Appreciate the update on this. I was not aware it was already a
submitted/resolved bug.


On 1/4/2024 06:07, Tam S. B. wrote:
> This is a MSVC STL bug that got fixed recently. The fix is expected to
> be shipped in VS 2022 17.10 preview 3.
>
> The PR that fixes the bug is https://github.com/microsoft/STL/pull/4421
>
> ------------------------------------------------------------------------
> *From:* Std-Proposals <std-proposals-bounces_at_[hidden]> on
> behalf of David Armour via Std-Proposals <std-proposals_at_[hidden]>
> *Sent:* Sunday, March 31, 2024 9:41
> *To:* std-proposals_at_[hidden] <std-proposals_at_[hidden]>
> *Cc:* David Armour <dave.armour_at_[hidden]>
> *Subject:* [std-proposals] Width Formatting using std::format with
> custom locale [user defined do_get_separator()]
> 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.
>
> 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.
>
> 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.
>
>
> ********* CODE BELOW *********
> #include <format>
> #include <iomanip>
> #include <iostream>
> #include <locale>
>
> class MyNumPunct : public std::numpunct<char>
> {
> protected:
> char do_thousands_sep() const override
> {
> return ',';
> }
> std::string do_grouping() const override
> {
> return "\03";
> }
> };
>
> int main()
> {
> double dblValue1 = 1000.0;
> double dblValue2 = 1000000.0;
>
> // This is used as a ruler to easily see if the formatting is correct.
> std::cout <<
> "123456789012345678901234567890123456789012345678901234567890" <<
> std::endl;
>
> // Standard locale
> std::cout << std::format("{:20.2f}{:20.2f}", dblValue1, dblValue2) <<
> std::endl;
>
> // Custom locale with thousands separator.
> // Error in formatting. Numbers are pushed to the right by the commas.
> std::cout << std::format(std::locale(std::locale(""), new
> MyNumPunct), "{:20.2Lf}{:20.2Lf}", dblValue1, dblValue2) << std::endl;
>
> std::cout.imbue(std::locale(std::locale(""), new MyNumPunct));
> std::cout << std::fixed << std::setprecision(2) << std::setw(20) <<
> dblValue1 << std::setw(20) << dblValue2 << std::endl;
> return 0;
> }
>
> ******* CODE ENDS *******
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals