C++之纯虚函数

class  类名
{
    virtual  函数返回值类型  函数名  (参数列表) = 0;    //即，抽象类不去实现函数体
}；

#include<iostream>

constexpr double PI = 3.1415926;

using std::endl;
using std::cout;

class Figure			//抽象类
{
public:
	//定义纯虚函数
	virtual double getArea() const = 0                      //获取面积
	{	
		cout << "Hello World" << endl;	//实际上C++允许我们在抽象类中实现纯虚函数的函数体
		return 0;
	}
	virtual double getCircum() const = 0;					//获取周长
};

class Rect : public Figure
{
public:
	Rect(double a,double b);
	virtual double getArea()const
	{
		Figure::getArea();		//由于抽象类不能实例化，只能这样在派生类中调用。不能在类外调用
		return a * b;	
	}
	virtual double getCircum()const
	{
		return 2 * (a + b);
	}
private:
	double a;
	double b;
};

Rect::Rect(double a, double b)
{
	this->a = a;
	this->b = b;
}

class Triangle : public Figure
{
public:
	Triangle(double a,double b,double c);
	virtual double getArea()const
	{
		//利用海伦公式计算三角形面积
		double p = (a + b + c) / 2;
		double tmp = p * (p - a) * (p - b) * (p - c);
		return pow(tmp, 0.5);
	}
	virtual double getCircum()const
	{
		return a + b + c;
	}
private:
	double a;
	double b;
	double c;
};

Triangle::Triangle(double a, double b, double c)
{
	this->a = a;
	this->b = b;
	this->c = c;
}

class Circle : public Figure
{
public:
	Circle(double r);
	virtual double getArea()const
	{
		return PI * r * r;
	}
	virtual double getCircum()const
	{
		return 2 * PI * r;
	}
private:
	double r;
};

Circle::Circle(double r)
{
	this->r = r;
}


int main()
{
	//Figure f;			---->error，抽象类不能实例化对象。

	Figure* p_rect = new Rect(4, 6);
	Figure* p_triangle = new Triangle(3, 3, 3);
	Figure* p_circle = new Circle(10);

	cout << "矩形面积:" << p_rect->getArea() << "     矩形周长:" << p_rect->getCircum() << endl;
	cout << "三角形面积:" << p_triangle->getArea() << "     三角形周长:" << p_triangle->getCircum() << endl;
	cout << "圆形面积:" << p_circle->getArea() << "     圆形周长:" << p_circle->getCircum() << endl;
	

	return 0;
}