NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/std/c++/775

Internet Message Format | 1996-08-06 | 3.9 KB

From: [e-mail] (Steve Clamage)
Message-ID: <4ikmol$[e-mail]>
X-Original-Date: 18 Mar 1996 22:06:13 GMT
Path: in2.uu.net!bounce-back
Date: 19 Mar 96 01:16:50 GMT
Approved: [e-mail]
Newsgroups: comp.std.c++
Subject: Re: FOR comp.std.c++: problems with I/O except
Organization: Sun Microsystems Inc.
References: <[e-mail]>
Reply-To: [e-mail]
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In article [e-mail], Nico Josuttis <[e-mail]> writes:
>Hi,
>Trying the new feature of throwing stream exceptions
>i wrote the following program:

     [ trimmmed ]

>    try {
>        int value, sum;
>
>        /* while not EOF
>         * read and add value
>         */
>        sum = 0;
>        while (cin >> value) {
>            sum += value;
>        }
>
>        cout << "sum: " << sum << endl;
>
>    }
>    catch (ios::failure error) {
>        cerr << "Error: " << error.what() << endl;
>        exit (EXIT_FAILURE);
>    }

>But it didn't work as I expected, because also on EOF
>I ALWAYS got the failbit exception!

>The problem is, that istream::ipfx() always calls setstate(failbit),
>if after any preparation good() is false.
>So even if only the eofbit is set (due to the skip of whitespace),
>the failbit is also set.

That is the desired behavior, and your program should take it into account.

For example, suppose the file ends with whitespace (which it often will,
if only because of a final newline before the EOF). You attempt to read
an int value, whitespace is skipped, and EOF is reached before any
valid characters are found. That is a failure, by definition. The failure
cannot be predicted before starting the input, since characters are
available at the start. As it happens, you are not interested in any
of the characters which are available.

Now suppose that EOF is already true when starting the input. Again by
definition, input fails, since no characters are available for reading.

The ipfx() function is called as part of starting an input operation.
You are asking for input, and to be notified if the input fails. It fails
-- no input available -- and you are notified.

When you have a loop like
    while( cin >> value ) { ... }
it will ALWAYS end by failing, since the loop only ends by finding improper
input or reading past EOF, and reading past EOF is a failure. Thus,
you probably do not want to use an exception to detect the end of the
loop.

> ... In general I need a chance to handle eof without getting
>failbit exceptions.

I think it is more likely that you don't want to throw an exception on
failure unless failure is not expected to happen. When reading input up
to the end, failure will always occur. A schema like the following
gives you the control you want without using exceptions:

    while( cin >> value ) {
        ... do something with value
    }
    if( cin.eof() ) ... no more input available
    else ... error in input

More likely, you would write a single test like this:
    if( ! cin.eof() ) ... error in input

Exceptions are best used for those cases where you don't expect
them to be triggered, and you need to escape to a higher level in the
program when they do occur. Exceptions give you a way to escape without
having to pass error status explicitly through a chain of function calls.
Don't use exceptions for normal loop control; they are inefficient for
that purpose and cause you to write more complicated code instead of
less complicated code.

---
Steve Clamage, [e-mail]
---
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