C++ Array vs vector

Question

when using C++ vector, time spent is 718 milliseconds, while when I use Array, time is almost 0 milliseconds.

Why so much performance difference?

int         


        

Answer 1

To get a fair comparison I think something like the following should be suitable:

#include 
#include 
#include 
#include 
#include 


int main()
{
  static size_t const size = 7e6;

  timeval start, end;
  int sum;

  gettimeofday(&start, 0);
  {
    std::vector v(size, 1);
    sum = std::accumulate(v.begin(), v.end(), 0);
  }
  gettimeofday(&end, 0);

  std::cout << "= vector =" << std::endl
        << "(" << end.tv_sec - start.tv_sec
        << " s, " << end.tv_usec - start.tv_usec
        << " us)" << std::endl
        << "sum = " << sum << std::endl << std::endl;

  gettimeofday(&start, 0);
  int * const arr =  new int[size];
  std::fill(arr, arr + size, 1);
  sum = std::accumulate(arr, arr + size, 0);
  delete [] arr;
  gettimeofday(&end, 0);

  std::cout << "= Simple array =" << std::endl
        << "(" << end.tv_sec - start.tv_sec
        << " s, " << end.tv_usec - start.tv_usec
        << " us)" << std::endl
        << "sum = " << sum << std::endl << std::endl;
}

In both cases, dynamic allocation and deallocation is performed, as well as accesses to elements.

On my Linux box:

$ g++ -O2 foo.cpp 
$ ./a.out 
= vector =
(0 s, 21085 us)
sum = 7000000

= Simple array =
(0 s, 21148 us)
sum = 7000000

Both the std::vector<> and array cases have comparable performance. The point is that std::vector<> can be just as fast as a simple array if your code is structured appropriately.

---

On a related note switching off optimization makes a huge difference in this case:

$ g++ foo.cpp 
$ ./a.out 
= vector =
(0 s, 120357 us)
sum = 7000000

= Simple array =
(0 s, 60569 us)
sum = 7000000

Many of the optimization assertions made by folks like Neil and jalf are entirely correct.

HTH!

EDIT: Corrected code to force vector destruction to be included in time measurement.