目录

抽象类

抽象类界面

前奏

修改Makefile

引出抽象类界面

---

抽象类

看以下例子，对于Human类中的虚函数，后面加上"=0"就变为了纯虚函数，连空函数都不需要提供，纯虚函数不需要定义，一个类中如果含有纯虚函数就称为抽象类，在main函数中抽象类不能有实例对象

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

using namespace std;

class Human {
private:
	int a;
public:
	virtual void eating(void) = 0; 
	virtual void wearing(void) = 0;
	virtual void driving(void) = 0;
	virtual ~Human() { cout<<"~Human()"<<endl; }
	virtual Human* test(void) {cout<<"Human's test"<<endl; return this; }
};

class Englishman : public Human {
public:
	void eating(void) { cout<<"use knife to eat"<<endl; }
	void wearing(void) {cout<<"wear english style"<<endl; }
	void driving(void) {cout<<"drive english car"<<endl; }
	virtual ~Englishman() { cout<<"~Englishman()"<<endl; }
	virtual Englishman* test(void) {cout<<"Englishman's test"<<endl; return this; }
};


class Chinese : public Human {
public:
	void eating(void) { cout<<"use chopsticks to eat"<<endl; }
	void wearing(void) {cout<<"wear chinese style"<<endl; }
	void driving(void) {cout<<"drive chinese car"<<endl; }
	virtual ~Chinese() { cout<<"~Chinese()"<<endl; }
	virtual Chinese* test(void) {cout<<"Chinese's test"<<endl; return this; }
};



int main(int argc, char **argv)
{
	//Human h; 
	Englishman e;
	Chinese c;

	return 0;
}

如果Chinese类中的driving函数注释了，则Chinese也是一个抽象类，不能被实例化，若子类没有覆写所有的纯虚函数，则子类还是抽象类

class Chinese : public Human {
public:
...
	//void driving(void) {cout<<"drive chinese car"<<endl; } 
...
};

对于Chinese再延伸出派生类，在子类中覆写driving函数就可以进行实例化

class Guangximan : public Chinese {  
	void driving(void) {cout<<"drive guangxi car"<<endl; }
};

 

抽象类界面

一个程序由多个人编写，分为应用编程和类编程，将上述程序分为类编程Human、English和Chinese，和应用编程main.cpp

前奏

如下程序中，English和Chinese都是Human的派生类，因此我们可以把共性的东西抽出来，例如name，这样派生类盘大的时候就可以节省代码

/* Human.h */
#ifndef _HUMAN_H
#define _HUMAN_H

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

using namespace std;

class Human {
private:
	char *name;

public:
	void setName(char *name);
	char *getName(void);
	virtual void eating(void){cout<<"use hand to eat"<<endl;}
	virtual void wearing(void){}
	virtual void driving(void){}
	
};

#endif

/* Human.cpp */
#include "Human.h"

void Human::setName(char *name) 
{
	this->name = name;
}

char *Human::getName(void) 
{
	return this->name;
}

/* Englishman.h */
#ifndef _ENGLISHMAN_H
#define _ENGLISHMAN_H

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

#include "Human.h"

using namespace std;

class Englishman : public Human {
public:
	void eating(void);
	void wearing(void);
	void driving(void);
	~Englishman();
};

#endif


/* Englishman.cpp */
#include "Englishman.h"

void Englishman::eating(void) 
{ 
	cout<<"use knife to eat"<<endl; 
}

void Englishman::wearing(void) 
{
	cout<<"wear english style"<<endl; 
}

void Englishman::driving(void) 
{
	cout<<"drive english car"<<endl; 
}

Englishman::~Englishman() 
{ 
	cout<<"~Englishman()"<<endl; 
}

/* Chinese.h */
#ifndef _CHINESE_H
#define _CHINESE_H

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

#include "Human.h"

using namespace std;

class Chinese : public Human{
public:
	void eating(void);
	void wearing(void);
	void driving(void);
	~Chinese();
};

#endif

/* Chinese.cpp */
#include "Chinese.h"

void Chinese::eating(void) 
{ 
	cout<<"use chopsticks to eat"<<endl; 
}

void Chinese::wearing(void) 
{
	cout<<"wear chinese style"<<endl; 
}

void Chinese::driving(void) 
{
	cout<<"drive chinese car"<<endl; 
}

Chinese::~Chinese() 
{ 
	cout<<"~Chinese()"<<endl; 
}

/* main.cpp */
#include "Human.h"
#include "Englishman.h"
#include "Chinese.h"

void test_eating(Human *h)
{
	h->eating();
}


int main(int argc, char **argv)
{
	Englishman e;
	Chinese c;

	Human* h[2] = {&e, &c};
	int i;
	for (i = 0; i < 2; i++)
		test_eating(h[i]); //需要在human里面实现多态 加上virtual

	return 0;
}

执行结果如下，对于test_eating函数实现多态，在基类加上virtual就可以了

use knife to eat
use chopsticks to eat
~Chinese()
~Englishman()

将程序在Human中将函数编写为纯虚函数，则Human类变为抽象类，这样Human节省空间，连空函数都不需要，同时可以防止Human类的实例化，重新编译执行程序，依然可以实现多态

class Human {
private:
	char *name;

public:
	void setName(char *name);
	char *getName(void);
	virtual void eating(void) = 0;
	virtual void wearing(void) = 0;
	virtual void driving(void) = 0;
	
