抽象类的需要注意的问题

 

 

带有纯虚函数的类称为抽象类

 

纯虚函数与抽象类的作用：利用它可以创建模板；

优点：能够实现代码重复使用；

 

抽象类派生的派生类必须对这个抽象类中的纯虚函数重载，即使什么功能也不做，也要重载它，不然没有函数实体。

如果不在people.h中添加一下代码：

virtual void play()

{

    ;

}

会出现的错误信息是：

error C2259: 'people' : cannot instantiate abstract class due to following members:
        d:\c text\5.22\practice3\people.h(5) : see declaration of 'people'

warning C4259: 'void __thiscall animal::play(void)' : pure virtual function was not defined
        d:\c text\5.22\practice3\animal.h(20) : see declaration of 'play'

 

 

#ifndef _ANIMAL_H_
#define _ANIMAL_H_

#include <iostream.h>
#include <string.h>
#include <stdio.h>

class animal
{
public:
    char *name;
public:
    animal(char *name1)
    {
        name=new char[strlen(name1)+1];
        strcpy(name,name1);
    }
    virtual void sleep()=0;
    virtual void eat()=0;
    virtual void play()=0;
    ~animal()
    {
        delete name;
    }
};

#endif

在抽象类（基类）中，定义了三个纯虚函数。这时需要在其派生类中重载纯虚函数。

 

 

#ifndef _PEOPLE_H_
#define _PEOPLE_H_
#include "animal.h"

class people:public animal
{
private:
    int age;
public:
    people(char *name1,int age1):animal(name1)
    {
        age=age1;
        printf("Hello, I am %s!\n",name1);
    }
    virtual void sleep()
    {
        printf("people %s is sleeping....!\n",name);
    }
    virtual void eat()
    {
        printf("people %s is eating....!\n",name);
    }
    virtual void play()
    {
        ;
    } //纯虚函数play需要重载,即使什么功能也不做，也要重载它，不然没有函数实体

    void play_with(animal &any)
    {
        printf("people %s is playing with %s ....!\n",name,any.name);
        any.play();
    }
};

#endif

 

 

#ifndef _CAT_H_
#define _CAT_H_
#include "animal.h"

class cat:public animal
{
public:
    cat(char *name1):animal(name1)
    {
        printf("Hello, I am %s! miao....\n",name1);
    }
    virtual void sleep()
    {
        printf("cat %s is sleeping....!\n",name);
    }
    virtual void eat()
    {
        printf("cat %s is eating....!\n",name);
    }
    virtual void play()
    {
        printf("cat %s is playing....! miaomiao..\n",name);
    }
};

#endif

 

#include "animal.h"
#include "cat.h"
#include "dog.h"
#include "people.h"

int main()
{
    people Lee("Lee ming",20);
    cat kitty("kitty");
    dog bruce("Bruce");
    Lee.play_with(kitty);
    Lee.play_with(bruce);
    return 0;
}