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Linked Lists

JSCFaith · 2001-05-16T22:13:29

Hey, I am making a asteroids clone and I have a class called "Bullet." Right now I have it so that when you click the space bar, the "Ship" class calls the "Bullet::CreateBullet." The only problem is that when It tries to add the bullet to the linked list it crashes. Here is some of the code. // This function creates a bullet dynamically. I am I using the 'new' command correctly? void Bullet::CreateBullet(float x, float y, int Direction) { Bullet *NewBullet; NewBullet = new Bullet;// This calls the constructor correct? // Set the bullets properties NewBullet->SetPosition(x,y); NewBullet->SetObjectState(ALIVE); NewBullet->SetBulletVelocity(Direction); NewBullet->SetSize(4, 4); NewBullet->SetAnimationProperties(8, 8, 1, 1, DDSCAPS_SYSTEMMEMORY, "Data\\Art\\Weapons\\bullet"); NewBullet->SetFrame(1); } // The class Bullet's constructor adds bullet to linked list Bullet() { Type = BULLET; Damage = 5; DistanceFlown = 0; // Add to linked List if( pBulletFirst == 0 ) { // this is the first node pBulletFirst = this; pBulletPrev = NULL; } else { pBulletLast->pBulletNext = this; this->pBulletPrev = pBulletLast; } pBulletLast = this; this->pBulletNext = NULL; } Well, I think the problem is in the constructor, but maybe I am using the 'new' thing wrong. I have never used it before. Well thanks a lot guys. Later. Edited by - JSCFaith on May 12, 2001 7:37:36 PM

General and Gameplay Programming Programming

Started by JSCFaith May 12, 2001 06:37 PM

29 comments, last by JSCFaith 23 years, 9 months ago

JSCFaith

Author

122

May 14, 2001 07:37 AM

Hey,

Yeah, that is what I have started to do. I created a little program is dos using all my classes. Also, I have changed the code so that BulletManage puts each object in the linked list. I didn't use the std thing, but wrote my own linked list code. Still the same problem so I will continue to try to find the fault. Well thanks a lot. I have learned a lot about OOP in general in these last 2 days. One question though, should I have all my pBulletFirst, Last, Next, and Prev in the Manager or Bullet Class?

Later,
James

Edited by - JSCFaith on May 14, 2001 8:40:46 AM

DeVore

122

May 14, 2001 08:00 AM

The manager should contain the head and tail of the list - pBulletFirst and Last. As for the prev and next pointers, if you were being really anal, you could create a Node class which contained them and have Bullet inherit from Node. Personally, I think that is over engineering for your project so I would have the prev and next pointers in the bullet class.

-DeVore

ps: can you post your bullet deletion code - ie/ RemoveDeadBullets(...)

Stoffel

250

May 14, 2001 03:21 PM

quote:
Original post by DeVore
The manager should contain the head and tail of the list - pBulletFirst and Last. As for the prev and next pointers, if you were being really anal, you could create a Node class which contained them and have Bullet inherit from Node. Personally, I think that is over engineering for your project so I would have the prev and next pointers in the bullet class.

It wouldn''t be over-engineering, it would just be plain wrong.

Bullet should not know it is in a list.

Read the above line until you understand that it is true or until your eyes fall out.

What this means is: there should be nothing in bullet.h or bullet.cpp that has anything to do with any list variables or functions. You shouldn''t even include the header files for whatever list management you''re using, and this precludes inheriting from some node class. The reason is, you should be able to put Bullets in a list, a vector, a heap, a hash table, or a static array. If you make Bullet inherit from Node, you''ll have to have ListBullet : public ListNode, HashBullet : public HashNode, etc., etc. This is a very bad thing.

The best way to do it, and the way STL does it, is to have your list manager include a nested class that describes a node. Pseudo-code:

  // list.h#include "bullet.h"class List{private:  class Node  {    Node *next, *prev;    Bullet data; // in-size bullet member  };};

Using STL''s list class, this is templated so you can make a list of anything. The restriction this places on Bullet (i.e. the contained data) is that it has to have a valid copy constructor, and that at least one copy of the object will occur (when it is inserted). But this allows you to use anything in the list and do so without wasting any memory (two pointers per object) and encapsulating all of the list functionality in a completely separate class.

