Re: Problem with Linked List
"oceanspell" <randomuser5000@gmail.com> wrote in message
news:1189976472.525301.187410@o80g2000hse.googlegroups.com...
I made a linked list that can add an item, go to the first item, and
get the current, next, and previous item in the list. When I compile
it, there are no errors, but it overloads. Here is the code:
//fnamelink.h------------------------------------------------------
class flink
{
public:
char data[_MAX_DIR+1];
flink* previous;
flink* next;
flink()
{
previous = NULL;
next = NULL;
}
};
class fnamelink
{
private:
flink* first;
public:
fnamelink()
{ first = NULL; }
void addItem(char* foldername);
char* getCurrent();
char* getPrevious();
char* getNext();
char* getFirst();
void deleteLink();
~fnamelink()
{
getFirst();
deleteLink();
}
};
//
fnamelink.cpp-------------------------------------------------------------------------------------------------
#include <string.h>
#include <malloc.h>
#include "StdAfx.h" //<--Includes #include "fnamelink.h"
void fnamelink::addItem(char* foldername)
{
flink* newlink = new flink;
memset(newlink->data, NULL, sizeof(newlink->data));
strcpy(newlink->data, foldername);
newlink->previous = first;
if(first)
first->next = newlink;
first = newlink;
}
char* fnamelink::getCurrent()
{
flink* current = first;
if(current != NULL)
return current->data;
else
return NULL;
}
char* fnamelink::getPrevious()
{
flink* current = first;
current = current->previous;
if(current->previous == NULL)
{
first = current;
return NULL;
}
if(current->previous != NULL)
return current->data;
else
return NULL;
}
char* fnamelink::getNext()
{
flink* current = first;
if((current = current->next) == NULL)
{
return NULL;
}
if(current != NULL)
return current->data;
else
return NULL;
}
char* fnamelink::getFirst()
{
flink* current = first;
if(current)
{
=> while(current->previous != NULL)
{
current = current->previous;
}
}
first = current;
if(first)
return first->data;
else
return NULL;
}
void fnamelink::deleteLink()
{
int numToDel = 0;
flink* current = first;
if(current->next)
{
while(current->next != NULL)
{
current = current->next;
first = current;
numToDel++;
}
while((numToDel) >= 0)
{
numToDel--;
first = first->previous;
free(current);
current = first;
}
free(current);
}
else
{
first = first->previous;
free(current);
}
}
The '=>' I placed in the code was the exception the debugger pointed
out, but I can't seem to find a problem with it. Any help would be
greatly appreciated.
You are attempting to dereference a NULL pointer. Lets look at one of your
methods:
char* fnamelink::getPrevious()
{
flink* current = first;
current = current->previous;
if(current->previous == NULL)
{
first = current;
return "NULL";
}
if(current->previous != NULL)
return current->data;
else
return "NULL";
}
flink* current = first;
at this point first may be NULL if there are no items in the list.
current = current->previous;
if first was NULL, you just attempted to use a NULL pointer. This will
abend (abnormal end). Now, lets presume we have 1 item in the list, so
first was not null. But since there's only one item, previous will be NULL.
So current is now NULL. So we get to the next line:
if(current->previous == NULL)
Ooops, current is NULL Again, you are attempting to derefence a NULL
pointer. You didn't check if current was NULL first.
These types of errors are all over the code in all the functions.