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Virtual Functions

popcorn · 2002-07-26T04:19:08

When creating derived classes sometimes you have to use virtual functions so that your program will know the correct function to call. I have done this in the following code which uses stockitem as a base class, visualitem is derived from this class and then visualmusic is derived from visual item: Header Files: ============= STOCKITEM #if !defined(STOCKITEMH) #define STOCKITEMH #include <strng.h> class StockItem { protected: String stockNo; String description; int quantity; float price; public: StockItem(); StockItem(String sNo, String desc, int qty, float pr); ~StockItem(); void SetStockNo(String sNo); void SetDescription(String desc); void SetQuantity(int qty); void SetPrice(float pr); //Fuctions are const as they do not change the class in any way String GetStockNo() const; String GetDescription() const; int GetQuantity() const; float GetPrice() const; virtual void Display() const; }; #endif VISUALITEM #if !defined(VISUALITEMH) #define VISUALITEMH #include <strng.h> #include "StockItem.h" class VisualItem : public StockItem { private: String media; public: VisualItem(); VisualItem(String sNo, String desc, int qty, float pr, String med); ~VisualItem(); void SetMedia(String med); String GetMedia() const; void Display() const; }; #endif VISUALMUSIC #if !defined(VISUALMUSICH) #define VISUALMUSICH #include <strng.h> #include "VItem.h" class VisualMusic : public VisualItem { private: String artist; public: VisualMusic(); VisualMusic(String sNo, String desc, int qty, float pr, String med, String art); ~VisualMusic(); void SetVmusicArtist(String art); String GetVmusicArtist() const; void Display() const; }; #endif QUESTION As you can see I have made the Display() member function of the stockitem class a virtual function so that the the correct display function is called depending on the type of object created but should I also make the Display() member function of visualitem a virtual function since visualmusic is derived from this class, or is the way that I am currently using okay? Since it is not currently a virtual function does that mean that I am just overriding the function? How about them apples?

General and Gameplay Programming Programming

Started by popcorn July 24, 2002 03:44 PM

11 comments, last by popcorn 21 years, 9 months ago

smanches

122

July 25, 2002 02:10 PM

Thanks. I don''t know what I was thinking. I think I need more coffee.

Stephen Manchester
Senior Technical Lead
Virtual Media Vision, Inc.
stephen@virtualmediavision.com
(310) 930-7349

Stephen ManchesterSenior Technical LeadVirtual Media Vision, Inc.stephen@virtualmediavision.com(310) 930-7349

Sneftel

1,788

July 25, 2002 05:37 PM

quote:Original post by smanches
I think I need more coffee.

A good decision, in any situation.

EDIT: seriously, tho, I know what you were thinking. C++ has so little syntactic sugar in the language that it's downright bizarre for the virtual keyword in a derived class not to have any effect whatsoever. I assume the option of the useless "virtual" was put in the language to assuage the ego of some committee member who wanted C++ to look more like his pet OO language.

Don't listen to me. I've had too much coffee.

[edited by - sneftel on July 25, 2002 6:39:40 PM]

SabreMan

504

July 26, 2002 04:19 AM

quote:Original post by Sneftel
it''s downright bizarre for the virtual keyword in a derived class not to have any effect whatsoever.

It''s also useful in certain circumstances. Consider:

  #include <iostream>struct VirtualDtor{	virtual ~VirtualDtor()	{}};struct StaticDtor{};template<class DestructionPolicy>struct Thing : public DestructionPolicy{	void f()	{		std::cout << "f()" << std::endl;	}};class AggregatedThing{private:	Thing<StaticDtor> st;};class DerivedThing : public Thing<VirtualDtor>{};int main(){	AggregatedThing at;	DerivedThing dt;}

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