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From: geiser@apollo.HP.COM (Wayne Geiser)
Newsgroups: comp.lang.c++
Subject: while (cin >> ch) ... when does it stop?
Message-ID: <4cecce18.20b6d@apollo.HP.COM>
Date: 20 Sep 90 17:44:00 GMT
Sender: root@apollo.HP.COM
Reply-To: geiser@apollo.HP.COM (Wayne Geiser)
Organization: Hewlett-Packard Apollo Division - Chelmsford, MA
Lines: 67

This is a "Should I do as they say or do as they do?" question.

On page 93 of "The C++ Primer," Stanley Lippman presents a program. 
For brevity, I've reduced it to the one below:

#include <iostream.h>

main() {

    char ch;
    int ct = 0;

    while (cin >> ch)
        ct++;

    cout << ct << " characters\n";
    return 0;
}

He intends (and explains) that this should read characters from the
standard input device until eof (among other things) and then the
condition in the while statement should fail.  In fact, on pages
383-384 he says:

"A more general method of reading from the input stream is to make the
 extraction operation the truth condition of a while loop.  For
 example,

        char ch;
        while ( cin >> ch )
            // ...

 reads a character at a time from standard input.  When end-of-file is
 encountered, the truth condition evaluates as false and the loop
 terminates."

The man page for IOS (this is from the AT&T version 2.1 documentation
set) on page 3 says:

"Two operators are defined to allow convenient checking of the error
 state of an ios: operator!() and operator void*().  The latter
 converts an ios to a pointer so that it can be compared to zero.  The
 conversion will return 0 if failbit or badbit is set in an error
 state, and will return a pointer value otherwise."

Note that it will not be set on end-of-file.

I have tried this program on both the AT&T cfront (versions 2.0 and
2.1) and it works are Lippman describes (returns false on eof).  I have
also tried this on Turbo C++ and it loops forever.  Borland's
documentation reads very much like the AT&T man page.

Thoughts?

BTW, I know to use cin.get(ch) to make this work in both environments.


                                Wayne.

Wayne Geiser
Apollo Computer, Inc. - A subsidiary of Hewlett Packard
geiser@apollo.hp.com

"Nolan Ryan is pitching much better now that he has his curve ball
 straightened out."

        - Joe Garagiola.