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Subject: Re: IOStreams behavior definition
From: Peter C++ (peter.cpp_at_[hidden])
Date: 2020-06-30 12:53:14


Width is only considered for the very next IO operation. Your C++ training should have told you so. See also cppreference.com

sent from a mobile device so please excuse strange words due to autocorrection.
Peter Sommerlad
peter.cpp_at_[hidden]
+41-79-432 23 32

> On 30 Jun 2020, at 18:35, Philip Prindeville via Std-Discussion <std-discussion_at_[hidden]> wrote:
>
> Hi.
>
> I’m trying to figure out what the appropriate behavior of std::setw() and std::ios_base::width() should be on a istream. It doesn’t seem to be well defined.
>
> If I have:
>
> istringstream is(“0123456789”);
> unsigned u1, u2;
>
> is >> setw(2) >> u1 >> u2;
>
> cout << u1 << ‘ ‘ << u2 << endl;
>
> I’d expect the output to be “1 23\n” with “4” being the next character on the input stream.
>
> Not the case.
>
> g++ and clang++ both try to consume all available digits into the first argument regardless of width, even overflowing if there are too many digits.
>
> In short, there’s no way to do columnar input if there are no delimiter characters between numbers.
>
> You’d thing this would be a fairly common/obvious usage scenario but it doesn’t seem to be well defined in the IOStreams specification.
>
> http://www.cplusplus.com/reference/iomanip/setw/
>
> Refers to both input and output streams but only describes output behavior concretely.
>
> http://www.cplusplus.com/reference/ios/ios_base/width/
>
> Its homologue width() on the other hand makes no mention of input at all.
>
> Can some clarity be shed on this?
>
> Thanks,
>
> -Philip
>
> --
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> Std-Discussion_at_[hidden]
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