};

修改Makefile

将应用编程和类编程分开，将类编程编译为动态库，在修改类编程的时候，就不需要重新编译应用编程

Human: main.o libHuman.so
	g++ -o $@ $< -L./ -lHuman

%.o : %.cpp
	g++ -fPIC -c -o $@ $<

libHuman.so : Englishman.o Chinese.o Human.o
	g++ -shared -o $@ $^

clean:
	rm -f *.o Human	

同时需要指定一下动态库环境变量为当前路径，执行结果和上述一样

export LD_LIBRARY_PATH=./

先来修改程序，修改English.h和English.cpp加上一个数组表示地址

/* English.h */
#ifndef _ENGLISHMAN_H
#define _ENGLISHMAN_H

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

#include "Human.h"

using namespace std;

class Englishman : public Human {
private:
	char address[100];
	int age;
public:
	void eating(void);
	void wearing(void);
	void driving(void);
	Englishman();
	Englishman(char *name, int age, char *address);
	~Englishman();
};

#endif

/* English.cpp */
#include "Englishman.h"

void Englishman::eating(void) 
{ 
	cout<<"use knife to eat"<<endl; 
}

void Englishman::wearing(void) 
{
	cout<<"wear english style"<<endl; 
}

void Englishman::driving(void) 
{
	cout<<"drive english car"<<endl; 
}

Englishman::~Englishman() 
{ 
	cout<<"~Englishman()"<<endl; 
}

Englishman::Englishman() {}
Englishman::Englishman(char *name, int age, char *address)
{
	setName(name);
	this->age = age;
	memset(this->address, 0, 100);
	strcpy(this->address, address);
}

/* main.cpp */
#include "Human.h"
#include "Englishman.h"
#include "Chinese.h"

void test_eating(Human *h)
{
	h->eating();
}

int main(int argc, char **argv)
{
	Englishman e("Bill", 10, "sfwqerfsdfas");
	Chinese c;

	Human* h[2] = {&e, &c};
	int i;
	for (i = 0; i < 2; i++)
		test_eating(h[i]);
	
	return 0;
}

重新通过make来编译生成库执行程序没有问题，当我们修改以下语句的时候，再调用刚才生成的动态库执行程序，程序崩溃

/* English.h */
char address[1000];

/* English.cpp */
memset(this->address, 0, 1000);

引出抽象类界面

对于上述情况，我们让应用编程只和Human类打交道，将容易变化的类隔离开，先来修改程序

在Chinese.cpp和English.cpp中都加上一个接口

/* Chinese.cpp */

...
Human& CreateChinese(char *name, int age, char *address)
{
	return *(new Chinese()); //在Chinese类中没有提供name，这里就不传递参数
}


/* English.cpp */

...
Human& CreateEnglishman(char *name, int age, char *address)
{
	return *(new Englishman(name, age, address));
}

 然后在Human.h中声明

#ifndef _HUMAN_H
#define _HUMAN_H

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

using namespace std;

class Human {
private:
	char *name;

public:
	void setName(char *name);
	char *getName(void);
	virtual void eating(void) = 0;
	virtual void wearing(void) = 0;
	virtual void driving(void) = 0;
	
};

Human& CreateEnglishman(char *name, int age, char *address);
Human& CreateChinese(char *name, int age, char *address);

#endif

在main.cpp中就只需要包含Human.h头文件

#include "Human.h"
//#include "Englishman.h"
//#include "Chinese.h"

void test_eating(Human *h)
{
	h->eating();
}


int main(int argc, char **argv)
{
	Human& e = CreateEnglishman("Bill", 10, "sfwqerfsdfas");
	Human& c = CreateChinese("zhangsan", 11, "beijing");

	Human* h[2] = {&e, &c};
	int i;
	for (i = 0; i < 2; i++)
		test_eating(h[i]);

	return 0;
}

这样重新编译，再根据上面的方法去修改address的大小，直接执行程序依然正常运行，这样，中间引入了一个相对固定的Human.h，就是所谓的抽象类界面

完善程序加上析构函数，修改各类，同时将析构函数编写为虚函数，实现调用的时候是自己的析构函数，不应该写成纯虚函数，否则在派生类中需要有一个"~Human()"命名的析构函数

class Chinese : public Human{
public:
	void eating(void);
	void wearing(void);
	void driving(void);
	virtual ~Chinese();
};

...

class Englishman : public Human {
private:
	char address[100];
	int age;
public:
	void eating(void);
	void wearing(void);
	void driving(void);
	Englishman();
	Englishman(char *name, int age, char *address);
	virtual ~Englishman();
};

...

class Human {
private:
	char *name;

public:
	void setName(char *name);
	char *getName(void);
	virtual void eating(void) = 0;
	virtual void wearing(void) = 0;
	virtual void driving(void) = 0;
	virtual ~Human() {cout<<"~Human"<<endl;}
};

...

int main(int argc, char **argv)
{
	Human& e = CreateEnglishman("Bill", 10, "sfwqerfsdfas");
	Human& c = CreateChinese("zhangsan", 11, "beijing");

	Human* h[2] = {&e, &c};
	int i;
	for (i = 0; i < 2; i++)
		test_eating(h[i]);

	delete &e;
	delete &c;	
	return 0;
}