C将2D数组转换为双指针会打印出意外的值 [英] C converting 2D Array to double pointer prints unexpected value

问题描述

以下程序为什么打印9而不是17?

The following program is printing 9 instead of 17 why?

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;
}

推荐答案

数组的数组与指向指针的指针不同.

An array of arrays is not the same as a pointer to a pointer.

原因是内存布局完全不同.

Reason being that the memory layout is completely different.

对于数组数组:

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

对于指向指针的指针:

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

---

随着数组衰减为指针，您可以通过以下方式获得指向数组的指针:

---

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

int (*ptr)[2] = arr;

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