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第五十三课、被遗弃的多重继承(上)
一、c++的多重继承
1、c++支持编写多重继承的代码
(1)、一个子类可以拥有多个父类
(2)、子类拥有所有父类的成员变量
(3)、子类继承所有父类的成员函数
(4)、子类对象可以当做任意父类对象使用
2、多重继承产生的问题一:通过多重继承得到的对象拥有不同的地址
#include<iostream> using namespace std; class BaseA { protected: int ma; public: BaseA(int a) { ma = a; } int GetA() { return ma; } }; class BaseB { protected: 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)//在初始化列表显示调用构造函数,构造顺序先父母,后他人,再自己 { mc = c; } int GetC() { return mc; } void print() { cout << "ma = " << ma << "," << "mb = " << mb << "," << "ma = " << 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;//赋值兼容性 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;//输出这个,说明同一对象拥有不同的地址 } cout << "pa = " << pa << endl; cout << "pb = " << pb << endl; cout << "paa = " << paa << endl; cout << "pbb = " << pbb << endl; return 0; } //输出结果 /* sizeof(Derived) = 12 ma = 1,mb = 2,ma = 3 d.getA() = 1 d.getB() = 2 d.getC() = 3 pa->getA() = 1 pb->getB() = 2 Error pa = 0xbfa104f4//发现地址值相差4,还是指向同一个对象,但是指向同一个对象的不同位置 pb = 0xbfa104f8 paa = 0xbfa104f4 pbb = 0xbfa104f8 */
二、多重继承产生的问题二:当多重继承出现闭合时产生冗余的成员
(1)、虚继承能够解决数据冗余问题
(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
#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//虚继承 { public: Teacher(string name, int age) : People(name, age) { } }; class Student : virtual public People//虚继承 { 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)//最终子类必须直接调用最顶层父类的构造函数 { } }; int main() { Doctor d("Delphi", 33); d.print(); return 0; }
三、小结
1、c++支持多继承的编程方式
2、多继承容易带来问题
(1)、可能出现同一个对象地址不同的情况
(2)、虚继承可以解决数据冗余的问题
(3)、虚继承使得架构设计可能出现问题
第五十三课、被遗弃的多重继承(上)
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