1.我们在c语言中会经常碰到强制类型转换。
在这，我介绍一种结构pointer类型转换，但是有前提（有点类似于c++中的继承中的子父对象的cast）。
简单的介绍一下：
首先我们要知道一个结构的指针，并且 在这个结构体中，第一个结构成员必须也是一个结构体(最好是结构体类型).
那么我们可以这个结构体指针转换为指向这个结构体中第一个成员结构体的指针。
直接看代码：
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/* struct transform for struct point
2 * author lkk
3 * time 2015-5-2
4 * inclcude a struct point
5 */
6 /*
7 * first a main struct vx_image( the key to include sub-struct) point
8 * we need transform the struct type to place first in the main struct
9 * some other struct
10 *
11 */
12
13 #include <stdio.h>
14 // define vx_ref
15 typedef struct _vx_ref{
16 int a;
17 }vx_ref_t;
18 typedef struct _vx_ref *vx_ref;
19 //define vx_scale
20 typedef struct _vx_scale{
21 int ab;
22 }vx_scale_t;
23 typedef struct _vx_scale *vx_scale;
24 //define vx_image include the two struct vx_ref vx_scale
25 typedef struct _vx_image {
26 vx_ref_t ab;
27 vx_scale_t ac;
28 int b;
29 }vx_image_t;
30 typedef struct _vx_image *vx_image;
31 //the main
32 void main()
33 {
34 vx_image a; //define a point to vx_image pointer
35 a->ab.a = 1;// put to assignment of sub_struct
36 a->ac.ab = 2;
37 printf("********the old value*********\n");
38 printf("the main struct value is %d %d:\n",a->ab.a,a->ac.ab);
39 printf("the transform first structure\n");
40 vx_ref p = (vx_ref)a;// make the main struct pointer point to sub_strcut
41 // printf("the transform second structure\n");
42 printf("the is %d\n",p->a);//output
43 // printf("we try\n");
44 // vx_scale q = (vx_scale)a;
45 // printf("the is %d ",a->ab);//output
46
47 }
48 // the conclusion
49 //in the main struct include some sub_strcut
50 //we can use first sub_struct pointer to cast(强制转换) the main structure pointer
51 // example : sub_strcut pointer = (sub_struct) (the main structure pointer)
52 //so we get sub_struct pointer.
53 //only by first point
运行结果：
********the old value*********
the main struct value is 1 2:
the transform first structure
the is 1
刚好和自己的想法是一样的。