C++ 将 time_t 转换为 int
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Converting time_t to int
提问by Yogi
I want to convert a given time into epoch(time_t) and vice versa. can anyone tell what is the routine or algorithm for this?
我想将给定的时间转换为纪元（time_t），反之亦然。谁能说出这的例程或算法是什么？
Thanks
谢谢
Update
更新
epoch_strt.tm_sec = 0;
epoch_strt.tm_min = 0;
epoch_strt.tm_hour = 0;
epoch_strt.tm_mday = 1;
epoch_strt.tm_mon = 1;
epoch_strt.tm_year = 70;
epoch_strt.tm_isdst = -1;
double nsecs = difftime(curtime, basetime);//current time from system, I converrting it to struct tm
but this always return 32399 for some reason.
但这总是出于某种原因返回 32399 。
回答by Luchian Grigore
You should cast it to a long intinstead of int.
您应该将其转换为 along int而不是int。
long int t = static_cast<long int> time(NULL);
An intmight not be enough to hold the time, for example, on my platform, time_tis a typedefof __int64.
一个int可能不足以保存时间，例如，我的平台上，time_t是typedef的__int64。
回答by bames53
Whatever you're doing with time_t, you'll probably be best off using the <chrono>library. Then you can convert to and from time_t, if necessary. Hopefully you can just do what you need in <chrono>
无论您使用 做什么time_t，最好使用该<chrono>库。然后，您可以time_t根据需要在 和 之间进行转换。希望你可以做你需要的<chrono>
#include <chrono>
int main() {
auto a = std::chrono::system_clock::now()
time_t b = std::chrono::system_clock::to_time_t(a);
auto c = std::chrono::system_clock::from_time_t(b);
}
Update:
更新：
The C++ standard library doesn't yet have an API for dates as good as <chrono>is for time. You can use the Boost date library or you can fall back on the C date library:
C++ 标准库还没有像<chrono>时间一样好的日期 API 。您可以使用 Boost 日期库，也可以使用 C 日期库：
#include <ctime>
#include <chrono>
#include <iostream>
int main() {
std::tm epoch_start = {};
epoch_start.tm_sec = 0;
epoch_start.tm_min = 0;
epoch_start.tm_hour = 0;
epoch_start.tm_mday = 1;
epoch_start.tm_mon = 0;
epoch_start.tm_year = 70;
epoch_start.tm_wday = 4;
epoch_start.tm_yday = 0;
epoch_start.tm_isdst = -1;
std::time_t base = std::mktime(&epoch_start);
auto diff = std::chrono::system_clock::now() - std::chrono::system_clock::from_time_t(base);
std::chrono::seconds s = std::chrono::duration_cast<std::chrono::seconds>(diff);
std::cout << s.count() << '\n';
}
回答by Rob?
To convert a struct tmto a time_t, use mktime. To convert a time_tto a struct tm, use either gmtimeor localtime.
要将 a 转换struct tm为 a time_t，请使用mktime. 要将 a 转换time_t为 a struct tm，请使用gmtime或localtime。
Sample:
样品：
#include <iostream>
#include <ctime>
int main () {
std::time_t now = std::time(0);
std::tm *now_tm = gmtime(&now);
std::tm tomorrow_tm(*now_tm);
tomorrow_tm.tm_mday += 1;
tomorrow_tm.tm_hour = 0;
tomorrow_tm.tm_min = 0;
tomorrow_tm.tm_sec = 0;
std::time_t tomorrow = std::mktime(&tomorrow_tm);
double delta = std::difftime(tomorrow, now);
std::cout << "Tomorrow is " << delta << " seconds from now.\n";
}
UPDATE 2更新 2
#include <iostream>
#include <ctime>
#include <cassert>
// Display the difference between now and 1970-01-01 00:00:00
// On my computer, this printed the value 1330421747
int main () {
std::tm epoch_strt;
epoch_strt.tm_sec = 0;
epoch_strt.tm_min = 0;
epoch_strt.tm_hour = 0;
epoch_strt.tm_mday = 1;
epoch_strt.tm_mon = 0;
epoch_strt.tm_year = 70;
epoch_strt.tm_isdst = -1;
std::time_t basetime = std::mktime(&epoch_strt);
std::time_t curtime = std::time(0);
long long nsecs = std::difftime(curtime, basetime);
std::cout << "Seconds since epoch: " << nsecs << "\n";
assert(nsecs > 42ll * 365 * 24 * 60 * 60);
}
