c++ - mktime 仅处理 Clang 上的闰年？

Question

在this answer我提议 marihikari使用 mktime 的标准功能而不是尝试实现他自己的公历系统。

我编写这个函数是为了演示如何使用 mktime 来完成此任务:

bool leap_year(int year) {
    tm bar = { 0, 0, 0, 29, 1, year - 1900 };

    mktime(&bar);

    return bar.tm_mday == 29 && bar.tm_mon == 1 && bar.tm_year == year - 1900;
}

测试这个:

cout << "2000: " << leap_year(2000) << "\n2001: " << leap_year(2001) << "\n2004: " << leap_year(2004) << "\n1900: " << leap_year(1900) << "\n2100: " << leap_year(2100) << endl;

在 Clang 3.7.0 中产生了正确的结果:

2000: 1
2001: 0
2004: 1
1900: 0
2100: 0

但是 gcc 5.1.0 中的结果不正确:

2000: 1
2001: 0
2004: 1
1900: 1
2100: 1

Visual Studio 2015 中的结果不正确:

2000: 1
2001: 0
2004: 1
1900: 1
2100: 0

我认为这是 gcc 5.1.0 和 Visual Studio 2015 中的错误？

Answer 1

mktime :

Converts local calendar time to a time since epoch as a time_t object. time->tm_wday and time->tm_yday are ignored. The values in time are permitted to be outside their normal ranges.
...
If the conversion is successful, the time object is modified. All fields of time are updated to fit their proper ranges.

mktime将返回:

Time since epoch as a time_t object on success or -1 if time cannot be represented as a time_t object.

但未指定实现必须做出哪些努力才能转换 tm .所以只要转换时间或者static_cast<time_t>(-1)返回 mktime 的要求已满。

意思是下面的函数will work on all platforms正确支持 mktime :

bool leap_year(int year) {
    tm bar = { 0, 0, 0, 29, 1, year - 1900 };

    return static_cast<time_t>(-1) != mktime(&bar) && bar.tm_mday == 29 && bar.tm_mon == 1 && bar.tm_year == year - 1900;
}