[C++知识库]继承的概念和意义

实验演示：
组合关系的描述

#include <iostream>
#include <string>

using namespace std;

class Memory
{
public:
    Memory()
    {
        cout << "Memory()" << endl;
    }
    ~Memory()
    {
        cout << "~Memory()" << endl;
    }
};

class Disk
{
public:
    Disk()
    {
        cout << "Disk()" << endl;
    }
    ~Disk()
    {
        cout << "~Disk()" << endl;
    }   
};

class CPU
{
public:
    CPU()
    {
        cout << "CPU()" << endl;
    }
    ~CPU()
    {
        cout << "~CPU()" << endl;
    }    
};

class MainBoard
{
public:
    MainBoard()
    {
        cout << "MainBoard()" << endl;
    }
    ~MainBoard()
    {
        cout << "~MainBoard()" << endl;
    }    
};

class Computer
{
    Memory mMem;
    Disk mDisk;
    CPU mCPU;
    MainBoard mMainBoard;
public:
    Computer()
    {
        cout << "Computer()" << endl;
    }
    void power()
    {
        cout << "power()" << endl;
    }
    void reset()
    {
        cout << "reset()" << endl;
    }
    ~Computer()
    {
        cout << "~Computer()" << endl;
    }
};

int main()
{   
    Computer c;
    
    return 0;
}

类之间的组合关系的特点：
(1)将其它类的对象作为当前类的成员使用
(2)当前类的对象与成员对象的声明期相同
(3)成员对象在其他用法上与普通对象完全一致

继承关系：父子关系 （父类(基类)） （子类(派生类)

面向对象中的继承指类之间的父子关系：
(1)子类拥有父亲的所有属性和行为
(2)子类是一种特殊的父类
(3)子类对象可以当做父类对象使用
(4)子类中可以添加父亲没有的方法和属性

C++中通过下面的方式描述继承关系：

class Parent
{
    int mv;
public:
    void method()
    {

    }
};

class Child : public Parent  //描述继承关系
{

};

实验演示：继承体验

#include <iostream>
#include <string>

using namespace std;

class Parent
{
    int mv;
public:
    Parent()
    {
        cout << "Parent()" << endl;
        mv = 100;
    }
    void method()
    {
        cout << "mv = " << mv << endl;
    }
};

class Child : public Parent  //描述继承关系
{
public:
    void hello()
    {
        cout << "Hello" << endl;
    }
};

int main()
{
    Child c;
    Parent p1 = c; //子类对象可以当做父类对象使用
    c.method();
    c.hello();
    return 0;
}

重要规则：
(1)子类是一个特殊的父类
(2)子类对象可以直接初始化父类对象
(3)子类对象可以直接赋值给父类对象
(4)子类对象可以当做父类对象使用

继承的意义：继承是C++中代码复用的重要手段。通过继承，可以获得父类的所有功能，并且可以在子类中重写已有功能，或者添加新功能

编程实验：继承的强化体验

#include <iostream>
#include <string>

using namespace std;

class Memory
{
public:
    Memory()
    {
        cout << "Memory()" << endl;
    }
    ~Memory()
    {
        cout << "~Memory()" << endl;
    }
};

class Disk
{
public:
    Disk()
    {
        cout << "Disk()" << endl;
    }
    ~Disk()
    {
        cout << "~Disk()" << endl;
    }   
};

class CPU
{
public:
    CPU()
    {
        cout << "CPU()" << endl;
    }
    ~CPU()
    {
        cout << "~CPU()" << endl;
    }    
};

class MainBoard
{
public:
    MainBoard()
    {
        cout << "MainBoard()" << endl;
    }
    ~MainBoard()
    {
        cout << "~MainBoard()" << endl;
    }    
};

class Computer
{
    Memory mMem;
    Disk mDisk;
    CPU mCPU;
    MainBoard mMainBoard;
public:
    Computer()
    {
        cout << "Computer()" << endl;
    }
    void power()
    {
        cout << "power()" << endl;
    }
    void reset()
    {
        cout << "reset()" << endl;
    }
    ~Computer()
    {
        cout << "~Computer()" << endl;
    }
};

class HPBook : public Computer
{
    string os;
public:
    HPBook()
    {
        os = "Windows 8";
    }
    void install(string os)
    {
        os = os;
    }
    void OS()
    {
        cout << os << endl;
    }
};

class MacBook : public Computer
{
public:
    void OS()
    {
        cout << "Mac os" << endl;
    }
};
int main()
{   
    Computer c;

    cout << endl;

    HPBook hp;
    hp.power();
    hp.install("Ubuntu 16.04");
    hp.OS();

    cout << endl;

    MacBook mac;
    mac.OS();

    return 0;
}