C++20chrono：如何将time_point与month_day进行比较？

html5 • • 问答

是否有一个现代和优雅的方式来确定月和日在time_point可变匹配给定的month_day变量？

例如，我想知道今天是否是圣诞节。我有以下代码：

#include <chrono>

bool IsTodayChristmas()
{
    using namespace std::chrono;

    constexpr month_day Christmas = {December / 25};
    auto Now = system_clock::now();

    // return Now == Christmas; // How to?
}

现代而优雅：我的意思是如果可能的话，我宁愿不使用旧的 C 类型（类似于std::time_t和std::tm）和字符串比较（类似于std::put_time）。

任何帮助，将不胜感激。

回答

您可以转换system_clock::now()为std::chrono::year_month_day通过型std::chrono::sys_days。在实践中，这可能看起来像

#include <chrono>

bool IsTodayChristmas() {
    using namespace std::chrono;

    constexpr month_day Christmas = {December / 25};
    auto Now = year_month_day{floor<days>(system_clock::now())};

    // either
    return Now == Christmas / Now.year();
    // or
    return Now.month() / Now.day() == Christmas;
}

正如 Howard Hinnant 指出的那样，这将决定 UTC 的圣诞节。您更有可能在本地时区的圣诞节之后：要这样做，我们必须首先转换Now到我们的本地时区：（std::chrono::current_zone据我所知，libstdc++ 或 libc++ 尚未提供注释。）

bool IsTodayChristmas() {
    using namespace std::chrono;

    constexpr month_day Christmas = {December / 25};
    auto Now_local = current_zone()->to_local(system_clock::now());
    auto Today = year_month_day{floor<days>(Now_local)};

    return Today == Christmas / Today.year();
}

  • I'm rooting for you! 🙂
  • If you need to know if it is Christmas in some place other than your local time, or UTC, then change `auto Now_local = current_zone()->to_local(system_clock::now());` to `auto Now_local = locate_zone("Australia/Sydney")->to_local(system_clock::now());` and everything else can stay the same. In dealing with time, getting the question right is often harder than getting the answer right. 🙂
  • Upvoted. 🙂 Nice job. Yes, working directly with the `time_zone` `current_zone()` is a good way to do this. But using `zoned_time` is also a good way. `zoned_time` is essentially a higher-level convenience API over working with `time_zone` directly. But in this case doesn't buy that much convenience (nor does it cost much). Either way, getting the current local time, and flooring it to days-precision is the correct way to get the current local date and compare it to Christmas.
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