A pure virtual function is a function declared in a base class that has no definition relative to the base. As a result, any derived class must define its own version—it cannot simply use the version defined in the base. To declare a pure virtual function, use this general form: virtual type func-name(parameter-list) = 0.
If a class has at least one pure virtual function, then that class is said to be abstract. An abstract class has one important feature: there can be no objects of that class. To prove this to yourself, try removing the override of area( ) from the Triangle class in the preceding program. You will receive an error when you try to create an instance of Triangle.
/*TwoDshape.h */---base class
#include <iostream>
#include <cstring>
using namespace std;
class TwoDshape
{
protected:
double width;
double height;
char name[20];
public:
TwoDshape(void);
TwoDshape(double w, double h,char *n);
TwoDshape(double x,char *n);
void ShowDim();
double GetWidth();
double GetHeight();
char* GetName();
virtual double Area()=0;
};
/*TwoDshape.cpp*/
TwoDshape::TwoDshape(void)
{
width=height=0.0;
strcpy(name,"unknow");
}
TwoDshape::TwoDshape(double w, double h,char *n)
{
width=w;
height=h;
strcpy(name,n);
}
TwoDshape::TwoDshape(double x,char *n)
{
width=height=x;
strcpy(name,n);
}
void TwoDshape::ShowDim()
{
cout<<"width and height are:"<<width<<" and "<<height;
}
double TwoDshape::GetWidth()
{
return width;
}
double TwoDshape::GetHeight()
{
return height;
}
char* TwoDshape::GetName()
{
return name;
}
/*Triangle*/---derived class
#include "TwoDshape.h"
class Triangle: public TwoDshape
{
char style[20];
public:
Triangle()
{
strcpy(style,"unknown");
}
Triangle(char *str,double w,double h):TwoDshape(w,h,"triangle")
{
width=w;
height=h;
strcpy(style,str);
}
Triangle(double x):TwoDshape(x,"triangle")
{
width=height=x;
strcpy(style,"isosceles");
}
double Area()
{
return GetWidth()*GetHeight()/2;
}
};
/*Main*/
#include "Triangle.h"
#include <iostream>
using namespace System;
int main(array<System::String ^> ^args)
{
char ch;
TwoDshape *shape=&Triangle("right",8.0,12.0);
cout<<shape->GetName();
cout<<shape->Area();
cin>>ch;
return 0;
}
