C语言实现继承和多态

上面一篇博客中写了《如何实现 C 的函数重载》，本文将进一步讲讲如何实现 C 语言的继承和多态，其实大致的思想已经在前面的一篇博客《200 行 C 代码实现插件式 NOSQL 存储服务器(一)》中体现过了，主要就是如何灵活运用C的函数指针这一特性！

等等，C本身是不支持继承和多态的，那还煞费苦心实现 C 的继承和多态干嘛呢？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》。

好了，貌似扯远了，现在我们就来动手实现C语言的继承与多态，我们还是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. * *        Created:  12/31/2012 11:36:43 AM * *         Author:  Fu Haiping (forhappy), [e-mail] *        Company:  ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */#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. * *        Created:  12/31/2012 12:27:27 PM * *         Author:  Fu Haiping (forhappy), [e-mail] *        Company:  ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */#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. * *        Created:  12/31/2012 12:52:26 PM * *         Author:  Fu Haiping (forhappy), [e-mail] *        Company:  ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */#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. * *        Created:  12/31/2012 12:44:05 PM * *         Author:  Fu Haiping (forhappy), [e-mail] *        Company:  ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */#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. * *        Created:  12/31/2012 12:52:26 PM * *         Author:  Fu Haiping (forhappy), [e-mail] *        Company:  ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */#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. * *        Created:  12/31/2012 01:00:43 PM * *         Author:  Fu Haiping (forhappy), [e-mail] *        Company:  ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */#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...");}

当然还有一点点 Makefile 啦：

all:mainmain:main.o dog.o cat.o animal-base.o    gcc -o $@ $^main.o:main.ccat.o:cat.cdog.o:dog.canimal-base.o:animal-base.c.PHONY:cleanclean:    rm main main.o dog.o cat.o animal-base.o

最后执行结果为：

I'm a dog, I eat bonesI'm a dog, I can jump 5 steps one timeI'm a dog, I talk my language wuang wuang wuang...I'm a cat, I eat fishI'm a cat, I can jump 3 steps one timeI'm a cat, I talk my language miao miao miao...

转载自:http://www.cnblogs.com/haippy/archive/2012/12/31/2840501.html