代码改变世界

如何实现 C 语言的继承和多态

2012-12-31 13:18  Haippy  阅读(11518)  评论(4编辑  收藏  举报

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), haipingf@gmail.com
 *        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), haipingf@gmail.com
 *        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), haipingf@gmail.com
 *        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), haipingf@gmail.com
 *        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), haipingf@gmail.com
 *        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), haipingf@gmail.com
 *        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: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