How to format floating point numbers with decimal comma using the fmt library?

Question

I want to use the fmt library to format floating point numbers.

I try to format a floating point number with decimal separator ',' and tried this without success:

#include <iostream>
#include <fmt/format.h>
#include <fmt/locale.h>

struct numpunct : std::numpunct<char> {
  protected:    
    char do_decimal_point() const override
    {
        return ',';
    }
};

int main(void) {
    std::locale loc;
    std::locale l(loc, new numpunct());
    std::cout << fmt::format(l, "{0:f}", 1.234567);
}

Output is 1.234567. I would like 1,234567

Update:

I browsed the source of the fmt library and think that the decimal separator is hard coded for floating point numbers and does not respect the current locale.

I just opened an issue in the fmt library

Possible duplicate of [c++: Format number with commas?](https://stackoverflow.com/questions/7276826/c-format-number-with-commas) — pandorafalters, Jul 03 '19 at 08:59
@pandorafalters: I want to use the new fmt library. So I guess it is not a duplicate — schoetbi, Jul 03 '19 at 09:03
@pandorafalters that's using `,` as a thousands separator, not as the decimal separator — Caleth, Jul 03 '19 at 09:04
Don't you have to imbue the output stream? E.g. https://wandbox.org/permlink/s4PdRDA8pYzbaDvl vs. https://wandbox.org/permlink/2LwYr8ZmuhwUYy26 — Bob__, Jul 03 '19 at 09:21
@Bob__: Does not work since the formatting is not done by the ostream operator but the fmt library. — schoetbi, Jul 03 '19 at 09:22
Also looks like this was fixed in `fmt` 5.3 see [release notes](https://github.com/fmtlib/fmt/releases/tag/5.3.0). Which version do you use? — Marek R, Jul 03 '19 at 09:51
@MarekR: No, fmt only respects the locale when using n for integers to chose which thousand separator should be used. (https://github.com/fmtlib/fmt/releases/tag/5.3.0) Improved Locale Support. I want to format a floating point number and this is done using the grisu algorithm. And there the decimal separator is hard coded. See above location. — schoetbi, Jul 03 '19 at 09:54
@MarekR: Did it already: https://github.com/fmtlib/fmt/issues/1218 — schoetbi, Jul 03 '19 at 09:57
"does not respect the current locale" Global state is evil. They however should accept an optional locale argument. — n. 1.8e9-where's-my-share m., Jul 03 '19 at 10:10
@n.m. some global state is less evil than others. Not all development occurs in the US — Caleth, Jul 03 '19 at 10:55

score 5 · Accepted Answer · edited May 16 '21 at 02:21

5

The fmt library made the decision that passing a locale as first argument is for overwriting the global locale for this call only. It is not applied to arguments with the f format specifier by design.

To format floating point number using locale settings the format specifier L has to be used, for example:

std::locale loc(std::locale(), new numpunct());
std::cout << fmt::format(loc, "{0:L}", 1.234567);

The L format specifier supports floating-point arguments as of revision 1d3e3d.

edited May 16 '21 at 02:21

vitaut

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answered Jul 03 '19 at 11:58

schoetbi

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@Bob__: I updated the solution and added a link to [#1084](https://github.com/fmtlib/fmt/issues/1084). – schoetbi Jul 04 '19 at 13:13

How to format floating point numbers with decimal comma using the fmt library?

Update:

1 Answers1