1.c++中的多重继承
a.c++支持编写多重继承的代码
b.一个子类可以拥有多个父类
c.子类拥有父类的成员变量
d.子类继承所有父类的成员函数
e.子类对象可以当做任意杜磊对象使用(退化)
语法规则:
class Derived : public BaseA, public BaseB, public BaseC { // 多重继承的本质与单继承相同 }
2.多重继承的问题一
多重继承时得到的对象可能拥有不同的地址。
#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 = " << getC() << ", " << endl; } }; int main() { cout << "sizeof(Derived) = " << sizeof(Derived) << endl; // 12, 子类是父类的叠加 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; // 把b对象的地址给pa指针 (退化) BaseB* pb = &d; // 把b对象的地址给pb指针 这里是同一个对象理论上来说,地址应该是一样的。 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,这表示。pa和pb从d中获得到地址不同 } cout << "pa = " << pa << endl; cout << "pb = " << pb << endl; cout << "paa = " << paa << endl; cout << "pbb = " << pbb << endl; return 0; }
解析:
多重继承时得到的对象可能拥有不同的地址。如下图所示。
pa为BaseA指针。它得到的地址为叠加时,它的成员在子类中存储的位置。
同理:pb为BaseB指针。它的得到的地址为叠加时,它的成员在子类中的存储位置
解决办法:无
3.多重继承的问题二
多重继承可能产生冗余的成员
eg:
#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 : public People { public: Teacher(string name, int age) : People(name, age) { } }; class Student : 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) { } }; int main() { Doctor d ("Huang", 24); d.print(); // d.Teacher::print(); // 可行,分开引用 // d.Student::print(); return 0; }
解析:这里把People这个类叠加了两次。在Tacher和Student中各自叠加两次后。在Doctor中叠加。所以会出现两个print函数。函数调用时,系统不知道到底要调用Tacher中叠加的print函数还是Student中的。
当多重继承关系出现闭合时,将产生数据冗余问题。
解决办法:虚继承
a.虚继承能够解决数据冗余问题
b.中间层父类不在关心顶层的初始化(但是还是得正常写)
c.最终子类必须直接调用顶层父类的构造函数
eg:
class People { }; // 因为继承关系在这里开始闭合(这里继承最顶层父类),所以在这里使用虚继承 class Tacher : virtual public People { }; // 因为继承关系在这里开始闭合(这里继承最顶层父类),所以在这里使用虚继承 class Student : virtual public People { }; class Doctor : public Tacher, public Student { public: Doctor (string name, int age) : Teacher(name, age), Student(name, age), People(name, age) { } };