c++ - std::vector 与 std::array 性能对比

Question

我正在查看新的计时库 (C++11) 并尝试使用它。我写了以下两个程序:

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;
}

数组.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;
}

我得到数组程序为 9 毫秒， vector 程序为 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？

Ps: 我使用的是 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

Answer 1

我将您的代码更改为:

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，两者都同样快。