原来代码可以这么写 --记性不好的matthew

今天在google groups 的comp.lang.c++邮件列表里看到这么一段代码，真是有意思，我是第一次看到这种代码。

#include <iostream>

int main()
{
  int arr[3] = {1, 2, 3};

  for (int i = 0; i < 3; ++i)
    std::cout << i[arr] << " ";

  std::cout << std::endl;
 
  for (int j = 0; j < 3; ++j)
      std::cout << arr[j] << " ";

  std::cout << std::endl;

  return 0;
}

据说是利用了加法的交换性和传递性（commutativity and transitivity of addition）。

推导过程：
[quote]
.......

Using the transitivity of addition (meaning that A+B == B+A) we can
get some pretty ugly, but valid, syntax when indexing into arrays:

So arr[N] == *(arr + N) then we apply the transitivity rule on the
left hand giving *(N + arr) and then go back to the array-form gives
N[arr].

So given array arr then 2[arr] will give the third element in arr. And
then we replace the N with a variable and we get something like this:

int main()
{
  int arr[3] = {1, 2, 3};
  for (int i = 0; i < 3; ++i)
    std::cout << i[arr];
  return 0;
}
[/quote]

原文：
http://groups.google.com/group/comp.lang.c++/browse_thread/thread/599be6c63ddaba57?hl=en