JSCFaith

Author

122

May 14, 2001 05:58 PM

Ok.. Well I read your post and am trying to get what you said I should to do, to work. I created a manager class that includes everything related to the list including the nested class. How would I use the Node class? Should I declare it like this:

        class BulletNode  {     BulletNode() {  }  ~BulletNode() {  }    BulletNode *pBulletNext, *pBulletPrev; // Should these be bullets or should they be// like in your pseudocode as 'BulletNodes'?  Bullet data; // Don't understand what this is for.  };

Here is my entire BulletManager Class. Tell me if I am doing this correct.

        class BulletManager {	public:	BulletManager() {		pBulletFirst = NULL;		pBulletLast = NULL;	}        // You said I need a copy constructor, correct?        BulletManager(const BulletManager &) {             // What should I do in here? Never really needed              // a copy constructor before        }	~BulletManager() {	}	void CreateBullet(float x, float y, int Range, int Damage, int Direction);	void CheckBulletCollisions();	void DrawBullets(LPDIRECTDRAWSURFACE7 PrimarySurface);	void RemoveDeadBullets();	void MoveBullets();	void ManageBullets(LPDIRECTDRAWSURFACE7 PrimarySurface);protected:        // These should not be static right? That way I can have    //multiple lists	Bullet *pBulletFirst;	Bullet *pBulletLast;	class BulletNode  {           BulletNode() {        }        ~BulletNode() {        }          BulletNode *pBulletNext, *pBulletPrev; // Should these be bullets or should they be        // like in your pseudocode as 'BulletNodes'?        Bullet data; // Don't understand what this is for.         };};

Ok, is that correct? Well my next question is this. How would I use pNext, pPrev and stuff in the 'BulletNode' class with the RemoveDeadBullets function. Also, do you really need that 'Bullet _data;' thing? What was it for?

Oh yeah, and DeVoir, here is the RemoveDeadBullets thing, It doesn't work with the changes I made in the class above though.

     void BulletManager::RemoveDeadBullets() {	cout<<"In Remove Bullets.\n";	for(Bullet *pS = pBulletFirst; pS; pS = pBulletLast) {		if(pS->GetObjectState() == DEAD) {			if (pS->pBulletPrev == NULL) 				pBulletFirst = pS->pBulletNext;			else				pS->pBulletPrev->pBulletNext = pS->pBulletNext;			if (pS->pBulletNext == NULL)				pBulletLast = pS->pBulletPrev;			else				pS->pBulletNext->pBulletPrev = pS->pBulletPrev;			delete pS;		}	}	}

Ok, well I hope I don't sound too stupid, lol. I really don't feel right asking all these questions. So if you are getting annoyed with me, don't answer. I still have so many questions. I will try to get the changes you suggested I make to work, but we will see. BTW, It doesn't crash anymore, I found the error, but I still want to try to get my code more Object Oriented. Right now its just a bunch of classes all mangled together with each class not having any real purpose.

Well thanks a million guys. I am learning a lot, too bad I let my friend borrow my C++ book. Oh well, you live, you learn.

Later,
James

Edited by - JSCFaith on May 14, 2001 7:06:51 PM

Edited by - JSCFaith on May 14, 2001 7:13:14 PM

Anonymous

May 14, 2001 06:33 PM

I just reread through your post. I think I understand what the ''Bullet data;'' thing is for. Didn''t notice the "(i.e. the contained data)" Well, I will do some more programming to try to fix all my classes. Damn, I really didn''t know what I was doing. My classe are a mess... ah!!!!

Later.

