C++ 20的计时:如何比较时间点和月日?

你可以转换 system_clock::now() std::chrono::year_month_day std::chrono::sys_days . 实际上,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,这将决定UTC的圣诞节。你更有可能在圣诞节后在当地时区:要做到这一点,我们必须首先转变 Now 进入我们当地的时区:(注 std::chrono::current_zone (据我所知,libstdc++或libc++尚未提供。)

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();
}