（了解）类前置声明实现接口和实现分离 必须使用指针 且不能作为基类

/////////Man.h///////////

#pragma once

//#include "person.h"        //去掉

class Man : public Person

{

public:

    Man(void);

    ~Man(void);

private:

    Person child;

};

/////////Man.h///////////

#pragma once

//#include "person.h"   //去掉

class Person;           //加入

class Man:public Person

{

public:

    Man(void);

    ~Man(void);

private:

    Person child;

};

error C2504: “Person” : 未定义基类

error C2504: “Person” : 未定义基类

/////////Man.h///////////

#pragma once

//#include "person.h"   //去掉

class Person;           //加入

class Man:public Person

{

public:

    Man(void);

    ~Man(void);

private:

    Person *child;      //改为指针

};

/////////Man.h///////////

#pragma once

//#include "person.h"   //去掉

class Person;           //加入

class Man               //去掉:Person

{

public:

    Man(void);

    ~Man(void);

private:

    Person *child;      //改为指针

};

error C2504: “Person” : 未定义基类

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