C++,从午夜开始获得毫秒的最快方法

Question

这是经常运行并且必须快速运行的代码,因此我想就最快的实现方式获得一些意见.注意,我实际上需要完整的毫秒分辨率到一定程度的准确度(所以秒*1000是不够的).对于这个项目,使用Boost是可以的.

编辑:目标平台是x64_86 CentOS5,也希望能够依赖OS时钟,所以我也可以在一个没有连续运行的程序中使用它.

Answer 1

C++有一个用于处理时间的计时库:

#include <chrono>
#include <iostream>
#include <ctime>

std::chrono::system_clock::duration duration_since_midnight() {
    auto now = std::chrono::system_clock::now();

    time_t tnow = std::chrono::system_clock::to_time_t(now);
    tm *date = std::localtime(&tnow);
    date->tm_hour = 0;
    date->tm_min = 0;
    date->tm_sec = 0;
    auto midnight = std::chrono::system_clock::from_time_t(std::mktime(date));

    return now-midnight;
}

int main()
{
    auto since_midnight = duration_since_midnight();

    auto hours = std::chrono::duration_cast<std::chrono::hours>(since_midnight);
    auto minutes = std::chrono::duration_cast<std::chrono::minutes>(since_midnight - hours);
    auto seconds = std::chrono::duration_cast<std::chrono::seconds>(since_midnight - hours - minutes);
    auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(since_midnight - hours - minutes - seconds);
    auto microseconds = std::chrono::duration_cast<std::chrono::microseconds>(since_midnight - hours - minutes - seconds - milliseconds);
    auto nanoseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(since_midnight - hours - minutes - seconds - milliseconds - microseconds);

    std::cout << hours.count() << "h ";
    std::cout << minutes.count() << "m ";
    std::cout << seconds.count() << "s ";
    std::cout << milliseconds.count() << "ms ";
    std::cout << microseconds.count() << "us ";
    std::cout << nanoseconds.count() << "ns\n";
}

这取决于您的实现,您获得的分辨率究竟是什么.VS 11 beta声称分辨率为100ns,但我不能说它有多精确.

11h 51m 57s 285ms 699us 600ns

Answer 2

例如，我会尝试创建一个计时器来检查每秒的时间。如果到午夜的时间少于 K 秒：

while(True)
{
    if (checkTime() >= MIDNIGHT))
    {
        break; 
    }
}