iterator vs reverse_iterator

Question

I\'m using std::map to store a lot of elements (pairs of elements) and I have a \"little\" doubt. What is more efficient to iterate all elements over my

Answer 1

For the record, dereferencing reverse_iterator on std::map and std::set containers is twice as slow as using iterator -- with both -O3 gcc 3.4.6 and MSVC on Intel/AMD processors (almost 3x as slow on PPC architectures.) Same holds for const_reverse_iterator vs. const_iterator. This is due to the fact that reverse_iterator actually points to the tree node immediately following the tree node to be dereferenced, hence the extra work. std::vector iterators exhibit a much milder difference (reverse_iterator is only ~30% slower on PPC, virtually indistinguishable on Intel/AMD.) Incidentally, a std::vector iterator is about 20x faster than a std::map or std::set iterator.

#include 
#include 
#include 
#ifdef _WIN32
#include 
#else
#include 
#endif
#include 

#define CONTAINER std::set< int >

double
mygettime(void) {
# ifdef _WIN32
  struct _timeb tb;
  _ftime(&tb);
  return (double)tb.time + (0.001 * (double)tb.millitm);
# else
  struct timeval tv;
  if(gettimeofday(&tv, 0) < 0) {
    perror("oops");
  }
  return (double)tv.tv_sec + (0.000001 * (double)tv.tv_usec);
# endif
}


int main() {
  int i, x = 0;
  CONTAINER bla;
  for (i = 0; i < 10000; bla.insert(bla.end(), i++)) ;

  double t1 = mygettime();

  for (i = 0; i < 100; ++i) {
    for (CONTAINER::iterator it = bla.begin(); it != bla.end(); ++it) {
      x ^= *it;
    }
  }

  printf("forward: %f\n", mygettime() - t1);

  double t2 = mygettime();

  for (i = 0; i < 100; ++i) {
    for (CONTAINER::reverse_iterator it = bla.rbegin(); it != bla.rend(); ++it) {
      x ^= *it;
    }
  }

  printf("reverse: %f\n", mygettime() - t2);

  return 0;
}