在前面的文章中，我们讨论了如何实现通用类型的链表，方法是用void *类型的指针，指向数据。那么还有其他的方法吗（不考虑内核链表）？
答案是肯定的。用零长数组也可以实现。
struct node_info
{
struct node_info *next;
struct node_info *prev;
char data[0];
};这里的最后一个元素，是元素个数为0的数组。其不占用任何空间，甚至是一个指针的空间都不占！
注意：在标准C和C++中，长度为0的数组是被禁止使用的。不过在GNU C中，存在一个非常奇怪的用法，那就是长度为0的数组。
在一个结构体的最后 ，定义一个长度为0的数组，就可以使得这个结构体是可变长的。对于编译器来说，这个长度为0的数组并不占用空间，因为数组名本身不占空间，它只是一个偏移量， 数组名这个符号本身代
表了一个不可修改的地址常量
先来看看整个代码的头文件吧
#pragma once
struct node_info
{
struct node_info *next;
struct node_info *prev;
char data[0];
};
struct student
{
char name[20];
unsigned char age;
};//for test
//有头双向循环链表
struct dlist_info
{
struct node_info *head;
void (*add_head)(struct dlist_info *info,
const void *data, size_t size);
void (*add_tail)(struct dlist_info *info,
const void *data, size_t size);
void (*del)(struct node_info *node);
struct node_info* (*find)(struct dlist_info *info,
int (*compare)(void *dest_data, void *key_data), void *key_data);
void (*for_each_safe)(struct dlist_info *info,void (*todo)(struct node_info *));
};
int dlist_init(struct dlist_info *info);
void dlist_destroy(struct dlist_info *info);
#define node_init(node)
do
{
(node)->next = (node);
(node)->prev = (node);
}while(0)
#define dlist_is_empty(info)
((info)->head->next == (info)->head)
接下来我们实现一些方法
1.头插
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include "dlist.h"
/* 有头循环双链表*/
static void dlist_add_head(struct dlist_info *info,
const void *my_data, size_t size)
{
assert(info != NULL && my_data != NULL);
if (size == 0) {
return ;
}
struct node_info *new_node = (struct node_info *)malloc(sizeof(struct node_info) + size);
if (new_node == NULL) {
fprintf(stderr, "out of memory
");
return ;
}
//数据域，内存拷贝
memmove(new_node->data, my_data, size);
//指针域修改
new_node->next = info->head->next;
new_node->prev = info->head;
info->head->next = new_node;
new_node->next->prev = new_node;
}size 表示数据域占用了多少个字节。memmove(new_node->data, my_data, size); 这句话把用户的数据拷贝到了结构体的最后。关于指针域的修改，是不是有点绕呢？没有关系，画图就明白了。
2.尾插
static void dlist_add_tail(struct dlist_info *info,
const void *my_data, size_t size)
{
assert(info != NULL && my_data != NULL);
if (size == 0) {
return ;
}
struct node_info *new_node = (struct node_info *)malloc(sizeof(struct node_info) + size);
if (new_node == NULL) {
fprintf(stderr, "out of memory
");
return ;
}
//数据域，内存拷贝
memmove(new_node->data, my_data, size);
//指针域修改
new_node->next = info->head;
new_node->prev = info->head->prev;
info->head->prev->next = new_node;
info->head->prev = new_node;
}3.删除
static void dlist_del(struct node_info *node)
{
assert(node != NULL);
node->next->prev = node->prev;
node->prev->next = node->next;
node_init(node);
free(node);
}因为申请空间的时候是带着size一起申请的，所以这里的释放就全部释放了，不存在内存泄漏。
4.查找
static struct node_info *dlist_find(struct dlist_info *info,
int (*key)(void *dest_data, void *key_data), void *key_data)
{
assert(info != NULL && key != NULL);
if (dlist_is_empty(info)) {
fprintf(stderr, "dlist is empty
");
return NULL;
}
struct node_info *cur = NULL;
for (cur = info->head->next; cur != info->head;
cur = cur->next) {
if (key(cur->data, key_data) != 0) {
return cur;
}
}
return NULL;
}
回调函数需要用户自己实现，不用多说。
5.安全遍历
static void dlist_for_each_safe(struct dlist_info *info,
void (*todo)(struct node_info *))
{
assert(info != NULL && todo != NULL);
struct node_info *cur = NULL;
struct node_info *Next = NULL;
for (cur = info->head->next; cur != info->head;
cur = Next) {
Next = cur->next;
todo(cur);
}
}6.构造和析构
int dlist_init(struct dlist_info *info)
{
