[英]Lost about std::filesystem's clock conversions, in C++17 lacking clock_cast
A while ago, I managed to do this, to go from filesystem to system clocks, thanks to SO:不久前,我设法做到了,从文件系统到系统时钟到 go,这要归功于 SO:
int64_t toUSsecSinceEpochUTC(std::filesystem::file_time_type ftime) {
// see https://stackoverflow.com/a/35282833/103724
using namespace std::chrono;
auto sys_now = system_clock::now();
auto fil_now = decltype(ftime)::clock::now(); // i.e. C++20's file_clock
auto sys_ftime = time_point_cast<system_clock::duration>(ftime - fil_now + sys_now);
auto sys_ftime_usec = time_point_cast<microseconds>(sys_ftime);
return sys_ftime_usec.time_since_epoch().count();
}
But now I want to do the reverse, and I'm struggling... here's my feeble attempt:但现在我想做相反的事情,我正在挣扎......这是我微弱的尝试:
std::filesystem::file_time_type fromUSsecSinceEpochUTC(int64_t usec_utc) {
std::filesystem::file_time_type ftime;
using namespace std::chrono;
auto sys_now = system_clock::now();
auto fil_now = decltype(ftime)::clock::now(); // i.e. C++20's file_clock
std::chrono::microseconds usec(usec_utc);
ftime = ????
return ftime;
}
MSVC2019 complains on auto res = sys_now - usec + fil_now;
MSVC2019 抱怨
auto res = sys_now - usec + fil_now;
with和
error C2676: binary '+': 'std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<std::chrono::system_clock::rep,std::chrono::system_clock::period>>' does not define this operator or a conversion to a type acceptable to the predefined operator
ie the code from https://stackoverflow.com/a/35282833/103724 between steady and system clocks does not appear to work for the filesystem clock.即稳定时钟和系统时钟之间的https://stackoverflow.com/a/35282833/103724中的代码似乎不适用于文件系统时钟。 Although it could be just me not following.
虽然可能只是我没有跟随。
Any <chrono>
expert who might be able to help?任何
<chrono>
专家可以提供帮助吗?
To fill out fromUSsecSinceEpochUTC
you would:要填写
fromUSsecSinceEpochUTC
,您将:
std::filesystem::file_time_type
fromUSsecSinceEpochUTC(int64_t usec_utc) {
std::filesystem::file_time_type ftime;
using namespace std::chrono;
auto sys_now = system_clock::now();
auto fil_now = decltype(ftime)::clock::now(); // i.e. C++20's file_clock
microseconds usec(usec_utc);
time_point<system_clock, microseconds> tp_sys{usec};
return tp_sys - sys_now + fil_now;
}
That being said, the relationship between the epoch of system_clock
and file_clock
is going to be a constant.话虽如此,
system_clock
和file_clock
的时代之间的关系将是一个常数。 I believe the Windows file_clock
epoch is 1601-01-01 00:00:00 UTC.我相信 Windows
file_clock
纪元是 1601-01-01 00:00:00 UTC。 The difference between that and the system_clock
epoch (1970-01-01 00:00:00 UTC) is 13,4774 days or 3'234'576h
.这与
system_clock
时期(1970-01-01 00:00:00 UTC)之间的差异是 13,4774 天或3'234'576h
。
This knowledge allows you to write conversions (for Windows) without calls to now()
.这些知识使您无需调用
now()
。
std::filesystem::file_time_type
to_file_time(std::chrono::system_clock::time_point sys_tp)
{
using namespace std::literals;
return std::filesystem::file_time_type{sys_tp.time_since_epoch() + 3'234'576h};
}
std::chrono::system_clock::time_point
to_sys_time(std::filesystem::file_time_type f_tp)
{
using namespace std::literals;
return std::chrono::system_clock::time_point{f_tp.time_since_epoch() - 3'234'576h};
}
Above I've taken advantage of the a-prior knowledge that file_clock::duration
is the same type as system_clock::duration
on Windows.上面我利用了先验知识,即
file_clock::duration
::duration 与 Windows 上的system_clock::duration
类型相同。 Add casts as necessary or desired.根据需要或需要添加演员表。
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