On Sun, 31 Mar 2024 at 10:55, Jonathan Wakely <cxx_at_[hidden]> wrote:

>
>
> On Sun, 31 Mar 2024 at 10:42, David Armour via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> I encountered an issue using std::format to format numbers which I
>> wanted to have a custom comma thousand separator in the display.
>>
>> When using the customised locale with MSVC I discovered the added commas
>> pushed the formatted number to the right making the number no longer
>> conform to the width specifier.
>>
>> I tested the same code with GCC and found std::format() to conform
>> correctly.
>>
>> I also tested both MSVC and GCC using std::cout.imbue() to make the
>> stream format the numbers to my custom locale. In this instance it
>> worked correctly on both compilers.
>>
>> I tried to find some info about this problem and it seems that for
>> streams, the standard requires the use of std::setw() to include the
>> custom locale but for std::format it is left to the compiler implementers.
>>
>
> Why did you conclude that?
>
> It seems obvious to me that any digit separators inserted for a
> locale-specific form should be included in the estimated field width.
>
> "The estimated field width is the number of field width units that are
> required for the formatted sequence of characters produced for a format
> argument independent of the effects of the width option."
>
>
>
>
>> I would suggest that the standard forces the same requirement to manage
>> the custom locale in the width specifier for std::format as well as for
>> imbue() on streams.
>>
>
> I think it already requires the behaviour you expect. If MSVC doesn't
> count digit separators in the estimated field width, that seems like a bug.
>
>
>
>>
>> A code sample that reproduces the problem is found below. As mentioned,
>> this needs to be compiled on MSVC to see the problem. I have not tested
>> any other compilers to see if they have the same behaviour as MSVC.
>>
>
For your example code, LLVM's libc++ produces the same output as GCC.