day10_多继承

多继承

C++允许一个类可以有多个父类(不建议使用，会增加程序设计复杂度)

using namespace std;

class Student {
public:
	int m_score;
	Student(int score = 0) :m_score(score) { }
	void study() {
		cout << "Student::study() - score = " << m_score << endl;
	}
	~Student() {
		cout << "~Student" << endl;
	}
};

class Worker {
public:
	int m_salary;
	Worker(int salary = 0) :m_salary(salary) { }
	void work() {
		cout << "Worker::work() - salary = " << m_salary << endl;
	}
	~Worker() {
		cout << "~Worker" << endl;
	}
};

class Undergraduate : public Student, public Worker {
public:
	int m_grade;
	Undergraduate(
		int score = 0, 
		int salary = 0, 
		int grade = 0) :Student(score), Worker(salary), m_grade(grade) {

	}
	void play() {
		cout << "Undergraduate::play() - grade = " << m_grade << endl;
	}
	~Undergraduate() {
		cout << "~Undergraduate" << endl;
	}
};

int main() {
	{
		Undergraduate ug;
		ug.m_score = 100;
		ug.m_salary = 2000;
		ug.m_grade = 4;
		ug.study();
		ug.work();
		ug.play(); 
	}

	cout << sizeof(Undergraduate) << endl;

	getchar();
	return 0;
}

多继承_虚函数

如果子类继承的多个父类都有虚函数，那么子类对象就会产生对应的多张虚表

#include <iostream>
using namespace std;

class Student {
public:
	virtual void study() {
		cout << "Student::study()" << endl;
	}
};

class Worker {
public:
	virtual void work() {
		cout << "Worker::work()" << endl;
	}
};

class Undergraduate : public Student, public Worker {
public:
	void study() {
		cout << "Undergraduate::study()" << endl;
	}
	void work() {
		cout << "Undergraduate::work()" << endl;
	}
	void play() {
		cout << "Undergraduate::play()" << endl;
	}
};

int main() {
	/*Undergraduate ug;
	ug.study();
	ug.work();
	ug.play();*/

	Student *stu = new Undergraduate();
	stu->study();

	Worker *worker = new Undergraduate();
	worker->work();

	cout << sizeof(Undergraduate) << endl;

	getchar();
	return 0;
}

同名函数

#include <iostream>
using namespace std;

class Student {
public:
	virtual void study() {
		cout << "Student::study()" << endl;
	}
};

class Worker {
public:
	virtual void work() {
		cout << "Worker::work()" << endl;
	}
};

class Undergraduate : public Student, public Worker {
public:
	void study() {
		cout << "Undergraduate::study()" << endl;
	}
	void work() {
		cout << "Undergraduate::work()" << endl;
	}
	void play() {
		cout << "Undergraduate::play()" << endl;
	}
};

int main() {
	/*Undergraduate ug;
	ug.study();
	ug.work();
	ug.play();*/

	Student *stu = new Undergraduate();
	stu->study();

	Worker *worker = new Undergraduate();
	worker->work();

	cout << sizeof(Undergraduate) << endl;

	getchar();
	return 0;
}

同名

#include <iostream>
using namespace std;

class Student {
public:
	int m_age;
};

class Worker {
public:
	int m_age;
};

class Undergraduate : public Student, public Worker {
public:
	int m_age;
};

int main() {
	Undergraduate ug;
	ug.m_age = 10;
	ug.Student::m_age = 20;
	ug.Worker::m_age = 30;
	ug.Undergraduate::m_age = 40;

	cout << sizeof(Undergraduate) << endl;

	getchar();
	return 0;
}

//class Student {
//public:
//	void eat() {
//		cout << "Student::eat()" << endl;
//	}
//};
//
//class Worker {
//public:
//	void eat() {
//		cout << "Worker::eat()" << endl;
//	}
//};
//
//class Undergraduate : public Student, public Worker {
//public:
//	void eat() {
//		cout << "Undergraduate::eat()" << endl;
//	}
//};
//
//int main() {
//	Undergraduate ug;
//	ug.eat();
//	ug.Worker::eat();
//	ug.Student::eat();
//	ug.Undergraduate::eat();
//
//	getchar();
//	return 0;
//}

菱形继承

• 菱形继承带来的问题
  • 最底下子类从基类继承的成员变量冗余、重复
  • 最底下子类无法访问基类的成员，有二义性

虚继承

• 虚继承可以解决菱形继承带来的问题

// 非虚继承：(10 + 10 + 3) * 4 = 23 * 4
// 虚继承：(10 + 2 + 2 + 1) * 4 = 15 * 4
class Person {
public:
	int m_age;
	int m_age1;
	int m_age2;
	int m_age3;
	int m_age4;
	int m_age5;
	int m_age6;
	int m_age7;
	int m_age8;
	int m_age9;
};

class Student : virtual public Person {
public:
	int m_score;
};

class Worker : virtual public Person {
public:
	int m_salary;
};

class Undergraduate : public Student, public Worker {
public:
	int m_grade;
};

int main() {
	/*Undergraduate ug;
	ug.m_grade = 10;
	ug.m_score = 20;
	ug.m_salary = 20;*/

	cout << sizeof(Student) << endl;

	getchar();
	return 0;
}

多继承的应用

// 多继承：增加程序的复杂度

// 多继承有一个很好的用途：
// 一个类实现多个接口（Java）
// 一个类遵守多份协议（OC）
// 一个类继承多个抽象类（C++）

class JobBaomu {
	virtual void clean() = 0;
	virtual void cook() = 0;
};

class JobTeacher {
	virtual void playBaseball() = 0;
	virtual void playFootball() = 0;
};

class SalesMan : public JobBaomu {
	void clean() {

	}

	void cook() {

	}
};

class Student : public JobBaomu, public JobTeacher {
	void clean() {

	}

	void cook() {

	}

	void playBaseball() {

	}

	void playFootball() {

	}
};

int main() {
	// 兼职中心
	// 招聘一些人来做兼职
	// 兼职的岗位很多种（保姆、老师、开挖掘机）
	// 应聘的人种类也很多（学生、XXX）

	getchar();
	return 0;
}