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From: cline@cheetah.ece.clarkson.edu (Marshall Cline)
Newsgroups: comp.lang.c++
Subject: Re: while (cin >> ch) ... when does it stop?
Message-ID: <CLINE.90Sep24232239@cheetah.ece.clarkson.edu>
Date: 25 Sep 90 04:22:39 GMT
References: <4cecce18.20b6d@apollo.HP.COM>
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Reply-To: cline@sun.soe.clarkson.edu (Marshall Cline)
Organization: (I don't speak for the) ECE Dept, Clarkson Univ, Potsdam, NY
Lines: 53
In-Reply-To: geiser@apollo.HP.COM's message of 20 Sep 90 17:44:00 GMT
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In article <4cecce18.20b6d@apollo.HP.COM> geiser@apollo.HP.COM (Wayne Geiser) writes:
...
> #include <iostream.h>
> main() {
>     char ch;
>     int ct = 0;
>     while (cin >> ch)
>	   ct++;
>     cout << ct << " characters\n";
>     return 0;
> }
...
> 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

The `operator void*()' is the one you're getting in the above loop.
The while loop wants a truth value, which means either an `int' which
can be compared against `0', or a pointer that can be compared against
the zero pointer.  Since one (and only one) of these two choices is
available, it chooses to use `operator void*()'.

> 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.

I documented this bug in TC++'s library in my review for The C++ Report
(the issue was this last July, I believe).  Unfortunately due to copy
limitations (as well as ambiguity in the AT&T specs), it had to be
dropped.  I still have it recorded in my TC++ bug list (anonymous ftp
from sun.soe.clarkson.edu [128.153.12.3] in ~ftp/pub/Turbo-C++/bug-list).
The ambiguity with the AT&T specs involves whether it is `bad()', or
`fail()' or some other mechanism that ought to be triggered on `eof()'.
Regardless, everyone (including Borland) seems to agree that the idiom
you reproduced above ought to work; it's just the exact mechanics of
getting it to work that are dubious.

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

I'm sure you're aware that operator>> skips whitespace but get(ch) doesn't.

Marshall Cline

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Marshall Cline / Asst.Prof / ECE Dept / Clarkson Univ / Potsdam, NY 13676
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