C ++ chrono :: duration_cast始终输出"0秒"，

问题描述：

这是我的第一个问题，我也是C ++的新手，但我会尽力做到尽可能具体.请告诉我是否要模糊:

This is my very first question here and I'm also a complete newbie at C++, but I'll do my best to be as specific as possible. Please tell me if I'm being to vague:

我正在尝试测量使用chrono和duration_cast进行排序的方法(合并排序)对给定的整数数组进行排序所花费的时间.这是有问题的代码段:

I am trying to measure the time it takes for a sorting method (merge sort) to sort a given array of integers by using chrono and duration_cast. Here is the code snippet in question:

    auto t1 = std::chrono::high_resolution_clock::now();
    mergesort(sortingArray, temp, 0, num - 1);
    auto t2 = std::chrono::high_resolution_clock::now();
    std::chrono::duration<double, std::milli> fp_ms = t2 - t1;
    std::cout << fp_ms.count() << " seconds\n";

无论我将它需要排序的数组多大，我得到的输出始终为"0秒".即使对一百万个整数进行排序并且有明显的执行时间，它仍然可以提供相同的输出.

And the output I get is always "0 seconds", no matter how big I make the array it needs to sort. Even when it sorts a million integers and there is a noticeable execution time, it still gives me the same output.

我基本上是按照此处给出的示例进行操作: http://en. cppreference.com/w/cpp/chrono/duration/duration_cast

I'm basically following the example given here: http://en.cppreference.com/w/cpp/chrono/duration/duration_cast

我正在使用我的mergesort函数而不是f().如何使它正确地衡量我的排序方法?

Only instead of f() I'm using my mergesort function. How can I make it measure my sorting method properly?

我正在使用minGW在Windows 10中通过Powershell进行编译.该命令如下所示:

I'm using minGW to compile via Powershell in Windows 10. The command looks like this:

g++ -std=c++11 .\Merge.cpp

答

TL; DR :在Windows上，std::chrono实现(libstdc ++)看起来很差，您将无法获得比秒更好的东西.

TL;DR: It looks like the std::chrono implementation (libstdc++) is quite poor on Windows and you won't get anything better than seconds.

长版:

libstdc ++ typedef s std::chrono::high_resolution_clock至std::chrono::system_clock.根据

libstdc++ typedefs std::chrono::high_resolution_clock to std::chrono::system_clock. According to the implementation a call to std::chrono::system_clock::now() will result in a call to one of the following, depending on the platform:

syscall(SYS_clock_gettime, CLOCK_REALTIME, ...)，这是Linux系统调用
clock_gettime(CLOCK_REALTIME, ...)，这是Windows不支持的POSIX系统调用
gettimeofday(...)，这是Windows不支持的POSIX函数
std::time()作为后备广告

syscall(SYS_clock_gettime, CLOCK_REALTIME, ...), which is a Linux system call
clock_gettime(CLOCK_REALTIME, ...), which is a POSIX syscall not supported by Windows
gettimeofday(...), which is a POSIX function not supported by Windows
std::time() as a fallback

因此，std::time()在Windows内部被调用.未指定std::time()的编码.但是，大多数系统遵循 POSIX规范:

Thus, std::time() is called internally on Windows. The encoding of std::time() is not specified; however, most systems follow the POSIX specification:

time()函数应返回自新纪元以来的时间值，以秒为单位.

Microsoft本身相同:

Microsoft itself does the same:

返回自1970年1月1日午夜起经过的时间( seconds )，如果发生错误，则返回-1.

Return the time as seconds elapsed since midnight, January 1, 1970, or -1 in the case of an error.

我想可以肯定地说，MingW的std::chrono不会使您获得更高的分辨率.

I think it is safe to say that you won't get a higher resolution with MingW's std::chrono.

对于您的问题，您有两种选择:

As for your problem, you have two options:

如果您的程序仅在Windows上运行，则可以使用 Boost.Chrono .它使用本地Windows API ，并且应该提供更好的分辨率.

If your program is running on Windows only you can build your own time measurement using QueryPerformanceCounter
If you want to keep it portable use Boost.Chrono. It uses the native Windows APIs and should offer a better resolution.

C ++ chrono :: duration_cast始终输出"0秒"，

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