深入解析pure virtual function call

    /** 
     * "pure virtual function call" on win32 platform 
     * filename: testWin32PVFC.cpp 
     */  
    #include <iostream>  
     
    #define PI 3.1415926  
    using namespace std;  
      
    class Shape  
    {  
    private:  
        double ValuePerSquareUnit;  
      
    protected:  
        Shape(double valuePerSquareUnit):  
            ValuePerSquareUnit(valuePerSquareUnit)  
        {  
            //error LNK2001: unresolved external symbol "public: virtual double __thiscall Shape::area(void)const " (?area@Shape@@UBENXZ)  
            //std::cout << "creating shape, area = " << area() << std::endl;  
            std::cout << "creating shape, value = " << value() << std::endl;  //indirectly call pure virtual function in constructor  
        }  
      
    public:  
        virtual double area() const = 0;  
      
        double value() const  
        {  
            return ValuePerSquareUnit * area();  
        }  
      
        virtual ~Shape()  
        {  
            printf("Shape::~Shape() is called");  
        }  
      
        double getPerSquareUnit()  
        {  
            return ValuePerSquareUnit;  
        }  
    };  
      
    class Rectangle : public Shape  
    {  
    private:  
        double Width;  
        double Height;  
      
    public:  
        Rectangle(double width, double height, double valuePerSquareUnit):  
            Shape(valuePerSquareUnit),Width(width),Height(height)  
        {  
        }  
      
        virtual ~Rectangle()  //can be removed  
        {  
        }  
      
        virtual double area() const  
        {  
            return Width * Height;  
        }  
      
    };  
      
    class Circle: public Shape  
    {  
        double Radius;  
      
    public:  
        Circle(double radius, double valuePerSquareUnit):  
            Shape(valuePerSquareUnit),Radius(radius)  
        {  
        }  
      
        virtual ~Circle()  //can be removed  
        {  
        }  
      
        virtual double area() const  
        {  
            return PI * Radius * Radius;  
        }  
    };  
      
      
    int main()  
    {  
        Rectangle* pr = new Rectangle(30, 20, 10);  
        Circle* pc = new Circle(15, 10);  
      
        //invoke Rectangle::area()  
        printf("rectangle: area = %.2f, PerSquareUnit = %.2f, value = %.2f/n", pr->area(), pr->getPerSquareUnit(), pr->value());  
        //invoke Circle::area()  
        printf("circle   : area = %.2f, PerSquareUnit = %.2f, value = %.2f/n", pc->area(), pc->getPerSquareUnit(), pc->value());  
      
        Shape* shape;  
        shape = pr;  
        printf("rectangle: area = %.2f, PerSquareUnit = %.2f, value = %.2f/n", shape->area(), shape->getPerSquareUnit(), shape->value());  
      
        shape = pc;  
        printf("circle   : area = %.2f, PerSquareUnit = %.2f, value = %.2f/n", shape->area(), shape->getPerSquareUnit(), shape->value());  
      
        return 0;  
    }