第五十三课、被遗弃的多重继承（上）

一、c++的多重继承

1、c++支持编写多重继承的代码

（1）、一个子类可以拥有多个父类

（2）、子类拥有所有父类的成员变量

（3）、子类继承所有父类的成员函数

（4）、子类对象可以当做任意父类对象使用

2、多重继承产生的问题一：通过多重继承得到的对象拥有不同的地址

#include<iostream>

using namespace std;

class BaseA
{
protected:    
    int ma;
public:
    BaseA(int a)
    {
         ma = a;       
    }

    int GetA()
    {
        return ma;
    }
};
class BaseB
{
protected:    
    int mb;
public:
    BaseB(int b)
    {
        mb = b;       
    }

    int GetB()
    {
        return mb;
    }
};

class Derived : public BaseA, public BaseB
{
    int mc;
public:
    Derived(int a, int b, int c) : BaseA(a), BaseB(b)//在初始化列表显示调用构造函数，构造顺序先父母，后他人，再自己
    {
        mc = c;       
    }

    int GetC()
    {
        return mc;
    }

    void print()
    {
        cout << "ma = " << ma << ","
             << "mb = " << mb << ","
              << "ma = " << mc << endl;    
    }
};

int main()
{
    cout << "sizeof(Derived) = " << sizeof(Derived) << endl;    // 12
    
    Derived d(1, 2, 3);
    
    d.print();
    
    cout << "d.GetA() = " << d.GetA() << endl;
    cout << "d.GetB() = " << d.GetB() << endl;
    cout << "d.GetC() = " << d.GetC() << endl;

    cout << endl;
    
    BaseA* pa = &d;//赋值兼容性
    BaseB* pb = &d;
    
    cout << "pa->GetA() = " << pa->GetA() << endl;
    cout << "pb->GetB() = " << pb->GetB() << endl;


     cout << endl;
    
    void* paa = pa;
    void* pbb = pb;
    
    
    if( paa == pbb )
    {
        cout << "Pointer to the same object!" << endl; 
    }
    else
    {
        cout << "Error" << endl;//输出这个，说明同一对象拥有不同的地址
    }
    
    cout << "pa = " << pa << endl;
    cout << "pb = " << pb << endl;
    cout << "paa = " << paa << endl;
    cout << "pbb = " << pbb << endl; 


       
    return 0;
}

//输出结果
/*
sizeof(Derived) = 12
ma = 1,mb = 2,ma = 3
d.getA() = 1
d.getB() = 2
d.getC() = 3

pa->getA() = 1
pb->getB() = 2

Error
pa = 0xbfa104f4//发现地址值相差4，还是指向同一个对象，但是指向同一个对象的不同位置
pb = 0xbfa104f8
paa = 0xbfa104f4
pbb = 0xbfa104f8
*/

二、多重继承产生的问题二：当多重继承出现闭合时产生冗余的成员

（1）、虚继承能够解决数据冗余问题

（2）、中间层父类不再关心顶层父类的初始化

（3）、最终子类必须直接调用顶层父类的构造函数

#include <iostream>
#include <string>

using namespace std;

class People
{
    string m_name;
    int m_age;
public:
    People(string name, int age)
    {
        m_name = name;
        m_age = age;
    }
    void print()
    {
        cout << "Name = " << m_name << ", "
             << "Age = " << m_age << endl;
    }
};

class Teacher : virtual public People//虚继承
{
public:
    Teacher(string name, int age) : People(name, age)
    {
    }
};

class Student : virtual public People//虚继承
{
public:
    Student(string name, int age) : People(name, age)
    {
    }
};

class Doctor : public Teacher, public Student
{
public:
    Doctor(string name, int age) : Teacher(name, age), Student(name, age), People(name, age)//最终子类必须直接调用最顶层父类的构造函数
    {
    }
};

int main()
{
    Doctor d("Delphi", 33);
    
    d.print();
    
    return 0;
}

三、小结

1、c++支持多继承的编程方式

2、多继承容易带来问题

（1）、可能出现同一个对象地址不同的情况

（2）、虚继承可以解决数据冗余的问题

（3）、虚继承使得架构设计可能出现问题