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.