std :: chrono :: high_resolution_clock不精确？

问题描述

所以我试图使用std :: chrono :: high_resolution_clock来计算需要执行多长时间。我想你可以找到开始时间和结束时间之间的差异...

So I was trying to use std::chrono::high_resolution_clock to time how long something takes to executes. I figured that you can just find the difference between the start time and end time...

为了检查我的方法工作，我做了以下程序：

To check my approach works, I made the following program:

#include <iostream>
#include <chrono>
#include <vector>

void long_function();

int main()
{
    std::chrono::high_resolution_clock timer;
    auto start_time = timer.now();

    long_function();

    auto end_time = timer.now();
    auto diff_millis = std::chrono::duration_cast<std::chrono::duration<int, std::milli>>(end_time - start_time);

    std::cout << "It took " << diff_millis.count() << "ms" << std::endl;
    return 0;
}

void long_function()
{
    //Should take a while to execute.
    //This is calculating the first 100 million
    //fib numbers and storing them in a vector.
    //Well, it doesn't actually, because it
    //overflows very quickly, but the point is it
    //should take a few seconds to execute.
    std::vector<unsigned long> numbers;
    numbers.push_back(1);
    numbers.push_back(1);
    for(int i = 2; i < 100000000; i++)
    {
        numbers.push_back(numbers[i-2] + numbers[i-1]);
    }
}

问题是，它只输出3000ms它显然不是真的。

The problem is, it just outputs 3000ms exactly, when it clearly wasn't actually that.

对于较短的问题，它只输出0ms ...我做错了什么？

On shorter problems, it just outputs 0ms... What am I doing wrong?

编辑：如果它是任何用途，我使用GNU GCC编译器与

If it's of any use, I'm using the GNU GCC compiler with -std=c++0x flag on

推荐答案

high_resolution_clock的分辨率取决于平台。

The resolution of the high_resolution_clock depends on the platform.

打印以下内容可以帮助您了解您使用的实现的解决方案。

Printing the following will give you an idea of the resolution of the implementation you use

    std::cout << "It took " << std::chrono::nanoseconds(end_time - start_time).count() << std::endl;

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