STL Vector

vector_iteration.cc

#include <iostream>
#include <iomanip>
#include <time.h>
#include <vector>

struct Item {
  int a;

  Item(int i): a(i) {
    std::cout << "ctor" << std::endl;
  }

  ~Item() {
    std::cout << "dtor" << std::endl;
  }

  Item(const Item& o): a(o.a) {
    std::cout << "copy ctor" << std::endl;
  }

  void operator=(const Item& o) {
    a = o.a;
    std::cout << "assign ctor" << std::endl;
  }
};


int main(int argc, char **argv) {
  int sum = 0;

  std::vector<Item> items;
  for (int i = 0; i < 10; i++)
    items.push_back(Item(i));

  clock_t s = clock();
  for (int i = 0; i < 10000000; i++) {
    // 1 第二
    for (std::vector<Item>::iterator it = items.begin(), end = items.end();
         it != end;
         ++it)
      sum += it->a;

  }
  clock_t e = clock();
  std::cout << sum << ";" << (double)(e - s)/CLOCKS_PER_SEC << std::endl;

  sum = 0;
  s = clock();
  for (int i = 0; i < 10000000; i++) {
    // 2 最优 在O3 Release下
    for (Item *it = &items[0], *end = &items[0] + items.size();
         it != end;
         ++it)
      sum += it->a;
  }
  e = clock();
  std::cout << sum << ";" << (double)(e - s)/CLOCKS_PER_SEC << std::endl;

  sum = 0;
  s = clock();
  for (int i = 0; i < 10000000; i++) {
    // 3
    Item* its = &items[0];
    for (int i = 0, size = items.size(); i < size; ++i)
      sum += its[i].a;
  }
  e = clock();
  std::cout << sum << ";" << (double)(e - s)/CLOCKS_PER_SEC << std::endl;
  return 0;
}