MacOS 上的 C++ : show date and time issue

Question

我买了一台 MacBook Pro，过去两天我一直在使用 MacOS。我一直在尝试使用 chrono 和 ctime 库编写输出日期和时间的 C++ 代码。 这段代码在我的 Windows 机器和 CentOS7 服务器上运行良好。但是，在我的 MacBook Pro 上它无法编译。 这是我尝试使用 G++ 编译时收到的错误消息:

main.cpp:19:61: error: no viable conversion from 'time_point<std::__1::chrono::steady_clock,
      duration<[...], ratio<[...], 1000000000>>>' to 'const
      time_point<std::__1::chrono::system_clock, duration<[...], ratio<[...], 1000000>>>'
    std::time_t date = std::chrono::system_clock::to_time_t(now);
                                                            ^~~
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/chrono:1340:28: note: candidate constructor
      (the implicit copy constructor) not viable: no known conversion from
      'std::__1::chrono::time_point<std::__1::chrono::steady_clock, std::__1::chrono::duration<long long,
      std::__1::ratio<1, 1000000000> > >' to 'const
      std::__1::chrono::time_point<std::__1::chrono::system_clock, std::__1::chrono::duration<long long,
      std::__1::ratio<1, 1000000> > > &' for 1st argument
class _LIBCPP_TEMPLATE_VIS time_point
                           ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/chrono:1340:28: note: candidate constructor
      (the implicit move constructor) not viable: no known conversion from
      'std::__1::chrono::time_point<std::__1::chrono::steady_clock, std::__1::chrono::duration<long long,
      std::__1::ratio<1, 1000000000> > >' to 'std::__1::chrono::time_point<std::__1::chrono::system_clock,
      std::__1::chrono::duration<long long, std::__1::ratio<1, 1000000> > > &&' for 1st argument
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/chrono:1359:5: note: candidate template
      ignored: could not match 'std::__1::chrono::system_clock' against 'std::__1::chrono::steady_clock'
    time_point(const time_point<clock, _Duration2>& t,
    ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/chrono:1566:53: note: passing argument to
      parameter '__t' here
    static time_t     to_time_t  (const time_point& __t) _NOEXCEPT;
                                                    ^
1 error generated.
basavyr@Roberts-MacBook-Pro simpleTest %

代码如下:

void getTime()
{
    auto now = std::chrono::high_resolution_clock::now();
    std::time_t date = std::chrono::system_clock::to_time_t(now);
    std::cout << std::ctime(&date);
}
int main()
{
getTime();
}

我认为这个问题与 MacOS 使用 CLANG 编译器这一事实有关？ 有什么想法可以解决这个问题吗？

谢谢!

Answer 1

只有当 std::high_resolution_clock 与 std::system_clock 的类型相同时，您的代码才有可能工作。无法保证来自不同时钟的时间点相同或可转换。

对于转换为只有秒分辨率的 time_t system_clock 就足够了:

void getTime()
{
    auto now = std::chrono::system_clock::now();
    std::time_t date = std::chrono::system_clock::to_time_t(now);
    std::cout << std::ctime(&date);
}
int main()
{
    getTime();
}