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9年前关闭。




为了量化类似 C 的数组和 C++ 中 Vectors 的性能差异，我编写了这个小程序。 https://github.com/rajatkhanduja/Benchmarks/blob/master/C%2B%2B/vectorVsArray.cpp

为了在共同点上比较它们，我决定测试随机访问和顺序访问。我添加了迭代器，只是为了比较它们(但这不是问题的重点)。

对于具有 7.7 GB RAM 且数组/vector 大小为 100 万的 64 位 Linux 机器，结果如下:-

#include <vector>
#include <iostream>
#include <cstdlib>
#include <ctime>
#include <sys/time.h>
#include <cassert>

#define ARR_SIZE 1000000

using std::string;

void printtime (struct timeval& start, struct timeval& end, string str);   

int main (void)
{
  int arr[ARR_SIZE];
  int tmp;
  struct timeval start, stop;

  srand (time (NULL));

  /* Writing data to array */
  gettimeofday (&start, NULL);
  for (int i = 0; i < ARR_SIZE; i++)
  {
    arr[i] = rand();
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to write to array."));

  /* Reading data from array */
  gettimeofday (&start, NULL);
  for (int i = 0; i < ARR_SIZE; i++)
  {
    tmp = arr[i];
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to read from array sequentially."));

  /* Reading data from array randomly*/
  gettimeofday (&start, NULL);
  for (int i = 0; i < ARR_SIZE; i++)
  {
    tmp = arr[rand() % ARR_SIZE];
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to read from array randomly."));


  int *darr = (int *) calloc (sizeof (int), ARR_SIZE);  

  /* Writing data to array */
  gettimeofday (&start, NULL);
  for (int i = 0; i < ARR_SIZE; i++)
  {
    darr[i] = rand();
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to write to dynamic array."));

  /* Reading data from array */
  gettimeofday (&start, NULL);
  for (int i = 0; i < ARR_SIZE; i++)
  {
    tmp = darr[i];
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to read from dynamic array sequentially."));

  /* Reading data from dynamic array randomly*/
  gettimeofday (&start, NULL);
  for (int i = 0; i < ARR_SIZE; i++)
  {
    tmp = darr[rand() % ARR_SIZE];
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to read from dynamic array randomly."));

  std::vector<int> v(ARR_SIZE);
  assert (v.capacity() == ARR_SIZE);

  /* Writing to vector using indices*/
  gettimeofday (&start, NULL);
  for (int i = 0; i < ARR_SIZE; i++)
  {
    v[i] = rand();
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to write to vector using indices."));
  assert (v.capacity() == ARR_SIZE);

  /* Reading from vector using indices*/
  gettimeofday (&start, NULL);
  for (int i = 0; i < ARR_SIZE; i++)
  {
    tmp = v[i];
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to read from vector using indices, sequentially."));

  /* Reading data from dynamic array randomly*/
  gettimeofday (&start, NULL);
  for (int i = 0; i < ARR_SIZE; i++)
  {
    tmp = v[rand() % ARR_SIZE];
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to read from vector using indices, randomly."));

  std::vector<int> v2(ARR_SIZE);

  /* Writing to vector using iterators*/
  gettimeofday (&start, NULL);
  std::vector<int>::iterator itr, itr_end;
  for (itr = v2.begin(), itr_end = v2.end(); itr != itr_end; itr++)
  {
    *itr = rand();
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to write to vector using iterators."));


  /* Reading from vector using iterators*/
  gettimeofday (&start, NULL);
  for (itr = v2.begin(), itr_end = v2.end(); itr != itr_end; itr++)
  {
    tmp = *itr;
  }
  gettimeofday (&stop, NULL);
  printtime (start, stop, string ("Time taken to read from vector using iterators."));

  return 0;
}

void printtime (struct timeval& start, struct timeval& end, string str)
{
  double start_time, end_time, diff;

  start_time = ((start.tv_sec) * 1000 + start.tv_usec/1000.0);
  end_time   = ((end.tv_sec) * 1000 + end.tv_usec/1000.0);
  diff = end_time - start_time;

  std::cout << str << " : " << diff << " ms" << std::endl;
}

最佳答案

当然不是一个明确的答案，但你正在一个循环中写入一个变量，这意味着编译器可以很容易地猜测顺序读取的最终结果应该是什么，从而优化循环。由于它显然没有这样做，我假设没有优化肯定不利于迭代器方法。其他数字太接近，无法得出结论。

关于C++ Array vs Vector 性能测试解释，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/10887668/