---- 整理自狄泰软件唐佐林老师课程

1. 问题

C++是否允许一个类继承自多个父类？

1.1 C++中的 多重继承

• 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

1.2 多重继承的语法规则

• 多重继承的本质和单继承相同

2. 多重继承问题一

• 编程实验：多重继承问题一

#include 
#include using namespace std;class BaseA
{int ma;
public:BaseA(int a){ma = a;}int getA(){return ma;}
};class BaseB
{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){mc = c;}int getC(){return mc;}void print(){cout << "ma = " << getA() << ", "<< "mb = " << getB() << ", "<< "mc = " << mc << endl;}
};int main()
{cout << "sizeof(Derived) = " << sizeof(Derived) << endl;    // 12Derived 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;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;}cout << "pa = " << pa << endl;cout << "pb = " << pb << endl;cout << "paa = " << paa << endl;cout << "pbb = " << pbb << endl; return 0;
}

• 通过 多重继承 得到的对象可能拥有 “不同的地址”

  • 解决方案：无

3. 多重继承问题二

• 多重继承可能产生冗余的成员

Doctor中有两个m_name、m_age

• 编程实验：多重继承问题二

#include 
#include 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
{
public:Teacher(string name, int age) : People(name, age){}
};class Student : virtual 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("Delphi", 33);d.print();return 0;
}

• 当多重继承关系出现闭合时将产生数据冗余的问题

  • 解决方案：虚继承
    

• 虚继承能够解决 数据冗余 问题

• 中间层父类 不再关心顶层父类的初始化（中间层父类不调用父类构造函数）

• 最终子类 必须直接调用 顶层父类的构造函数

问题：当架构设计中需要继承时，无法确定使用直接继承还是虚继承

4. 多重继承问题三

• 多重继承可能产生 多个虚函数表
  

• 编程实验：多重继承问题三

#include 
#include using namespace std;class BaseA
{
public:virtual void funcA(){cout << "BaseA::funcA()" << endl;}
};class BaseB
{
public:virtual void funcB(){cout << "BaseB::funcB()" << endl;}
};class Derived : public BaseA, public BaseB
{};int main()
{Derived d;BaseA* pa = &d;BaseB* pb = &d;BaseB* pbe = (BaseB*)pa;    // oops!!BaseB* pbc = dynamic_cast(pa);cout << "sizeof(d) = " << sizeof(d) << endl;cout << "Using pa to call funcA()..." << endl;pa->funcA();cout << "Using pb to call funcB()..." << endl;pb->funcB();cout << "Using pbc to call funcB()..." << endl;pbc->funcB();cout << endl;cout << "pa = " << pa << endl;cout << "pb = " << pb << endl;cout << "pbe = " << pbe << endl;cout << "pbc = " << pbc << endl;return 0;
}

需要进行强制类型转换时，C++中推荐使用新式类型转换关键字
解决方案：dynamic_cast

5. 正确的使用多重继承

• 工程开发中的“多重继承”方式：
  • 单继承某个类 + 实现（多个）接口

• 编程实验：正确的多继承方式

#include 
#include using namespace std;class Base
{
protected:int mi;
public:Base(int i){mi = i;}int getI(){return mi;}bool equal(Base* obj){return (this == obj);}
};class Interface1
{
public:virtual void add(int i) = 0;virtual void minus(int i) = 0;
};class Interface2
{
public:virtual void multiply(int i) = 0;virtual void divide(int i) = 0;
};class Derived : public Base, public Interface1, public Interface2
{
public:Derived(int i) : Base(i){}void add(int i){mi += i;}void minus(int i){mi -= i;}void multiply(int i){mi *= i;}void divide(int i){if( i != 0 ){mi /= i;}}
};int main()
{Derived d(100);Derived* p = &d;Interface1* pInt1 = &d;Interface2* pInt2 = &d;cout << "p->getI() = " << p->getI() << endl;    // 100pInt1->add(10);pInt2->divide(11);pInt1->minus(5);pInt2->multiply(8);cout << "p->getI() = " << p->getI() << endl;    // 40cout << endl;cout << "pInt1 == p : " << p->equal(dynamic_cast(pInt1)) << endl;cout << "pInt2 == p : " << p->equal(dynamic_cast(pInt2)) << endl;return 0;
}

• 一些有用的工程建议
  • 先继承自一个父类，然后实现多个接口
  • 父类中提供equal()成员函数
  • equal()成员函数用于判断指针是否指向当前对象
  • 与多重继承相关的强制类型转换使用dynamic_cast完成

6. 小结

• C++支持多重继承的编程方式
• 多重继承容易带来问题：
  • 可能出现“同一个对象的地址不同”的情况
  • 虚继承可以解决数据冗余的问题
  • 虚继承使得架构设计可能出现问题
• 多重继承中可能出现多个虚函数表指针VPTR
• 与多重继承相关的强制类型转换用dynamic_cast完成
• 工程开发中采用“单继承多接口”的方式使用多继承
• 父类提供成员函数用于判断是否指向当前对象