Thread: Access Violation / Seg Fault

There is only but one precious exception to this rule that I have encountered on a regular basis. Using the placement new operator. If you don't know what that even is, then you should not be toying with manual destructors.

Yes, it's true. http://yosefk.com/c++fqa/dtor.html#fqa-11.9
But I think we can safely assume that the OP doesn't need to know that at the moment.

[edit]

Should I write a list class and inherit my Angel and Mortal classes from that ?

That sounds like a plausible idea. However, it would make more sense to create a list class that can handle Angels and Mortals. Are you familiar with templates? [/edit]

Originally Posted by master5001

There is only but one precious exception to this rule that I have encountered on a regular basis. Using the placement new operator. If you don't know what that even is, then you should not be toying with manual destructors.

That is correct. Placement new is not a common operation in normal programs, so I'd say that it's fair to say "Never call the destructor", without adding the exception case.

--
Mats

This thread is suffering from too many programmers spoiling the top-down reading order. Some of us should stop posting, or go out and design a bulletin board that takes into account when users started typing their posts. Or automatically refreshes the page (in the background, without disturbing your typing) every ten or thirty seconds. Or something.

[edit] See what I mean? Okay.

dwks, I'm not familiar with templates, but I wasn't familiar with linked lists until this project, nor did I understand the importance of pointers. This is the largest C++ app I've written, and I haven't actually written in a few year prior to the last few weeks. So, I'm learning everything as I go.

Would you suggest templates then ? I've looked into inheritance, polymorphism and the like, and understand the concepts, so could probably go about writing a class to manage my lists. It probably wouldn't be much good, but it would at least be less messy :P

Maybe you should just make use of what you already know -- templates would suit the problem better, but hey.

Just call the inherited classes AngelList and MortalList or something like that, not just "Angel". It's a collection of Angels. [/edit]

Originally Posted by Korhedron

dwks, I'm not familiar with templates, but I wasn't familiar with linked lists until this project, nor did I understand the importance of pointers. This is the largest C++ app I've written, and I haven't actually written in a few year prior to the last few weeks. So, I'm learning everything as I go.

Would you suggest templates then ? I've looked into inheritance, polymorphism and the like, and understand the concepts, so could probably go about writing a class to manage my lists. It probably wouldn't be much good, but it would at least be less messy :P

I personally would avoid templates for now - yes, you can get very powerful programs in a few lines with templates, but it's also a more advanced concept.

--
Mats

There are TWO advantages of doing a list class:
1. You separate the list handling from your other code - so you can work on making the list class work correctly separately from making other code work correctly [you have a list component that is "known good", so you don't need to worry about it].

2. It teaches you how to use inheritance to achieve more complex stuff. This is of course a completely non-technical reason - but an equally good one when you are learning. [And if the purpose here wasn't to learn, then we'd be back at recommendin std::list].

--
Mats

Originally Posted by Korhedron

Just to return to this lists class :

All of my problems are arising from me accessing the previous or next pointer on a node that doesn't have a previous or next node. So, technically, all I need is to :

1. Make sure all pointers are NULL when declared
2. Check for NULL before doing any operation that uses these pointers.

Yeah, pretty much. Plus never calling destructors, but that's probably not something you'll do once you know you shouldn't.

I don't see how writing a list object will help. It'll remove about four lines of code in total ( the stuff duplicated in Angel and Mortal classes ), but I'm struggling to see that I can include code to manage my pointers inside the object itself. The objects are what makes the list, and the operations on pointers are done "between objects", not from inside an object... In this way, I'm very confused.

Some operations, like deleting a list item, you simply can't really do from "inside" one of the list objects. You do sort of need a controller class or function or whatever.

But I think you're right, a list object wouldn't help much in this case -- because it's a linked list. You can't really have a AngelList::delete() method for linked lists.

Deleting objects is the hard part, I think. You have to keep in mind the fact that the root object could change. (So maybe returning a new root from the delete() function or whatever would be a good idea.) The list could become completely empty.

One mistake I consistently made when I was creating my first linked lists, something that you're doing too, was assume that the linked list always had at least one object. It just confuses everything. Write your code carefully, so that the root object can be NULL.

[edit] Beaten again -- by two posts! What is it with this thread?

but the code from going from one node to the other

That's easy -- just call traverse() or whatever in the next node.

and the code for deleting a node - that can't fit inside the actual object

Yeah, that's a bit tougher. It would probably be easier to have an AngelList to do that . . . .
[/edit]

Something like this (actually, the Angel and Mortal are nodes, not lists - because the list is a separate thing.

Code:

class Node
{
    public:
       Node *next;
       Node *prev;
       Node() : next(0), prev(0) {};
};

class List
{
    private: 
       Node *first;
       Node *last;
    public:
       List() : first(0), last(0);
       void Insert(Node &);
       void Remove(Node &);
};

class Angel : public Node
{
   ... your angel definition goes here ...       
};

class Mortal : public Node
{
   ... 
}

List AngelList;
List MortalList;

--
Mats

Also, doing this gives you a natural progression to learning templates, because the problems you'll encounter working with this code are the main motivation behind templates.