两个例子分析 9章课后选择7题

7.当一个类的某个函数被说明为virtual时,该函数在该类的所有派生类中(A)

A. 都是虚函数

B. 只有被重新说明时才是虚函数

C. 只有被重新说明virtual时才是虚函数

D. 都不是虚函数

例一: 

#include<iostream.h>
const double pl=3.14;
class Shape         //定义一个图形类
{
public:
    virtual double area()=0;          //声明一个求面积的纯虚函数
    virtual void shapename()=0;        //声明一个输出图形名称的纯虚函数
};
class Circle:public Shape
{
private:
  double r;
public:
    Circle(){}
    Circle(double rr){r=rr;}
    virtual double area(){ return pl*r*r;}         //在派生类中实现求面积的功能
    virtual void shapename(){cout<<"this is a Circle.";}     //在派生类中输出图形的名称
};
class Rectangle:public Shape
{
private:
    double x,y;
public:
    Rectangle(){}
    Rectangle(double xx,double yy){x=xx;y=yy;}
    virtual double area(){ return x*y;}                       //在派生类中实现求面积的功能
    virtual void shapename(){cout<<"this is a Rectangle.";}   //在派生类中输出图形的名称

};
void main()
{
 Shape *p;
 Circle c(2);
 Rectangle r(3,4);
 p=&c;
 p->shapename();
 cout<<"area="<<p->area()<<endl;
 p=&r;
 p->shapename();
 cout<<"area="<<p->area()<<endl;
}

程序结果:this is a Circle.area=12.56

                 this is a rectangle.area=12

 

例二:

#include<iostream.h>
const double pl=3.14;
class Shape  

{
public:
    virtual double area()=0;     

    virtual void shapename()=0;  

 };
class Circle:public Shape
{
private:
  double r;
public:
 Circle(){}
 Circle(double rr){r=rr;}
    double area(){ return pl*r*r;}     

    void shapename(){cout<<"this is a Circle.";}  

};
class Rectangle:public Shape
{
private:
    double x,y;
public:
    Rectangle(){}
    Rectangle(double xx,double yy){x=xx;y=yy;}
    double area(){ return x*y;}                       

     void shapename(){cout<<"this is a Rectangle.";}  

};
void main()
{
 Shape *p;
 Circle c(2);
 Rectangle r(3,4);
 p=&c;
 p->shapename();
 cout<<"area="<<p->area()<<endl;
 p=&r;
 p->shapename();
 cout<<"area="<<p->area()<<endl;
}

程序结果:this is a Circle.area=12.56

                 this is a rectangle.area=12

 

注释:

定义派生类中的虚函数时,可以省略关键字virtual,但提倡不省略.

当一个类的某个函数被说明为virtual时,该函数在该类的所有派生类中都是虚函数.