【C++深度剖析教程31】被遗弃的多重继承

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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
子类继承父类所有的成员函数
子类对象可以当做任意父类对象使用

多重继承的语法规则：
这里写图片描述

多重继承的本质与单继承相同

编程示例;

#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;
}

运行结果：
这里写图片描述

分析以上程序我们就可以发现问题所在啦：
1、通过多重继承的对象可能拥有不同的地址
这里写图片描述
2、多重继承可能产生冗余的成员：

当多重继承关系闭合将产生数据冗余问题
解决办法是：
虚继承！！！

下面看一个解决冗余的例子：

#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;
}

运行结果为;
Name = Delphi, Age = 33

虚继承能够解决数据冗余的问题
中间层父类不再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

虽然我们解决的数据冗余，但是还有一个问题，在架构设计师，无法确定使用虚继承还是直接继承？？？

3、多重继承有可能会产生多个虚函数表
这里写图片描述

#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); //dynamic_cast会对指针进行修正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;
}

这里写图片描述

工程开发中的多继承方式：
这里写图片描述

#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;
}