Re: Error with ifstream and exceptions

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 8 Feb 2011 04:43:11 -0800 (PST)
Message-ID:
<0ca8961c-cd93-4ca6-95e3-36e8629ea171@d23g2000prj.googlegroups.com>
On Feb 8, 5:19 am, "Paul" <pchris...@yahoo.co.uk> wrote:

"James Kanze" <james.ka...@gmail.com> wrote in message

news:79b5c763-d6dc-4d79-b3e5-d4f4422bb5b7@t8g2000vbd.googlegroups.com...

On Feb 7, 12:59 am, Marco <net...@lavabit.com> wrote:

in the following code I get an error message after the file
is output. What did I do wrong?


Basically, I think you've misunderstood how IO works.

#include <iostream>
#include <string>
#include <fstream>
using namespace std;

int main() {
  ifstream fileIn;
  fileIn.exceptions ( ifstream::failbit | ifstream::badbit );


You don't *ever* (well, almost never) want exceptions on failbit


don't EVER? or almost never?


For text input, don't ever. For some very special cases of
binary input, it might be appropriate. But such cases will only
occur in very advanced programs.

in input. The normal idiom is to read a file until it
fails---the most frequent reason for failure is that you've
reached end of file.


EOF is not a 'failure', it's not only expected but guaranteed
to exist.


The standard says otherwise, and all compilers I've seen
implement this correctly. Trying to read beyond the end of file
results in failbit being set.

You try to encourage the OP to not use failbit but you are
wrong to suggest that this is the norm.


I'm not saying not to use failbit. I'm saying not to generate
an interrupt on failbit.

Even if it was the norm who gives a dam what the norm is , it
doesn't mean he needs to code this way.


Unless he wants his code to work.

  try {
    fileIn.open( "file" ); }
  catch ( ifstream::failure e ) {
    cout << "Error."; }

  string line;
  while ( getline( fileIn, line ) ) {


And when you've no more lines to read?

    cout << line << endl;
  }
  fileIn.close();
  return 0;
}


In practice, the only thing you'd ever use exceptions for with
IO is badbit, which should be set in case of a hardware problem
(disk read error, etc.).


This may be your practise, I don't see what anyone can gain
from ignoring failbit errors.


Who said anything about ignoring failbit? You don't seem to
understand English any better than you understand C++.

Hundreds of online examples use badbit or failbit masks, they
are not all wrong.


I've never seen an example which triggers an exception on
failbit.

--
James Kanze

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