C語言——指標與結構體 記憶體的動態分配

例一 ：

 1 #include <stdio.h>
 2 #include <string.h>
 3 
 4 struct Student {
 5     int sid ;
 6     char name[200] ;
 7     int age ;
 8 };   
 9   
10 void f(struct Student *p);
11 void g(struct Student *p);
12 void h(struct Student stu);
13 
14 int main()
15 {
16     struct Student stu ;
17 
18     f(&stu);
19     g(&stu);
20     h(stu);
21 
22     //printf("%d %s %d \n" ,stu.sid , stu.name , stu.age);
23 
24     return 0 ;
25 }
26 
27 void g(struct Student *p)    //用指標可以減小記憶體佔用 , 節省時間提高效率
28 {
29     printf("%d %s %d \n", p -> sid , p -> name , p -> age);
30 }
31 
32 void h(struct Student stu)  //值傳遞
33 {
34     printf("%d %s %d \n", stu.sid , stu.name ,stu.age);
35 }
36 
37 void f(struct Student *p)  //輸入函數
38 {
39     (*p).sid = 23 ;    //等價 p -> sid = 23 ;
40     strcpy( p -> name , "zhangsan");
41     p -> age = 34 ;
42 }

例二 ：

 1 //////////////////////////////////
 2 // ——注釋部分為第二種方法——///
 3 //////////////////////////////////
 4 #include <stdio.h>
 5 #include <malloc.h>
 6 
 7 struct Student
 8 {
 9     int sid ;
10     int age ;
11 };
12 
13 //struct Student *CreateStudent(struct Student **q);
14 struct Student *CreateStudent(void);
15 void ShowStudent(struct Student *);
16 
17 int main()
18 {
19     struct Student *p ;
20 
21     //p = CreateStudent(&p);
22     p = CreateStudent();
23     ShowStudent(p);
24 
25     return 0 ;
26 }
27 
28 //struct Student *CreateStudent(struct Student **q)
29 struct Student *CreateStudent(void)
30 {
31     /*
32     *q = (struct Student *)malloc(sizeof(struct Student));
33 
34     (*q) -> sid = 78 ;  // " -> "優先順序大於" * "
35     (*q) -> age = 23 ;
36     return *q;
37     */
38     struct Student *p = (struct Student *)malloc(sizeof(struct Student));
39 
40     p -> sid = 78 ;
41     p -> age = 23 ;
42 
43     return p ;
44 }
45 
46 void ShowStudent(struct Student *ps)
47 {
48     printf("%d %d \n" , ps -> sid , ps -> age);
49 }