Trying to use direct positioning in a text file.
Generally speaking, they don't call them streams for nothing;
you can get away with some direct positionning in a binary file,
and you can place a "bookmark" to go back to in a text file, but
globally, they are designed for streamed input, i.e. sequential
access. You speak of parsing: all of the parsing technologies I
know are designed to work with sequential input, so I'm not sure
why you want to seek.
If worse comes to worse, read large chunks (or all) of your file
into memory, and use random access there. If you're not afraid
of system dependant issues, you might even consider memory
mapping the file. (Note that in a memory mapped file, you will
see the system specific line terminators.)
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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