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多继承即一个子类可以有多个父类，它继承了多个父类的特性。

class <派生类名>:<继承方式1><基类名1>,<继承方式2><基类名2>,…
{
<派生类类体>
};

其中，访问修饰符继承方式是 public、protected 或 private 其中的一个，用来修饰每个基类，各个基类之间用逗号分隔，如上所示。

1. 子类与父类关系

多继承本质与单继承相同。子类拥有所有父类的成员变量，所有父类的成员函数，子类对象可以当做任意的父类对象使用。

#include <iostream>
#include <string>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;
}

1.2. 多继承问题

多继承在实际项目中需要按照合理的方式被使用，主要是多继承存在多种问题，会给架构设计带来问题。首先，上面例子中pa和pb地址两个指针地址值不一样时，它们也有可能指向的是同一个对象，这就是多重继承引入的问题。

其次，多继承还会产生如下冗余成员；

 

这种问题可以通过虚继承就不会调用父类的构造函数解决。但是，虚继承情况下Docter类需要直接调用顶层父类的构造函数，如第42行所示。在大型的工程中，我们可能很难找到顶层父类，虚继承解决了数据冗余问题，但是带来了项目管理问。

#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
{
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;
}

最后，多继承可能产生多个虚函数表，解决方案是通过dynamic_cast。

#include <iostream>
#include <string>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<BaseB*>(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;
}

2. 正确打开方式

单个继承某个类，同时继承多个接口。工程中可以在父类中提供equal()成员函数，equal()成员函数用于判断指针是否指向当前对象，与多继承相关的强制类型转换使用dynamic_cast。

#include <iostream>
#include <string>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<Base*>(pInt1)) << endl;cout << "pInt2 == p : " << p->equal(dynamic_cast<Base*>(pInt2)) << endl;return 0;
}