info->head = (struct node_info *)malloc(sizeof(struct node_info));
if (info->head == NULL) {
fprintf(stderr, "Error:Out of memory
");
return -1;
}
/*头节点空间的初始化*/
node_init(info->head);
/*函数指针的挂接*/
info->add_head = dlist_add_head;
info->add_tail = dlist_add_tail;
info->del = dlist_del;
info->find = dlist_find;
info->for_each_safe = dlist_for_each_safe;
return 0;
}
void dlist_destroy(struct dlist_info *info)
{
// 依次删除，直到为空
while (!dlist_is_empty(info)) {
dlist_del(info->head->next);
}
free(info->head);
}接下来是单元测试。
测试一下头插和尾插吧。
#include "uni_test.h"
#include "dlist.h"
#include <stdio.h>
void setup (void)
{
// will excute in every case
}
void teardown (void)
{
}
void print_student(struct node_info *node)
{
struct student *p = (struct student *)(node->data);
printf("Name: %15s Age:%d
",p->name,p->age);
}
int compare_student(void *dest,void *src)
{
struct student *p1 = dest;
struct student *p2 = src;
if(strcmp(p1->name,p2->name)==0)
return 1;
else
return 0;
}
START_TEST(my_dlist_1)
{
struct student students[8] = {{"WangDong",18},{"LiuMing",19},{"SunYazhou",21},{"ChenYu",27},{"LiuXuewei",28},
{"ZhangGuorong",47},{"LiuDehua",53},{"WangGuozhen",48}};
struct dlist_info list;
dlist_init(&list);
int i = 0;
for(;i<sizeof(students)/sizeof(students[0]);++i)
list.add_head(&list,students+i,sizeof(students[0]));
list.for_each_safe(&list,print_student);
printf("===========
");
dlist_destroy(&list);
}
END_TEST
START_TEST(my_dlist_2)
{
struct student students[8] = {{"WangDong",18},{"LiuMing",19},{"SunYazhou",21},{"ChenYu",27},{"LiuXuewei",28},
{"ZhangGuorong",47},{"LiuDehua",53},{"WangGuozhen",48}};
struct dlist_info list;
dlist_init(&list);
int i = 0;
for(;i<sizeof(students)/sizeof(students[0]);++i)
list.add_tail(&list,students+i,sizeof(students[0]));
list.for_each_safe(&list,print_student);
printf("===========
");
dlist_destroy(&list);
}
END_TEST
运行结果如图
Running suite(s): two_way_list_with_head
Name: WangGuozhen Age:48
Name: LiuDehua Age:53
Name: ZhangGuorong Age:47
Name: LiuXuewei Age:28
Name: ChenYu Age:27
Name: SunYazhou Age:21
Name: LiuMing Age:19
Name: WangDong Age:18
===========
Name: WangDong Age:18
Name: LiuMing Age:19
Name: SunYazhou Age:21
Name: ChenYu Age:27
Name: LiuXuewei Age:28
Name: ZhangGuorong Age:47
Name: LiuDehua Age:53
Name: WangGuozhen Age:48
===========
START_TEST(my_dlist_3)//遍历删除
{
struct student students[8] = {{"WangDong",18},{"LiuMing",19},{"SunYazhou",21},{"ChenYu",27},{"LiuXuewei",28},
{"ZhangGuorong",47},{"LiuDehua",53},{"WangGuozhen",48}};
struct dlist_info list;
dlist_init(&list);
int i = 0;
for(;i<sizeof(students)/sizeof(students[0]);++i)
list.add_tail(&list,students+i,sizeof(students[0]));
list.for_each_safe(&list,list.del);
list.for_each_safe(&list,print_student);
printf("===========
");
dlist_destroy(&list);
}
END_TEST运行结果是：
===========
查找并删除。
START_TEST(my_dlist_4)//查找并删除
{
struct student students[8] = {{"WangDong",18},{"LiuMing",19},{"SunYazhou",21},{"ChenYu",27},{"LiuXuewei",28},
{"ZhangGuorong",47},{"LiuDehua",53},{"WangGuozhen",48}};
struct dlist_info list;
dlist_init(&list);
int i = 0;
for(;i<sizeof(students)/sizeof(students[0]);++i)
list.add_tail(&list,students+i,sizeof(students[0]));
list.del(list.find(&list,compare_student,"ChenYu"));
list.for_each_safe(&list,print_student);
printf("===========
");
dlist_destroy(&list);
}
END_TESTName: WangDong Age:18
Name: LiuMing Age:19
Name: SunYazhou Age:21
Name: LiuXuewei Age:28
Name: ZhangGuorong Age:47
Name: LiuDehua Age:53
Name: WangGuozhen Age:48
===========