C语言本身是不支持继承和多态的，但其实在 C 的世界里，有一套非常有名的面向对象的框架，用的也非常广，那就是 GObject，它是整个图形界面开发库 GTK 的基石，在IBM developerWorks上有一篇很好的文章介绍 GObject《GObject对象系统》。另外，在 Linux 内核里面也大量使用了面向对象的思想，比如虚拟文件系统，设备驱动等模块，在 lwn上有两篇文章就讲到了内核中的面向对象，详细请看：《Object oriented design patterns in the kernel, part 1》，《Object oriented design patterns in the kernel, part 2》。
c语言里继承和多态的实现主要通过函数指针来实现，现在我们就来动手实现C语言的继承与多态，我们以比较经典的动物世界中的实例来举例：假设动物们（包括人）都会吃(Eat)，会走（Walk），会说（Talk）,而派生类为 dog（汪星人） 和 cat（喵星人），当然还可以是更多，dog 和 cat 都有自己独特的 eat, walk 和 talk 方式，那么大致的代码如下：
基类代码 animal-base.h|c:
/*
* =============================================================================
*
* Filename: animal-base.h
*
* Description: animal base class.
*
*
* =============================================================================
*/
#ifndef _ANIMAL_H_
#define _ANIMAL_H_ typedef struct animal_s_ animal_t;
typedef struct animal_ops_s_ animal_ops_t; /* 动物类，是所有动物类的基类,也是抽象类 */
struct animal_s_ {
char *name; /*< 动物的名称 */
animal_ops_t *animal_ops; /* 动物的基本行为 */
}; /* 动物的基本行为 */
struct animal_ops_s_ {
/* 动物吃了什么食物 */
void (*eat)(char *food);
/* 动物走了多少步 */
void (*walk)(int steps);
/* 动物在说什么 */
void (*talk)(char *msg);
}; /* 基类的构造函数，需要显示调用 */
extern animal_t * animal_init(char *name); /* 基类的有关操作，如吃，走，说等等 */
extern void animal_eat(animal_t *animal, char *food);
extern void animal_walk(animal_t *animal, int steps);
extern void animal_talk(animal_t *animal, char *msg); /* 基类的析构函数，需要显示调用 */
extern void animal_die(animal_t *animal); #endif /* _ANIMAL_H_ */
/*
* =============================================================================
*
* Filename: animal-base.c
*
* Description: animal base class.
*
* =============================================================================
*/
#include <assert.h>
#include <stdlib.h>
#include <string.h> #include "animal-base.h" /* 基类的构造函数，需要显示调用 */
animal_t * animal_init(char *name)
{
assert(name != NULL);
size_t name_len = strlen(name); animal_t *animal = (animal_t *)malloc(sizeof(animal_t)
+ sizeof(animal_ops_t) + name_len + 1);
memset(animal, 0, (sizeof(animal_t) + sizeof(animal_ops_t)
+ name_len + 1));
animal->name = (char *)animal + sizeof(animal_t);
memcpy(animal, name, name_len);
animal->animal_ops = (animal_ops_t *)((char *)animal
+ sizeof(animal_t) + name_len + 1); return animal;
} /* 基类的有关操作，如吃，走，说等等 */
void animal_eat(animal_t *animal, char *food)
{
animal->animal_ops->eat(food);
return;
} void animal_walk(animal_t *animal, int steps)
{
animal->animal_ops->walk(steps);
return;
} void animal_talk(animal_t *animal, char *msg)
{
animal->animal_ops->talk(msg);
return;
} /* 基类的析构函数，需要显示调用 */
void animal_die(animal_t *animal)
{
assert(animal != NULL); free(animal);
return;
}
汪星人 dog 类的实现代码：
#include "animal-base.h"
typedef struct dog_s_ dog_t;
struct dog_s_ {
animal_t base; /* 继承自 animal 基类 */
/* 以下还可以添加与 dog 相关的属性和方法(函数指针), 如: */
/* char *owner; // dog 的主人 */
/* void (*hunt)(const char *rabbit); // 猎兔犬 */
};
extern dog_t * dog_init();
extern void dog_die(dog_t * dog);
/*
* =============================================================================
*
* Filename: dog.c
*
* Description: dog class derived from animal base class.
