0xdddddddd != NULL?

[NixPhoeni]

I have a user-defined class Character, of which fighting is a member of type Character. Within the destructor, I have the following code:

Character *dp = new Character;
while(fighting!=NULL) // erase this linked list
{
dp = fighting;
fighting = fighting->next;
dp->next = NULL;
delete dp;
}

This works, until fighting gets the hex value of 0xdddddddd (I checked in the MSVC++ debugger). It goes through the loop and tries to dereference a NULL pointer when this occurs. How can I make this so that it doesn't go through the loop when fighting == NULL or 0xdddddddd? I tried the corresponding C++ code, but it says there is no operator that can convert from Character * to int (it may have said unsigned long, but I know it was an int). Any help is appreciated.

[Nick]

MSVC++ uses a different *representation* of a null pointer
to be 0xdddddddd to trap memory problems in debug releases.
I'm not sure of the specifics though you should know the value
of a null pointer and the representation can be different.

[Nick]

I think maybe I got it wrong, sill not very sure though, msvc++ sets a pointer to 0xdddddddd after deleting or freeing it. So you basically have dangling pointers and it would be a major
bug to check against 0xdddddddd.

[SilentStrike]

General recommendation. Use the STL list, rather than having characters a list item themselves. It will save a lot of headaches, and reusing well written code when applicable is always a good idea.

[no-one]

actually null pointers in VC always turn out 0xcccccccc for me so are you sure its valid memory?

[damyan]

Hi
You should rewrite slightly your code for destructor when have in mind that when you invoke the delete dp operator the destuctor for that object is invoked too and the fighting pointer is not valid anymore - it points to memory which is freed inside the destructor of of the dp object.

damyan

[nvoigt]

Not to create some misunderstandings here:

In MSVC, a NULL pointer is ALWAYS 0x00000000 !
Release build, debug build, hardisk upside down,
it's ALWAYS 0x00000000. No exceptions. Anything
else would be a horrible mess.

What you are expiriencing is an uninitialized pointer.
When you don't initialize a pointer, or don't set it to
a new value after you called delete on it, MSVC will
set it to a value like 0xcdcdcdcd to symbolize it has
no value. So you can see that a pointer is not
initialized instead of wondering why you program
crashes when you try to call a function on your
pointer 0x139abc87 which in reality is just a bunch
of random numbers.

So what you are getting at here is that you have a
pointer in your list that is either not initialized,
or has been deleted previously.

Character *dp = new Character; 
while(fighting!=NULL) // erase this linked list 
{ 
dp = fighting; 
fighting = fighting->next; 
dp->next = NULL; 
delete dp; 
}

Why does dp have to be a new Character, when the
first thing you do is lose it's value ( dp=fighting; ) ?

Another thing to check: Is the list always teminated
by an element that has NULL as a next value ?

Next Question: Is fighting INITIALIZED to NULL in your
constructor ? If not, it might fail at any instance in your
list.

Suggestion:
If you want to delete a list of classes, use this code:
Remember that when you call delete, the destructor
is called.

CCharacter::~CCharacter()
{
     // this calls the destructor of the instance 
     // that is stored in fighting, which in turn 
     // will delete it's predecessors in it's own
     // destructor
     if( fighting != NULL ) delete fighting;
     fighting = NULL;
}

Another thing that comes to mind: Make sure you have
each instance only once in this list.

Example:

A->B->C will be fine

A->B->A means trouble