C++学习笔记之多态性（2）

1. 运行了一段书上的代码，发现不一样
   使用的是vs2019

#include<iostream>
using namespace std;

class Base {
public:
	virtual void fun1() { cout << "Base::fun1()" << endl; }
	virtual ~Base() {}
};

class Derived1: public Base {
public:
	virtual void fun1() { cout << "Derived1::fun1()" << endl; }
	virtual void fun2() { cout << "Derived1::fun2()" << endl; }
};

class Derived2 : public Base {
public:
	virtual void fun1() { cout << "Derived2::fun1()" << endl; }
	virtual void fun2() { cout << "Derived2::fun2()" << endl; }
};

void fun(Base* b) {
	b->fun1();
	Derived1* d = dynamic_cast<Derived1*>(b);
	if (d != 0) d->fun2();
}

int main(){
	Base b;
	fun(&b);

	Derived1 d1;
	fun(&d1);

	Derived2 d2;
	fun(&d2);

	return 0;
}

运行结果

Base::fun1()
Derived1::fun1()
Derived1::fun2()
Derived2::fun1()

郑莉 董渊 何江舟编著的《C++语言程序设计（第四版）》338页说还有一行Derived2::fun2()，解释是 “d2是Derived2类型的对象，由于Derived2是Derived1的派生类，对指向d2的指针执行转换，也能够成功得到Derived1类型的指针”

但是运行结果却没有，因为类型不符合，返回的是nullptr。

2. 虚函数小实验

#include<iostream>
#include<typeinfo>
using namespace std;

class Base {
public:
	virtual ~Base() {}
	virtual void f1() {
		cout << "Base::f1" << endl;
	}
};
class Derived :public Base {
public:
	virtual void f1() {
		cout << "Derived::f1" << endl;
	}
};

int main(){

	Base b;
	b.f1();

	Derived d;
	d.f1();	

	return 0;
}

结果：

Base::f1
Derived::f1

2. 编写一个计数器Counter类，对其重载运算符“+”

#include<iostream>
using namespace std;

class Counter {
public:
	Counter(int a) { i = a; }
	Counter operator+( Counter b) const {
		return Counter(i + b.i);
	}
	void get() { cout << i << endl; }
private:
	int i;
};


int main(){
	Counter a(3),b(5),c(0);
	c = a + b;
	c.get();
	
	return 0;
}

3. 对类Point重载“++”（自增），“–”（自减）运算符，要求同时重载前缀和后缀的形式。

#include<iostream>
using namespace std;

class Point {
public:
	Point(int a) { i = a; }
	Point operator+( Point b) const {//双目运算符
		return Point(i + b.i);
	}
	Point& operator++();//前置运算符
    Point operator++(int);//后置运算符
	void get() { cout << i << endl; }
private:
	int i;
};
//前置运算符
Point &Point::operator++() {
	i++;
	return *this;
}
//后置运算符
Point Point::operator++(int) {
	Point old = *this;
	i++;
	return old;
}
int main(){
	Point a(3),b(5),c(0);
	c = a + b;
	c.get();

	++c;
	c.get();
	
	c++;
	c.get();
	
	c++.get();

	return 0;
}//“--”略

4. 编写抽象类Shape，由此派生出Rectangle和Circle，二者都有计算对象面积的函数getArea（）、计算对象周长的函数getPerim（）。

#include<iostream>
using namespace std;

class Shape {
public:
	virtual ~Shape() {}
	virtual double getArea() = 0;
	virtual double getPerim() = 0;
};

class Rectangle :public Shape {
public:
	Rectangle(double w, double h) { width = w; height = h; }
	virtual double getArea() {
		return width * height;
	}
	virtual double getPerim() {
		return 2*(width + height);
	}
private:
	double width;
	double height;
};

class Circle :public Shape {
public:
	Circle(double r) { radius = r; }
	virtual double getArea() {
		return 3.1415 * radius * radius;
	}
	virtual double getPerim() {
		return 2 * 3.1415 * radius;
	}
private:
	double radius;
};

int main(){
	Rectangle r(3, 4);
	cout << "Area of r : "<<r.getArea() << endl;
	cout << "Perim of r : " << r.getPerim() << endl;

	Circle c(5);
	cout << "Area of c : " << c.getArea() << endl;
	cout << "Perim of c : " << c.getPerim() << endl;

	return 0;
}

5. 在上题基础上，通过继承Rectangle得到一个新的类Square，然后在Shape中增加一个函数getVertaxCount（）const用来获得当前图形的顶点个数。用以下几种方法分别实现，并体会各自优劣。
   （1）使用dynamic_cast实现Shape::getVertexCount函数
   （2）使用typeid实现Shape::getVertexCount函数
   （3）将Shape::getVertexCount声明为纯虚函数，在派生类中给出具体实现。

#include<iostream>
using namespace std;

class Shape {
public:
	virtual ~Shape() {}
	virtual double getArea() = 0;
	virtual double getPerim() = 0;
	int getVertexCount1();//（1）使用dynamic_cast实现
	int getVertexCount2();//（2）使用typeid实现
	virtual int getVertexCount() = 0;//（3）纯虚函数，在派生类中实现
};

class Rectangle :public Shape {
public:
	Rectangle(double w, double h) { width = w; height = h; }
	virtual double getArea() {
		return width * height;
	}
	virtual double getPerim() {
		return 2*(width + height);
	}
	virtual int getVertexCount() {
		return 4;
	}
private:
	double width;
	double height;
};

class Square :public Rectangle {
public:
	Square(double w) :Rectangle(w, w) {}
	virtual int getVertexCount() {
		return 4;
	}
};

class Circle :public Shape {
public:
	Circle(double r) { radius = r; }
	virtual double getArea() {
		return 3.1415 * radius * radius;
	}
	virtual double getPerim() {
		return 2 * 3.1415 * radius;
	}
	virtual int getVertexCount() {
		return 0;
	}
private:
	double radius;
};

//（1）使用dynamic_cast实现
int Shape::getVertexCount1() {
	Rectangle* r = dynamic_cast<Rectangle*>(this);
	if (r != NULL)return 4;
	Circle *c = dynamic_cast<Circle*>(this);
	if (c != NULL)return 0;
	return -1;
}

int Shape::getVertexCount2() {
	const type_info &r = typeid(*this);
	if (r == typeid(Rectangle))return 4;
	else if (r == typeid(Square))return 4;
	
	else if (r == typeid(Circle))return 0;
	else return -1;
}

int main(){
	Rectangle r(3, 4);
	cout << "1.Rectangle:" << r.getVertexCount1() << endl;
	cout << "2.Rectangle:" << r.getVertexCount2() << endl;
	cout << "3.Rectangle:" << r.getVertexCount() << endl;

	Square s(6);
	cout << "1.Square:" << s.getVertexCount1() << endl;
	cout << "2.Square:" << s.getVertexCount2() << endl;
	cout << "3.Square:" << s.getVertexCount() << endl;

	Circle c(5);
	cout << "1.Circle:" << c.getVertexCount1() << endl;
	cout << "2.Circle:" << c.getVertexCount2() << endl;
	cout << "3.Circle:" << c.getVertexCount() << endl;

	return 0;
}