【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：

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;    // 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;
    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：

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

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

1. 虚继承能够解决数据冗余的问题
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

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

#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;
}

工程开发中的多继承方式：

#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;    // 100

    pInt1->add(10);
    pInt2->divide(11);
    pInt1->minus(5);
    pInt2->multiply(8);

    cout << "p->getI() = " << p->getI() << endl;    // 40

    cout << endl;

    cout << "pInt1 == p : " << p->equal(dynamic_cast<Base*>(pInt1)) << endl;
    cout << "pInt2 == p : " << p->equal(dynamic_cast<Base*>(pInt2)) << endl;

    return 0;
}

运行结果：

p->getI() = 100
p->getI() = 40

pInt1 == p : 1
pInt2 == p : 1

一些有用的工程建议：

1. 先继承自一个父类，然后实现多个接口
2. 父类中提供equal()成员函数
3. equal()成员函数用于判断当前指针是否指向当前对象
4. 与多重继承相关的强制类型转换用dynamic_cast完成

总结：

1. 多继承中可能出现多个虚函数表指针
2. 与多重继承相关的强制类型转换用dynamic_cast完成
3. 工程开发中使用单继承多接口的方式实现多继承
4. 父类提供成员函数用来判断指针是否指向当前对象