将 time_t 转换为 int

Question

我想将给定时间转换为 epoch(time_t)，反之亦然。谁能告诉这个的例程或算法是什么？

谢谢

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

但由于某种原因，这总是返回 32399。

Answer 1

您应该将其转换为 long int 而不是 int。

long int t = static_cast<long int> (time(NULL));

int 可能不足以保持时间，例如，在我的平台上，time_t 是 typedef 的 __int64。

Answer 2

无论您使用time_t 做什么，最好还是使用<chrono> 库。然后，如有必要，您可以与time_t 相互转换。希望您可以在<chrono>中做您需要的事情

#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);
}

更新：

C++ 标准库还没有像<chrono> 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';
}

Answer 3

要将struct tm 转换为time_t，请使用mktime。要将time_t 转换为struct tm，请使用gmtime 或localtime。

示例：

#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";
}

---

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);
}