On Thu, Jun 17, 2021 at 4:57 PM Peter Brett via SG16 <sg16@lists.isocpp.org> wrote:
Hi all,

The current proposed resolution for LWG3565 (https://wg21.link/LWG3565)
involves transcoding from the locale encoding to UTF-8.  This makes me a
little uncomfortable.

Is it possible instead to say that, if the string literal encoding is
UTF-8, then the effective locale is _as if_ the specified or global
locale was modified by replacing the associated codeset with UTF-8?

So, the following code:

    std::locale l1("Russian.1251");
    auto s = std::format(l1, "День недели: {:L}", std::chrono::Monday);

Would behave as if replaced by:

    std::locale l1("Russian.1251");
    std::locale l2(l1, std::locale("Russian.UTF-8"), locale::time);
    auto s = std::format(l2, "День недели: {:L}", std::chrono::Monday);

This would permit an implementation that has UTF-8 locale data available
to use it directly, rather than being required to use the 1251 codeset
locale data and transcode in order to conform to the standard.

                             Peter

P.S. How would one go about writing a locale object that customizes
chrono formatting with std::format?  Does anyone have a code sample?

The nl_langinfo CODESET element is an aspect of the LC_CTYPE facet. I don't think the C++ layer has much control over the mapping. The `localedef` is used in POSIX to define custom locales for use with C. The "charmap" format provides for custom names. Not all implementations require a mapping to the UCS to be provided.

My understanding is that the implementation of iconv_open need not support all charmaps used with locales on the system.
 
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