【C++】53.被遗弃的多重继承（上）

问题：

C++ 中是否允许一个类继承多个父类？

C++支持编写多重继承的代码

• 一个子类可以拥有多个父类
• 子类拥有所有父类的成员变量
• 子类继承所有父类的成员函数
• 子类对象可以当作任意父类对象使用

多重语法规则

class Derived ： public BaseA, public BaseB, public BaseC
{
};

多重继承的本质与单继承相同

示例：

class CBaseA
{
	int m_a;
	
public:
	CBaseA(int a)
	{
		m_a = a;
	}
	
	int getA()
	{
		return m_a;
	}
	
};

class CBaseB
{
	int m_b;
	
public:
	CBaseB(int b)
	{
		m_b = b;
	}
	
	int getB()
	{
		return m_b;
	}
	

};


class CDerived : public CBaseA, public CBaseB
{
	int m_c;
public:
	CDerived(int a, int b, int c): CBaseA(a), CBaseB(b)
	{
		m_c = c;
	}
	
	void print()
	{
		cout << "ma = " << getA() << ", "
        << "mb = " << getB() << ", "
        << "mc = " << m_c << endl;
	}
	
	
};
 
int main()
{
	
	CDerived d(1, 2, 3);
	
	d.print();
	
	CBaseA *pa = &d;
	CBaseB *pb = &d;
	
	cout << pa->getA() << endl;
	cout << pb->getB() << endl;
	
	cout << endl;
	
	void *paa = pa;
	void *pbb = pb;
	
	if( paa == pbb)
	{
		cout << " Pointer same adder " << endl;
	}
	else
	{
		cout << "Not Pointer same adder " << endl;
	}
	
	cout << "pa = " << pa << endl;
    cout << "pb = " << pb << endl;
    cout << "paa = " << paa << endl;
    cout << "pbb = " << pbb << endl; 

    
    return 0;
}

输出：

ma = 1, mb = 2, mc = 3
1
2

Not Pointer same adder 

pa = 0x7ffd70e33930
pb = 0x7ffd70e33934
paa = 0x7ffd70e33930
pbb = 0x7ffd70e33934

分析：父类指针指向同一个同一个子类，但是地址却不一样

多重继承的问题一

通过多重继承得到的对象可以拥有 不同的地址（没有对应的解决方案）

Derived d(1, 2, 3);
BaseA* pa = &d;
BaseB* pb = &d;

指向相同的对象，但是对象的地址却不一样，好比一个指向头，一个指向胸口

多重继承的问题二

多重继承可能产生冗余的成员

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 :  public People
{
public:
    Teacher(string name, int age) : People(name, age)
    {
    }
};

class Student :  public People
{
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)
    {
    }
};

int main()
{
    Doctor d("Kevin", 24);
    
    d.print();
    
    return 0;
}

错误信息：

Test.cpp:144:7: error: request for member ‘print’ is ambiguous
     d.print();

当多重继承关系出现闭合时将参生数据冗余的问题

• 解决方法：虚继承

class People {}；
class Teacher : virtual public People {};
class Student : virtual  public People {};
class Doctor : public Teacher, public Student {};

• 虚继承能够解决数据冗余问题
• 中间层父类不在关心顶层父类的初始化
• 最终子类必须直接调用顶层父类的构造函数

目前来看只有C++语言支持多继承

问题：

当架构设计中需要继承时，无法确认使用直接继承还是虚继承！

虚继承在项目管理上带来了难度，多继承不适合实际的项目开发。

小结

• C++支持多重继承的编程方式
• 多重继承容易带来的问题

1. 可能出现同一个对象的地址不同的情况
2. 虚继承可以解决数据冗余问题，虚继承的使架构设计可能出现问题