JSCFaith

Author

122

May 14, 2001 06:33 PM

JSCFaith

Author

122

May 15, 2001 01:49 PM

Hey,

I have been doing some research, but have come up empty handed. I really only have one obstical in my way. You said I should have a nested class in the Manager class. It should contain the next and prev pointers. My question is, if I don''t inherite from Bullet, how does BulletNode access anything? How would I move about the list and use different members of class Bullet. For example: Before, when I had the Bullet class manage the linked list. I would call a function called ''DrawBullets'' which, like the name implies, draws all the bullets that are alive. The code looked like follows:

  void DrawBullets(LPDIRECTDRAWSURFACE7 PrimarySurface) {     for(Bullet *pS = pBulletFirst; pS; pS = pS->pBulletNext) {           if(pS->ObjectState == ALIVE) {                pS->DrawBullet(PrimarySurface);           }      }}

The code above will not work with the nested class; because, as you know, the ''pNext, pPrev'' do not point to a ''Bullet'' object. They point to a ''BulletNode'' object. So, how would I do the same exact thing as above with this new class layout? I read a few tutorials on STL, hoping they might have some information on the question I am asking. Of course, I did not want to use STL. I still want to try to code my own linked list. Well, if you could help, that would be great.

Later.

Big B

337

May 15, 2001 03:00 PM

while your pNext and pPrev do not point to Bullets, your BulletNode should.

Here''s the Linked list I am using for my project. It looks incredibly similiar to what you seem to be doing.

  //declarationclass node{public:node *next;node *prev;obj *data;node(obj *o=NULL){	next = NULL;	prev = NULL;	data = o;}~node(){	delete data;	data = NULL;};};class list{public:	node *head;	list(){head = NULL;};	~list();	bool add(obj *o);	void remove(node *p,bool del=true);	void removeall();	void update();	void draw();};//an example of how to update a node''s infovoid list::draw(){node *p = head;while(p != NULL){	if(p && p->data)	{		p->data->draw();		p = p->next;	}}}  //end list::draw

In the above example, p is a node in the list, or a BulletNode for you. p looks at its data, which is a Bullet and says "draw yourself". Then p becomes the next node in the list, and the cycle continues until there are no more nodes.

I''m not sure what research you have done, but here''s a link to a tut. on Linked lists.

http://www.gamedev.net/reference/programming/features/uds1/

Anonymous

May 15, 2001 05:36 PM

Thanks a lot Big B and everyone else. I think I got it now. Let me ask one last question. I think I get what you told me Big B and I created a CreateBullet function. Does this look correct?

  // Here is my BulletManager Classclass BulletManager {public:	BulletManager() {		pFirst = NULL;		pLast = NULL;	}	~BulletManager() {	}	void CreateBullet(float x, float y, int Range, int Damage, int Direction);	void ManageBullets(LPDIRECTDRAWSURFACE7 PrimarySurface);	void RemoveDeadBullets();	protected:	BulletNode *pFirst;	BulletNode *pLast;	class BulletNode {	public:		BulletNode(Bullet *Object = NULL) {			pNext = NULL;			pPrev = NULL;			Data = Object;		}		~BulletNode() {			delete Data;			Data = NULL;		}		BulletNode *pNext;		BulletNode *pPrev;		Bullet     *Data;	};};// Here is my create bullet functionvoid BulletManager::CreateBullet(float x, float y, int Range, int Damage, int Direction) {	Bullet *NewBullet;	BulletNode *NewNode;	NewBullet = new Bullet;	NewNode = new BulletNode(NewBullet);	if(pFirst == NULL) {		pFirst = NewNode;		NewNode->pPrev = NULL;	}	else {		pLast->pNext = NewNode;		NewNode->pPrev = pLast;	}	pLast = NewNode;	NewNode->pNext = NULL;	NewNode->Data->SetSize(4,4);	NewNode->Data->SetBulletVelocity(Direction);	NewNode->Data->SetPosition( x, y);	NewNode->Data->SetRange(Range);	NewNode->Data->SetDamager(Damage);	}

BTW, do I need a copy constructor in the ''Bullet'' class for this to work? I haven''t tested the code yet.

Well thanks a lot everyone.

Later.

Snale

122

May 16, 2001 06:40 AM

You only seem to pass pointers to Bullet objects so you shouldn''t need a copy constructor. Copy constructors are required when you are passing a whole object to a function since the object will be copied to the stack in that case.

Snale
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Snale+--My humble and superior homepage

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