std::chrono::years (since C++20) duration</*signed integer type of at least 17 bits*/, std::ratio<31556952>>
使用libc++
,它似乎的强调存储std::chrono::years
就是short
其中签订16位。
std::chrono::years( 30797 ) // yields 32767/01/01
std::chrono::years( 30797 ) + 365d // yields -32768/01/01 apparently UB
cppreference上有错别字吗?
例:
#include <fmt/format.h>
#include <chrono>
template <>
struct fmt::formatter<std::chrono::year_month_day> {
char presentation = 'F';
constexpr auto parse(format_parse_context& ctx) {
auto it = ctx.begin(), end = ctx.end();
if (it != end && *it == 'F') presentation = *it++;
# ifdef __exception
if (it != end && *it != '}') {
throw format_error("invalid format");
}
# endif
return it;
}
template <typename FormatContext>
auto format(const std::chrono::year_month_day& ymd, FormatContext& ctx) {
int year(ymd.year() );
unsigned month(ymd.month() );
unsigned day(ymd.day() );
return format_to(
ctx.out(),
"{:#6}/{:#02}/{:#02}",
year, month, day);
}
};
using days = std::chrono::duration<int32_t, std::ratio<86400> >;
using sys_day = std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<int32_t, std::ratio<86400> >>;
template<typename D>
using sys_time = std::chrono::time_point<std::chrono::system_clock, D>;
using sys_day2 = sys_time<days>;
int main()
{
auto a = std::chrono::year_month_day(
sys_day(
std::chrono::floor<days>(
std::chrono::hours( (1<<23) - 1 )
)
)
);
auto b = std::chrono::year_month_day(
sys_day(
std::chrono::floor<days>(
std::chrono::minutes( (1l<<29) - 1 )
)
)
);
auto c = std::chrono::year_month_day(
sys_day(
std::chrono::floor<days>(
std::chrono::seconds( (1l<<35) - 1 )
)
)
);
auto e = std::chrono::year_month_day(
sys_day(
std::chrono::floor<days>(
std::chrono::days( (1<<25) - 1 )
)
)
);
auto f = std::chrono::year_month_day(
sys_day(
std::chrono::floor<days>(
std::chrono::weeks( (1<<22) - 1 )
)
)
);
auto g = std::chrono::year_month_day(
sys_day(
std::chrono::floor<days>(
std::chrono::months( (1<<20) - 1 )
)
)
);
auto h = std::chrono::year_month_day(
sys_day(
std::chrono::floor<days>(
std::chrono::years( 30797 ) // 0x7FFF - 1970
)
)
);
auto i = std::chrono::year_month_day(
sys_day(
std::chrono::floor<days>(
std::chrono::years( 30797 ) // 0x7FFF - 1970
) + std::chrono::days(365)
)
);
fmt::print("Calendar limit by duration's underlining storage:\n"
"23 bit hour : {:F}\n"
"29 bit minute : {:F}\n"
"35 bit second : {:F}\n"
"25 bit days : {:F}\n"
"22 bit week : {:F}\n"
"20 bit month : {:F}\n"
"16? bit year : {:F}\n"
"16? bit year+365d : {:F}\n"
, a, b, c, e, f, g, h, i);
}
[ Godbolt链接 ]
std::chrono::years( 30797 ) + 365d
无法编译。
years{30797} + days{365}
为204528013,单位为216s。
hours{2} + seconds{5}
。
duration
名称是复数: years
,months
,days
。历法的部件名称是单数: year
,month
,day
。 year{30797} + day{365}
是编译时错误。 year{2020}
是今年。 years{2020}
持续时间为2020年。
year
范围:eel.is/c++draft/time.cal.year#members-19years
范围: eel.is/c++draft/time.syn。year
是公民年度的“名称”,需要16位。years
是一个时间长度,与时间不同year
。一个可以减去两个year
,结果为typeyears
。years
必须能够保存的结果year::max() - year::min()
。