39 多重继承

目录

• 1 问题
• 2 C++ 中的多重继承
• 3 多重继承的问题

1 问题

• C++ 中是否允许一个类继承自多个父类？

2 C++ 中的多重继承

• C++ 支持编写多重继承

  • 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

• 多重继承的语法规则

  • 多重继承的本质与单继承相同

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

• 示例1：多重继承问题一

  • Demo

    #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;
    }
    

  • 编译运行

    sizeof(Derived) = 12
    ma = 1, mb = 2, mc = 3
    d.getA() = 1
    d.getB() = 2
    d.getC() = 3
    
    pa->getA() = 1
    pa->getB() = 2
    
    Error
    pa = 0xbff39374
    pb = 0xbff39378
    paa = 0xbff39374
    pbb = 0xbff39378
    

3 多重继承的问题

• 问题一：通过多重继得到的对象可能拥有不同的地址

  • 解决方案：无！

• 问题二：多重继承可能产生冗余的成员

  • 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：虚函数
  • 虚继承能够解决数据冗余的问题
  • 自建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