c++ - C 将二维数组转换为双指针打印意外值

Question

为什么下面的程序打印的是 9 而不是 17？

int main()
{
   int **ptr;
   int i=0, j=0;
   int arr[2][2]={{17,9},{7,19}};

   ptr=(int**)arr;
   printf("%d\n", ptr[0]);
   return 0;
}

Answer 1

数组的数组与指向指针的指针不同。

原因是内存布局完全不同。

对于数组的数组:

+-----------+-----------+-----------+-----------+| arr[0][0] | arr[0][1] | arr[1][0] | arr[1][1] |+-----------+-----------+-----------+-----------+

For a pointer to pointer:

+--------+--------+-----+| arr[0] | arr[1] | ... |+--------+--------+-----+  |        |  |        V  |      +-----------+-----------+-----+  |      | arr[1][0] | arr[1][1] | ... |  |      +-----------+-----------+-----+  V+-----------+-----------+-----+| arr[0][0] | arr[0][1] | ... |+-----------+-----------+-----+

---

As arrays decays to pointer, you can have a pointer to arrays though:

int (*ptr)[2] = arr;