C + + virtual functions

Default parameter values for virtual functions

If a virtual function is declared in the base class with a default variable value, the default variable value is always accepted from the base class template of the function when the function is called through a base class pointer.

The default value in the function derived class version does not work.

#include <iostream>classBox {Private :    protected :        DoubleLength =1.0; Doublewidth =1.0; DoubleHeigth =1.0;  Public: Box () {}; Box (DoubleLengthvalue,DoubleWidthvalue,Doubleheigthvalue) {Length=Lengthvalue; Width=Widthvalue; Heigth=Heigthvalue; }        voidShow_volume () {std::cout<<"Volume is"<< Volume () <<Std::endl; }                Virtual DoubleVolumeinti = -) {Std::cout<< I <<Std::endl; returnlength*width*Heigth; }};classCarton: PublicBox {Private :    protected :     Public: Carton () {}; Carton (DoubleLengthvalue,DoubleWidthvalue,Doubleheigthvalue): Box (Lengthvalue, Widthvalue, Heigthvalue) {}; DoubleVolumeinti = -)Override{std::cout<< I <<Std::endl; return 0.85*length*width*Heigth; }            };intMain () {Box abox{ -, -, +}; Carton acarton{ -, -, +}; Box* Pbox (&Acarton); Std::cout<<"Carton Volume is"<< Pbox->volume () <<Std::endl; return 0;}

$./virtual
50
Carton volume is 20400

objects that have virtual functions consume more bytes than objects that do not have virtual functions.

Pure virtual function

There is no virtual function defined in the base class, and the class containing the virtual function is called an abstract class.

C + + virtual functions