C语言之封装，继承，多态

本文参考：

• c语言面向对象之封装
• c面向对象之继承
• Linux源码分析之多态

---

一、封装

封装的本质就是将数据和方法集中到一个对象中，c++或者java使用的是class来实现。c语言中可以使用struct来实现同样的功能。比如下面的程序：

struct student
{int age;char *name;void (*read)(char *);
};void read_book(char *book)
{printf("book is :%s", book);
}int main(void)
{struct student s1;s1.age = 10;s1.name = strdup("xiaoming");s1.read = read_book;printf("name: %s age: %d\n", s1.name, s1.age);s1.read("math");free(s1.name)return 0;
}

二、继承

继承可以让子类使用父类的资源，减少重复。父类可以是将子类的共同点抽象出来，提升代码层次。
如何使用c来实现同样的效果，首先肯定还是需要struct来构建父类和子类，同时如果要达到is a的效果，从内存空间来看，也就意味着父类占据了子类内存地址的最开始位置。所以，我们可以按照下面的方式来构造继承。

struct parent
{void (*func1)(void);
};struct child
{struct parent pt;void (*func2)(void);
};void test(struct parent *p)
{p->func1();
}int main()
{struct child cd;test((struct parent *)&cd);
}

三、多态

多态就是在程序运行时，父类指针可以根据具体的子类对象来执行不同的函数行为。c++中一般通过虚函数来实现。c实现多态主要是通过结构体和函数指针。

#include <stdio.h>  // 定义基类  
struct Shape
{  void (*draw)(struct Shape*);  
};// 定义派生类  
struct Circle
{  struct Shape shape;  int radius;  
};struct Rectangle
{  struct Shape shape;  int width;  int height;  
};void drawCircle(struct Shape *shape)
{  printf("Drawing a circle...\n");struct Circle *circle = (struct Circle *)shape;printf("Circle radius is %d\n", circle->radius);
}void drawRectangle(struct Shape *shape)
{  printf("Drawing a rectangle...\n");struct Rectangle *rectangle = (struct Rectangle *)shape;printf("Rectangle width is %d\n", rectangle->width);printf("Rectangle height is %d\n", rectangle->height);
}int main()
{  struct Circle circle;circle.shape.draw = drawCircle;circle.radius = 10;struct Rectangle rectangle;  rectangle.shape.draw = drawRectangle;rectangle.width = 10;rectangle.height = 20;struct Shape *shape;shape = (struct Shape *)&circle;shape->draw(shape);shape = (struct Shape *)&rectangle;shape->draw(shape);return 0;  
}

在Linux内核中数量最多的就是驱动程序代码，因为硬件设备是多种多样的，每种设备都会有对应的驱动程序。Linux对上层的接口都是统一和抽象的，要保证相同的接口最终能够对应到特定的硬件设备，这就需要使用多态的技巧。

include/linux/i2c.h
这个结构体定义了不少函数指针，就是实现多态的关键struct i2c_driver {unsigned int class;/* Standard driver model interfaces */int (*probe)(struct i2c_client *, const struct i2c_device_id *);int (*remove)(struct i2c_client *);/* New driver model interface to aid the seamless removal of the* current probe()'s, more commonly unused than used second parameter.*/int (*probe_new)(struct i2c_client *);/* driver model interfaces that don't relate to enumeration  */void (*shutdown)(struct i2c_client *);/* Alert callback, for example for the SMBus alert protocol.* The format and meaning of the data value depends on the protocol.* For the SMBus alert protocol, there is a single bit of data passed* as the alert response's low bit ("event flag").* For the SMBus Host Notify protocol, the data corresponds to the* 16-bit payload data reported by the slave device acting as master.*/void (*alert)(struct i2c_client *, enum i2c_alert_protocol protocol,unsigned int data);/* a ioctl like command that can be used to perform specific functions* with the device.*/int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);struct device_driver driver;const struct i2c_device_id *id_table;/* Device detection callback for automatic device creation */int (*detect)(struct i2c_client *, struct i2c_board_info *);const unsigned short *address_list;struct list_head clients;bool disable_i2c_core_irq_mapping;
};
drivers/i2c/i2c-slave-eeprom.c