使用 std::chrono 的正确方法

Question

虽然我意识到这可能是许多相同的问题之一，但我似乎无法弄清楚如何正确使用 std::chrono。这是我拼凑起来的解决方案。

#include <stdlib.h>
#include <iostream>
#include <chrono>

typedef std::chrono::high_resolution_clock Time;
typedef std::chrono::milliseconds ms;

float startTime;

float getCurrentTime();

int main () {
    startTime = getCurrentTime();
    std::cout << "Start Time: " << startTime << "\n";

    while(true) {
        std::cout << getCurrentTime() - startTime << "\n";
    }

    return EXIT_SUCCESS;
}

float getCurrentTime() {
    auto now = Time::now();
    return std::chrono::duration_cast<ms>(now.time_since_epoch()).count() / 1000;
}

由于某种原因，这只会返回整数值作为差值，该差值以每秒 1 的速率递增，但从任意值开始，通常为负值。

我做错了什么？有更好的方法吗？

Answer 1

• 除非万不得已，否则不要逃避 chrono 类型系统。这意味着不要使用 .count()，除非是 I/O 或与旧版 API 交互。

这意味着：不要使用float 作为时间点。

• 不要打扰high_resolution_clock。这始终是system_clock 或steady_clock 的typedef。如需更便携的代码，请选择后者。

.

#include <iostream>
#include <chrono>

using Time = std::chrono::steady_clock;
using ms = std::chrono::milliseconds;

首先，您需要一个表示为float 和单位为seconds 的持续时间。你就是这样做的：

using float_sec = std::chrono::duration<float>;

接下来你需要一个time_point，它使用Time作为时钟，float_sec作为它的持续时间：

using float_time_point = std::chrono::time_point<Time, float_sec>;

现在您的getCurrentTime() 可以返回Time::now()。不要大惊小怪：

float_time_point
getCurrentTime() {
    return Time::now();
}

您的 main，因为它必须执行 I/O，因此负责将 chrono 类型解包为标量，以便它可以打印它们：

int main () {
    auto startTime = getCurrentTime();
    std::cout << "Start Time: " << startTime.time_since_epoch().count() << "\n";

    while(true) {
        std::cout << (getCurrentTime() - startTime).count() << "\n";
    }
}

Answer 2

这个程序做了类似的事情。希望它展示了 std::chrono 的一些功能（和方法）：

#include <iostream>
#include <chrono>
#include <thread>


int main()
{
    using namespace std::literals;
    namespace chrono = std::chrono;
    using clock_type = chrono::high_resolution_clock;

    auto start = clock_type::now();

    for(;;) {
        auto first = clock_type::now();

        // note use of literal - this is c++14
        std::this_thread::sleep_for(500ms);

        // c++11 would be this:
        //        std::this_thread::sleep_for(chrono::milliseconds(500));

        auto last = clock_type::now();
        auto interval = last - first;
        auto total = last - start;

        // integer cast
        std::cout << "we just slept for " << chrono::duration_cast<chrono::milliseconds>(interval).count() << "ms\n";

        // another integer cast
        std::cout << "also known as " << chrono::duration_cast<chrono::nanoseconds>(interval).count() << "ns\n";

        // floating point cast
        using seconds_fp = chrono::duration<double, chrono::seconds::period>;
        std::cout << "which is " << chrono::duration_cast<seconds_fp>(interval).count() << " seconds\n";

        std::cout << "  total time wasted: " << chrono::duration_cast<chrono::milliseconds>(total).count() << "ms\n";
        std::cout << "         in seconds: " << chrono::duration_cast<seconds_fp>(total).count() << "s\n";


        std::cout << std::endl;
    }
    return 0;
}

示例输出：

we just slept for 503ms
also known as 503144616ns
which is 0.503145 seconds
  total time wasted: 503ms
         in seconds: 0.503145s

we just slept for 500ms
also known as 500799185ns
which is 0.500799 seconds
  total time wasted: 1004ms
         in seconds: 1.00405s

we just slept for 505ms
also known as 505114589ns
which is 0.505115 seconds
  total time wasted: 1509ms
         in seconds: 1.50923s

we just slept for 502ms
also known as 502478275ns
which is 0.502478 seconds
  total time wasted: 2011ms
         in seconds: 2.01183s