std::vector vs std::array 性能 [英] std::vector vs std::array performance

问题描述

我正在查看新的 chrono 库 (C++11) 并尝试使用它.我写了以下两个程序:

I was looking at the new chrono library (C++11) and trying to use it. I wrote the two following programs:

vector.cpp

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

int main()
{
    std::vector<double> vector(1000000, 0.);

    auto start = std::chrono::high_resolution_clock::now();
    for(int i(0); i < vector.size(); i++)
    {
        vector[i] += 1.;
    }
    auto end = std::chrono::high_resolution_clock::now();

    std::cout << "Elapsed time: " << std::chrono::duration_cast<std::chrono::milliseconds>(end-start).count() << " milliseconds" << std::endl;

    return 0;
}

array.cpp

#include <iostream>
#include <array>
#include <algorithm>
#include <chrono>

int main()
{
    std::array<double, 1000000> array;

    std::fill(array.begin(), array.end(), 0.);

    auto start = std::chrono::high_resolution_clock::now();
    for(int i(0); i < array.size(); i++)
    {
        array[i] += 1.;
    }
    auto end = std::chrono::high_resolution_clock::now();

    std::cout << "Elapsed time: " << std::chrono::duration_cast<std::chrono::milliseconds>(end-start).count() << " milliseconds" << std::endl;

    return 0;
}

我为数组程序获得了 9 毫秒，为向量程序获得了 12 毫秒.std::vector 似乎比 std::array 慢 33%.我做得对吗?为什么会有这种差异?

I obtained 9 millisecond for the array program and 12 milliseconds for the vector program. The std::vector seems about 33% slower than the std::array. I'm doing it right? Why this difference?

Ps:我使用 GCC 4.7，Mac OS X 10.7.

Ps: I'm using GCC 4.7, Mac OS X 10.7.

g++-mp-4.7 -std=c++11 vector.cpp -o vector
g++-mp-4.7 -std=c++11 array.cpp -o array

推荐答案

我把你的代码改成这样:

I changed your code to this:

std::array<double, 1000000> array;

double total = 0;
std::fill(array.begin(), array.end(), 0.);

for (unsigned j = 0; j < 1000; ++j)
{
    auto start = std::chrono::high_resolution_clock::now();

    for (unsigned i = 0; i < array.size(); i++)
    {
        array[i] += 1.;
    }

    auto end = std::chrono::high_resolution_clock::now();
    total = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
}

std::cout << total << " for Array." << std::endl;

std::vector<double> vector(1000000, 0.);
total = 0;

for (unsigned j = 0; j < 1000; ++j)
{
    auto start = std::chrono::high_resolution_clock::now();

    for (unsigned i = 0; i < vector.size(); i++)
    {
        vector[i] += 1.;
    }

    auto end = std::chrono::high_resolution_clock::now();
    total = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
}

std::cout << total << " for Vector." << std::endl;

我使用 -O3 的结果:

8123 for Array.
8117 for Vector.

在我看来两者都一样快.

Seems to me that both are equally fast.

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