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Subject: Re: Wording strategy for Unicode std::format
From: Hubert Tong (hubert.reinterpretcast_at_[hidden])
Date: 2021-04-16 12:15:45


On Fri, Apr 16, 2021 at 1:08 PM Peter Brett via SG16 <sg16_at_[hidden]>
wrote:

> Hi all (esp. Victor),
>
> We discussed adding C++23 support for homogeneous formatting in UTF-8,
> UTF-16 and UTF-32. For C++23, we would like to allow UTF-8 format
> strings with UTF-8 substitutions, UTF-16 format strings with UTF-16
> substitutions, etc. In a future version of the standard (where UTF
> transcoding is guaranteed to be available) we would like to extend this
> to allowing e.g. UTF-32 substitutions into UTF-8 format strings.
>
> Victor: {fmt} already does exactly this, right?
>
> As far as I can tell, most of the wording is already in place for this,
> and it will only be necessary to mandate the addition of specific
> overloads and template specialisations.
>
> My current sticking point is the way we have specified the
> locale-specific form (with the `L` option). Take the `{L}` substitution
> for bool, for example. In P1892 I chose to specify this in terms of
> std::numpunct<charT>, but the standard only requires the standard
> library to provide numpunct<char> and numpunct<wchar_t> specializations.
> Similar problems arise for `L` with other standard format specifiers.
>
> How can we word this so as to make `{L}` substitutions for UTF-8/16/32
> formatting conditionally-supported, depending on whether the
> implementation provides the necessary specializations of <locale>
> facilities?
>

Isn't the "first problem" that the current wording potentially treats
"plain" char as UTF-8 regardless of locale?

>
> Advice appreciated.
>
> Peter
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