c++ - 给定一个字符串形式的日期/时间，知道当时是否为 DST 的最佳方法是什么？

Question

假设我得到了这样一个字符串:"2009-4-9"，表示 2009 年 4 月 9 日。假设对于初学者来说，我只关心本地时间(我也担心其他时区，但我会先解决简单的问题)。了解夏令时当时是否有效的最佳方法是什么？

您可以假设系统具有正确更新的时区文件，例如 /etc/localtime 并且我对可移植的解决方案很感兴趣。 c 或 c++ 也可以接受。

你也可以假设我只关心过去或现在的日期，从不关心 future 。

目前我有一个看起来像这样的“hack”(我知道 localtime_r 是一个库扩展，但它与我可以使其可移植的相同功能足够接近)

struct tm tm;
// convert the time string to a broken down structure in localtime
if(strptime("2009-4-9", "%Y-%m-%d", &tm)) {
    // convert the broken down structure into seconds since epoch
    const time_t t = mktime(&tm);

    // convert it back to a broken down structure, this seems pointless at first...
    // but libc will fill out fields such as tm_isdst
    localtime_r(&t, &tm);

    printf("DST : %d\n", tm.tm_isdst);
}

虽然这很有效(而且似乎是一种非常有效的方法)，但我觉得来回转换很愚蠢。有没有更好的办法？

Answer 1

您根本不需要调用 localtime_r()。 mktime() 标准化您传递给它的 struct tm，包括将 tm_isdst 设置为非负值(如果它被设置为 ) -1。所以你只需要:

struct tm tm = { 0 };
// convert the time string to a broken down structure in localtime
if(strptime("2009-4-9", "%Y-%m-%d", &tm)) {
    tm.tm_isdst = -1;
    // normalise the broken down structure - this calculates tm_isdst
    mktime(&tm);

    printf("DST : %d\n", tm.tm_isdst);
}

mktime() 的这种行为是 C 标准所要求的；例如，C99 说:

The mktime function converts the broken-down time, expressed as local time, in the structure pointed to by timeptr into a calendar time value with the same encoding as that of the values returned by the time function. The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to the ranges indicated above.²⁷⁶ On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to the ranges indicated above; the final value of tm_mday is not set until tm_mon and tm_year are determined.

脚注 276 明确解决了 tm_isdst 成员:

276) Thus, a positive or zero value for tm_isdst causes the mktime function to presume initially that Daylight Saving Time, respectively, is or is not in effect for the specified time. A negative value causes it to attempt to determine whether Daylight Saving Time is in effect for the specified time.