*
* =============================================================================
*/
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h> #include "dog.h" static void eat(char *food); static void walk(int steps); static void talk(char *msg); dog_t * dog_init()
{
dog_t *dog = (dog_t *)malloc(sizeof(dog_t));
animal_t *animal = (animal_t *)animal_init("doggggggggggggg");
memcpy(&(dog->base), animal, sizeof(animal_t)); dog->base.animal_ops->eat = eat;
dog->base.animal_ops->walk = walk;
dog->base.animal_ops->talk = talk; free(animal);
return dog;
} void dog_die(dog_t *dog)
{
/* nothing to do here. */
} static void eat(char *food)
{
printf("I'm a dog, I eat %s\n", food);
} static void walk(int steps)
{
printf("I'm a dog, I can jump %d steps one time\n", steps);
} static void talk(char *msg)
{
printf("I'm a dog, I talk my language %s\n", msg);
}
喵星人(cat 类) 的实现代码：
/*
* =============================================================================
*
* Filename: cat.h
*
* Description: cat class derived from animal base class.
*
* =============================================================================
*/
#include "animal-base.h" typedef struct cat_s_ cat_t; struct cat_s_ {
animal_t base; /* 继承自 animal 基类 */ /* 以下还可以添加与 cat 相关的属性和方法(函数指针), 如: */
/* char *owner; // cat 的主人 */
/* void (*hunt)(const char *rabbit); // 猎兔犬 */
}; extern cat_t * cat_init();
extern void cat_die(cat_t * cat);
/*
* =============================================================================
*
* Filename: cat.c
*
* Description: cat class derived from animal base class.
* =============================================================================
*/
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h> #include "cat.h" static void eat(char *food); static void walk(int steps); static void talk(char *msg); cat_t * cat_init()
{
cat_t *cat = (cat_t *)malloc(sizeof(cat_t));
animal_t *animal = (animal_t *)animal_init("cat");
memcpy(&(cat->base), animal, sizeof(animal_t)); cat->base.animal_ops->eat = eat;
cat->base.animal_ops->walk = walk;
cat->base.animal_ops->talk = talk; free(animal);
return cat;
} void cat_die(cat_t *cat)
{
/* nothing to do here. */
} static void eat(char *food)
{
printf("I'm a cat, I eat %s\n", food);
} static void walk(int steps)
{
printf("I'm a cat, I can jump %d steps one time\n", steps);
} static void talk(char *msg)
{
printf("I'm a cat, I talk my language %s\n", msg);
}
最后，测试代码如下：
/*
* =============================================================================
*
* Filename: main.c
*
* Description: main test.
*
*
* =============================================================================
*/
#include <stdio.h> #include "animal-base.h"
#include "dog.h"
#include "cat.h" int main(int argc, const char *argv[])
{
dog_t *dog = dog_init();
cat_t *cat = cat_init(); /* dog 类测试 */
animal_eat(dog, "bones");
animal_walk(dog, 5);
animal_talk(dog, "wuang wuang wuang..."); /* cat 类测试 */
animal_eat(cat, "fish");
animal_walk(cat, 3);
animal_talk(cat, "miao miao miao..."); }
还有Makefile ：
all:main main:main.o dog.o cat.o animal-base.o
gcc -o $@ $^ main.o:main.c cat.o:cat.c dog.o:dog.c animal-base.o:animal-base.c .PHONY:clean clean:
rm main main.o dog.o cat.o animal-base.o
最后执行结果为：
I'm a dog, I eat bones
I'm a dog, I can jump 5 steps one time
I'm a dog, I talk my language wuang wuang wuang...
I'm a cat, I eat fish
I'm a cat, I can jump 3 steps one time
I'm a cat, I talk my language miao miao miao...