Question: locale treated differently by fmt and std::cout

[mcharneyamp]

Pretty sure this is me being stupid; not a bug. Please see example here:

https://godbolt.org/z/rasnKzfae

When I uncomment line 22, I get what I want: no commas in hex number.

When I comment out line 22, I get commas in my hex numbers which is not what anyone would want.

What's the right way to do not get commas in hex numbers when using fmt and overloaded ostream operators?

Seems like std::out does not "see" the locale in the way fmt does. Is this because std::cout is constructed before main() sets std::locale::global(std::locale("en_US.UTF-8"));?

Thank you for your help. Apologies if this is a stupid question.

#include <cstdint>
#include <locale>
#include <ostream>
#include <iostream>
#include <iomanip>
#include <fmt/core.h>
#include <fmt/ostream.h>

class Foo {
public:
    Foo() = default;
    explicit Foo(std::uint64_t value) : contents(value) {}
    Foo(const Foo& other) : contents(other.contents) {}
    std::uint64_t value() const { return contents; }
    friend std::ostream& operator<<(std::ostream& os, const Foo& addr);
    std::uint64_t contents{0};
};

template <> struct fmt::formatter<Foo> : ostream_formatter {};
std::ostream& operator<<(std::ostream& os, const Foo& addr) {
    auto old_locale = os.getloc();
    os.imbue(std::locale("C"));  // line 22. comment this out and we get commas in hex number
    auto flags = os.setf(std::ios::hex, std::ios::basefield);
    os << addr.value();
    os.setf(flags);
    os.imbue(old_locale);
    return os;
}

int main(int argc, char** argv) {
    std::locale::global(std::locale("en_US.UTF-8"));
    Foo x(0xABCDEF00F);
    std::cout << "std cout: " << x << "\n";
    fmt::print("fmt     : {}\n", x);
}

[vitaut]

Currently ostream_formatter uses the locale passed into a formatting function (or global locale if there is none) but I think it's reasonable to switch the default to classic locale in

fmt/include/fmt/ostream.h

Line 95 in abdb7fd

if (loc) output.imbue(loc.get<std::locale>());

It will be more consistent with other defaults which are locale-independent.

A PR is welcome!

[mcharneyamp]

Thanks, Victor!

[vitaut]

Fixed in 8a39388.

[mcharneyamp]

Thanks, Victor!