小甲鱼-C++快速入门笔记(30)之虚继承

前一节的student示例程序看起来似乎已经解决了问题,但它存在着一些隐患。

首先，在TeachingStudent类的introduce（）方法里，我们不得不明确告诉编译器应该使用哪一个属性。

这对于classes属性来说是应该的，因为教一门课和上一门课有点本质的区别，而作为常识，助教生教的课程和他学的课程不可能一样。但是既然在TeachingStudent对象里可以继承两个不同的classes属性，那它是不是应该有两个不同的name属性呢？

答案是肯定的，TeachingStudent可以有两个不同的名字，但这肯定不是我们在设计这个类继承模型时所预期的。

#include <iostream>
#include <string>

using namespace std;

class Person
{
public:
	Person(string theName);

	void introduce();

protected:
	string name;
};

class Teacher:public Person
{
public:
	Teacher(string theName, string theClass);

	void teach();
	void introduce();

protected:
	string classes;
};

class Student:public Person
{
public:
	Student(string theName, string theClass);

	void attendClass();
	void introduce();

protected:
	string classes;
};

class TeachingStudent:public Student, public Teacher
{
public:
	TeachingStudent(string theName1, string theName2, string classTeaching, string classAttending);

	void introduce();

};

Person::Person(string theName)
{
	name = theName;
}

void Person::introduce()
{
	cout << "大家好,我是" << name << endl;
}

Teacher::Teacher(string theName, string theClass):Person(theName)
{
	classes = theClass;
}

void Teacher::teach()
{
	cout << name << "教" << classes << endl;
}

void Teacher::introduce()
{
	cout << "大家好,我是" << name << ",我教" << classes << endl;
}

Student::Student(string theName, string theClass):Person(theName)
{
	classes = theClass;
}

void Student::attendClass()
{
	cout << name << "加入" << classes << "学习.\n";
}

void Student::introduce()
{
	cout << "大家好,我是" << name << ",我在" << classes << "学习.\n";
}

TeachingStudent::TeachingStudent(string theName1,
								 string theName2,
								 string classTeaching,
								 string classAttending)
								 :Teacher(theName1, classTeaching), 
								 Student(theName2, classAttending)
{
}

void TeachingStudent::introduce()
{
	cout << "大家好,我是" << Student::name << ",我教" << Teacher::classes << ",";
	cout << "同时我在" << Student::classes << "学习.\n";
}

int main()
{
	Teacher teacher("小甲鱼", "C++入门班");
	Student student("迷途羔羊", "C++入门班");
	TeachingStudent teachingStudent("丁丁", "丹丹", "C++入门班", "C++进阶班");

	teacher.introduce();
	teacher.teach();
	student.introduce();
	student.attendClass();
	teachingStudent.introduce();
	teachingStudent.teach();
	teachingStudent.attendClass();

	return 0;
}

TeachingStudent类继承自Teacher和Student两个类,因而继承了两组Person类的属性,这在某些时候完全有道理,例如classes属性.但它也有可能引起麻烦,例如发生在name属性身上的情况。

因此提供了一个功能可以解决这个问题：虚继承 （virtual inheritance）

通过虚继承某个基类，就是在告诉编译器：从当前这个类再派生出来的子类只能拥有那个基类的一个实例

语法：

class Teacher：virtual public Person

{...}

这样做就解决了: 让Student和Teacher类都虚继承自Person类,编译器将确保从Student和Teacher类再派生出来的子类只能拥有一份Person类的属性。

#include <iostream>
#include <string>

using namespace std;

class Person
{
public:
	Person(string theName);

	void introduce();

protected:
	string name;
};

class Teacher:virtual public Person   //改动
{
public:
	Teacher(string theName, string theClass);

	void teach();
	void introduce();

protected:
	string classes;
};

class Student:virtual public Person   //改动
{
public:
	Student(string theName, string theClass);

	void attendClass();
	void introduce();

protected:
	string classes;
};

class TeachingStudent:public Student, public Teacher
{
public:
	TeachingStudent(string theName, string classTeaching, string classAttending);

	void introduce();

};

Person::Person(string theName)
{
	name = theName;
}

void Person::introduce()
{
	cout << "大家好,我是" << name << endl;
}

Teacher::Teacher(string theName, string theClass):Person(theName)
{
	classes = theClass;
}

void Teacher::teach()
{
	cout << name << "教" << classes << endl;
}

void Teacher::introduce()
{
	cout << "大家好,我是" << name << ",我教" << classes << endl;
}

Student::Student(string theName, string theClass):Person(theName)
{
	classes = theClass;
}

void Student::attendClass()
{
	cout << name << "加入" << classes << "学习.\n";
}

void Student::introduce()
{
	cout << "大家好,我是" << name << ",我在" << classes << "学习.\n";
}

TeachingStudent::TeachingStudent(string theName,
								 string classTeaching,
								 string classAttending)
								 :Teacher(theName, classTeaching), 
								 Student(theName, classAttending),
								 Person(theName)   //改动
{
}

void TeachingStudent::introduce()
{
	cout << "大家好,我是" << name << ",我教" << Teacher::classes << ",";
	cout << "同时我在" << Student::classes << "学习.\n";
}

int main()
{
	Teacher teacher("小甲鱼", "C++入门班");
	Student student("迷途羔羊", "C++入门班");
	TeachingStudent teachingStudent("丁丁", "C++入门班", "C++进阶班");

	teacher.introduce();
	teacher.teach();
	student.introduce();
	student.attendClass();
	teachingStudent.introduce();
	teachingStudent.teach();
	teachingStudent.attendClass();

	return 0;
}

有点没理解。