实验三虚函数与多态、纯虚函数一.实验目的1. 在掌握继承与派生关系的基础上，进一步理解虚函数与多态性的关系，实现运行时的多态。
2. 学会定义和使用纯虚函数二、实验内容1．范例：了解"单接口，多方法"的概念。
现有称为figure的基类，存放了各二维对象（三角形、矩形和圆形三个类）的各维数据，set_dim()设置数据，是标准成员函数。
show_area()为虚函数，因为计算各对象的面积的方法是不同的。
【程序】#include < iostream >using namespace std;class figure{protected:double x,y;public:void set_dim(double i,double j=0){ x=i; y=j; }virtual void show_area(){ cout<<"No area computation defined for this class.\n";}};class triangle:public figure{public:void show_area(){ cout<<"Triangle with height "<< x<<" and base "<< y<<" has an area of "<< x*0.5*y<< endl;}};class square:public figure{public:void show_area(){ cout<<"Square with dimensions "<< x<<" and "<< y<<" has an area of "<< x*y<< endl;}};class circle:public figure{public:void show_area(){ cout<<"Circle with radius "<< x<<" has an area of "<<3.14159*x*x<< endl;}};int main(){figure *p;triangle t;square s;circle c;p=&t;p->set_dim(10.0,5.0);p->show_area();p=&s;p->set_dim(10.0,5.0);p->show_area();p=&c; p->set_dim(10.0);p->show_area();return 0;}【要求】（1）建立工程，录入上述程序，调试运行并记录运行结果。
（2）修改上述程序，将virtual void show_area()中的virtual去掉，重新调试运行观察结果有何变化？为什么？在不使用关键字virtual后，基类指针p对show-area的访问p->show_area()没有针对所指对象的类型调用不同的函数，而是直接根据p的类型调用了基类的成员函数show-area。
（3）修改上述程序入口函数，使其动态建立三角形、矩形和圆形3个对象，通过基类指针访问这3个对象，然后释放这3个对象。
#include < iostream >using namespace std;class figure{protected:double x,y;public:void set_dim(double i,double j=0){ x=i; y=j; }virtual void show_area(){ cout<<"No area computation defined for this class.\n";}};class triangle:public figure{public:void show_area(){ cout<<"Triangle with height "<< x<<" and base "<< y<<" has an area of "<< x*0.5*y<< endl;}};class square:public figure{public:void show_area(){ cout<<"Square with dimensions "<< x<<" and "<< y<<" has an area of "<< x*y<< endl;}};class circle:public figure{public:void show_area(){ cout<<"Circle with radius "<< x<<" has an area of"<<3.14159*x*x<< endl;}};int main(){figure *p;triangle *p1; square *p2;circle *p3;p1=new triangle;p2=new square;p3=new circle;p=p1;p->set_dim(10.0,5.0); p->show_area();p=p2;p->set_dim(10.0,5.0); p->show_area();p=p3;p->set_dim(10.0); p->show_area(); delete p1; delete p2; delete p3;}（4）修改类定义中的析构函数，使之适应用户动态定义对 2、使用纯虚函数和抽象类对实验二中的题1进行改进。
#include < iostream >using namespace std;class figure{protected:double x,y;public:void set_dim(double i,double j=0){ x=i; y=j; }virtual void show_area(){cout<<"No area computation defined for this class.\n";}//////////////////////////////////////////在此处将基类的析构函数声明为虚析构就OK了virtual ~figure(){}////////////////////////////////////};class triangle:public figure{public:void show_area(){cout<<"Triangle with height "<< x<<" and base "<< y<<" has an area of "<< x*0.5*y<< endl;}};class square:public figure{public:void show_area(){cout<<"Square with dimensions "<< x<<" and "<< y<<" has an area of "<< x*y<< endl;}};class circle:public figure{public:void show_area(){cout<<"Circle with radius "<< x<<" has an area of"<<3.14159*x*x<< endl;}};int main(){figure *p;triangle t;square s;circle c;p=&t;p->set_dim(10.0,5.0);p->show_area();p=&s;p->set_dim(10.0,5.0);p->show_area();p=&c; p->set_dim(10.0);p->show_area();return 0;2、编程：自定义一个抽象类Element,提供显示、求面积等公共接口（虚函数），派生出Point、Line、Circle等图形元素类，并重新定义（override）这些虚函数，完成各自的任务。
在这里，Element是抽象基类，它不能提供具体的显示操作，应将其成员函数定义为纯虚函数。
只有采用指向基类的指针或对基类的引用进行调用，实现的才是动态绑定，完成运行时的多态性。
#include < iostream >using namespace std;const double PI=3.14159;class Element{public:virtual const char* Name() const =0;virtual double Area() const =0;};class Point:public Element{protected:double x,y;public:Point(double xv=0,double yv=0):x(xv),y(yv){}virtual const char* Name() const{return "Point";} virtual double Area() const{return 0;}};class Line:public Element{protected:double length;public:Line(double l=0):length(l){}virtual const char* Name() const{return "Line";}virtual double Area() const{return 0;}};class Circle:public Element{protected:double radius;public:Circle(double r):radius(r){}virtual const char* Name() const{return "Circle";} virtual double Area() const{return PI*radius*radius;} };void print(Element *p){cout<<p->Name()<<":\t"<<p->Area()<<endl;}int main(){Point A(1,1);print(&A);Line B(10);print(&B);Circle C(10);print(&C);return 0;}。