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[libc++][format][chrono] Subsecond part of duration is not correctly formatted #62082

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frederick-vs-ja opened this issue Apr 12, 2023 · 3 comments
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[libc++][format][chrono] Subsecond part of duration is not correctly formatted #62082

frederick-vs-ja opened this issue Apr 12, 2023 · 3 comments
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@mordante
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libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi.

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@frederick-vs-ja
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Example

#include <chrono>
#include <cstdint>
#include <format>
#include <iostream>

int main()
{
    std::cout << std::format("{0:%M:%S} {0}\n", std::chrono::duration<int, std::ratio<101, 103>>{40});
    std::cout << std::format("{0:%M:%S} {0}\n", std::chrono::duration<int, std::ratio<1, 1024>>{1511});
}

Current output (Godbolt link):

00:39.000023 40[101/103]s
00:01.0000000487 1511[1/1024]s

Expected output:

00:39.223300 40[101/103]s
00:01.4755859375 1511[1/1024]s

It seems that we should convert (via duration_cast) __fraction to precision before writing its count().

llvm-project/libcxx/include/__chrono/formatter.h

Line 87 in 9d4df97

auto __fraction = __value - chrono::duration_cast<chrono::seconds>(__value);

llvm-project/libcxx/include/__chrono/formatter.h

Lines 101 to 109 in 9d4df97

std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
_LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}.0f}"),
__fraction.count(),
chrono::hh_mm_ss<__duration>::fractional_width);
else
std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
_LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}}"),
__fraction.count(),
chrono::hh_mm_ss<__duration>::fractional_width);

@EugeneZelenko EugeneZelenko added libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. and removed new issue labels Apr 12, 2023
@mordante mordante self-assigned this Apr 12, 2023
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mordante commented May 6, 2023

https://reviews.llvm.org/D149868 should fix this.

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The author of the previous fix seems to be MIA, instead I created https://reviews.llvm.org/D154851

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The patch has landed.

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