实验三 虚函数与多态 纯虚函数(完整版)

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{
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->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;。