2019-04-10

C++ 計算程式執行時間

本篇 ShengYu 將介紹如何使用 C++ std::chrono 計算某段程式碼執行的時間差並且列印時間差，
計算程式執行時間有幾種方式，本篇介紹使用 C++11 的 std::chrono 的幾種寫法來計算時間差並且用 printf 或 cout 將時間差印出來。

C++11 用法

需要引入的標頭檔：<chrono>

在使用前，需要先宣告引用 chrono，以下範例分別示範 steady_clock 與 high_resolution_clock 的用法，
t1-t2 所花時間可使用 duration_cast 轉換，它會根據傳入的時間單位，回傳出對應的時間單位，

calculate-time.cpp

// g++ calculate-time.cpp -o a.out -std=c++11 -pthread
#include <iostream>
#include <chrono>
#include <thread>

int main() {
    std::chrono::steady_clock::time_point t1 = std::chrono::steady_clock::now();
    std::this_thread::sleep_for(std::chrono::milliseconds(200));
    std::chrono::steady_clock::time_point t2 = std::chrono::steady_clock::now();

    std::cout << "Waited " << std::chrono::duration_cast<std::chrono::microseconds>(t2 - t1).count() << " us.\n";
    std::cout << "Waited " << std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1).count() << " ms.\n";
    printf("Waited %ld ms.\n", std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1).count());

    auto t3 = std::chrono::high_resolution_clock::now();
    std::this_thread::sleep_for(std::chrono::milliseconds(200));
    auto t4 = std::chrono::high_resolution_clock::now();
    std::chrono::duration<double, std::milli> duration3 = t4 - t3;

    printf("Waited %f ms.\n", duration3.count());
    return 0;
}

輸出結果如下：

Waited 200072 us.
Waited 200 ms.
Waited 200 ms.
Waited 200.085996 ms.

參考
https://kheresy.wordpress.com/2013/12/27/c-stl-chrono/
C++ use std::chrono to measure execution of member functions in a nice way
https://stackoverflow.com/questions/52905915/c-use-stdchrono-to-measure-execution-of-member-functions-in-a-nice-way
深入理解std::chrono的時鐘Clock
https://www.cnblogs.com/zhongpan/p/7490657.html