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), [email protected] * 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), [email protected] * Company: ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */ #include#include #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), [email protected] * Company: ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */ #include#include #include #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), [email protected] * 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), [email protected] * Company: ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */ #include#include #include #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), [email protected] * Company: ICT ( Institute Of Computing Technology, CAS ) * * ============================================================================= */ #include#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: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...
查看完整的代码请移步：https://github.com/forhappy/c-recipes/tree/master/ooc