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学习C++编译环境:Linux
第五十三课 被遗弃的多重继承(上)
1.问题
C++中是否允许一个类继承自多个父类?允许
2.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!
53-1 多重继承问题一
#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
sizeof(Derived) = 12
ma =1,mb =2,mc =3
d.getA() = 1
d.getB() = 2
d.getC() = 3
pa->getA() = 1
pb->getB() = 2
Error
pa = 0x7fffe245f4b0
pb = 0x7fffe245f4b4
paa = 0x7fffe245f4b0
pbb = 0x7fffe245f4b4
3.多重继承问题一
通过多重继承得到的对象可能拥有“不同的地址”!!
解决方案:无
实质:指向同一个对象的不同位置(一个指向头,一个指向腿)
在这里插入图片描述
4.多重继承的问题二
多重继承可能产生冗余的成员
53-2 多重继承问题二
#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 + "1", age + 1), Student(name, age),People(name, age)
{
}
};
int main()
{
Doctor d("Delphi", 33);
//d.print();
d.Teacher::print();
d.Student::print();
return 0;
}
运行结果
Name = Delphi,Age = 33
Name = Delphi,Age = 33
无论运行哪种方式,两者结果是一样的。
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:虚继承
虚继承能够解决数据冗余问题
中间层父类不再关系顶层父类的初始化
最终子类必须直接调用顶层父类的构造函数
问题:当架构设计中需要继承时,无法确定使用直接继承还是虚继承!!!
牺牲效率,牺牲移植性,架构师不喜欢多继承,多重继承仅作学术研究。
小结
C++支持多重继承的编程方式
多重继承容易带来问题
可能出现“同一个对象的地址不同”的情况
虚继承可以解决数据冗余的问题
虚继承的使得架构设计可能出现问题