c++ 虚函数

#include <iostream>

class Box {
    private :
    protected :
        double length = 1.0;
        double width = 1.0;
        double heigth = 1.0;
    public :
        Box() {};
        Box(double lengthValue, double widthValue, double heigthValue) {
            length = lengthValue;
            width = widthValue;
            heigth = heigthValue;
        }

        void show_volume() {
            std::cout << "volume is " << volume() << std::endl;
        }
        
        virtual double volume(int i = 50) {
            std::cout << i << std::endl;
            return length*width*heigth;
        }
};

class Carton:public Box {
    private :
    protected :
    public :
        Carton() {};
        Carton(double lengthValue, double widthValue, double heigthValue) : Box(lengthValue, widthValue, heigthValue) {};

        double volume(int i = 500) override {
            std::cout << i << std::endl;
            return 0.85*length*width*heigth;
        }            
};

int main()
{
    Box aBox{20, 30, 40};
    Carton aCarton{20, 30, 40};
    
    Box* pBox(&aCarton);
    
    std::cout << "carton volume is " << pBox->volume() << std::endl;

    return 0;
}