Re: std::ws

From:
James Kanze <kanze.james@neuf.fr>
Newsgroups:
comp.lang.c++.moderated
Date:
9 Jul 2006 17:06:10 -0400
Message-ID:
<e8rcai$326$1@nntp.aioe.org>
Ulrich Eckhardt wrote:

I'm parsing some text with a stringstream and came across
something that struck me as odd. I'm testing if a string
contains a number (surrounded by optional whitespace) and
different stdlibs behave differently.

unsigned n;
in >> std::ws >> n >> std::ws;
if( !in.fail() && in.eof())
   ...//it's a number


Just a nit, but the first >> std::ws isn't necessary.

Using "10" as input, the problem I have is that after the
unsigned value is read the streamstate is eof. The following
'>> std::ws' then causes the streamstate to be fail|eof, at
least with some implementations.

My prerelease copy of the standard says explicitly that ws
sets eofbit but not failbit if no more characters are
available, but it doesn't explicitly mention what happens when
the eofbit is already set when it is invoked.

My guess is that it should detect eofbit and and simply return
instead of setting failbit, but I'd like to clarify that.


I'd say that that is certainly the intent. I can't see any
other way of reading ?27.6.1.4/1.

What is probably happening is that a sentry object is being
constructed. Strictly speaking, I don't see anything to forbid
this, although I don't think it corresponds to the intent of
manipulators. (Note that in this case, the >> operator is NOT
formatted input, which should fail if eosbit is set on entry.)

Just out of curiousity, could you indicate on which compilers it
fails. I use the idiom a lot, and I've never had any problem
(but I don't use a large variety of different compilers in my
current work).

--
James Kanze kanze.james@neuf.fr
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