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Subject: Re: IOStreams behavior definition
From: Philip Prindeville (philipp_subx_at_[hidden])
Date: 2020-06-30 16:35:10


> On Jun 30, 2020, at 12:59 PM, Gennaro Prota <gennaro.prota_at_[hidden]> wrote:
>
>
>> On Tue, Jun 30, 2020, 19:02 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.
>
> It will work as you expect if you extract into a std::string. Not all extractors take into account the width (the string extractor does). Alternatively, you can use the get() member, or, since C++20, extract into an array of chars.

Is it too late to get to-integer extractors to have properly defined behavior in C++20 that includes .width() or setw() ?

>
>>
>> [...]
>>
>> You'd think 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/
>
> If you really must use a non-primary source, I heartedly recommend cppreference.com over cplusplus.com (but, still, double check with the standard).

Ack.

-Philip


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