Linked List Using Classes

Question

I have created a linked list class VW with a function Build to add nodes to the list. However, when I run the code to build my linked list, the Build function throws an "access violation" error. The error is thrown by the call tmp->NextObj() in the Build function.

If the object of class VW is newly created and the Build is called for the first time, then the condition if(tmp==NULL) in the Build function should be true, and hence the program should not enter the else section during the first call to Build. What am I missing here? Could you please help.

   class VW
{
    World_Obj *head;
public:
    void Build(...);

};
 void VW::Build(...)
    {
        World_Obj *newObj;
        newObj = new World_Obj;
        newObj->SetData(...);
        newObj->SetNext(NULL);

        World_Obj *tmp;
        tmp = this->head;

        if (tmp == NULL)
            this->head = newObj;
        else
        {
            while (tmp->NextObj() != NULL)
            {
                tmp = tmp->NextObj();
            }
            tmp->SetNext(newObj);
        }
    }

int main()
{
    VW *g_w;    
    g_w = new VW;

          /* Reading arguments from input file*/

    g_w->Build(...);

}

Create a minimal example and don't use pictures that only contain text, they can't be indexed and searched. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Feb 17, 2016 at 7:19
Did you try to catch the crash in a debugger to help you locate where it happens? — Some programmer dude
– Some programmer dude, Commented Feb 17, 2016 at 7:22
Yes, the error occurs due to the call to tmp->NextObj() in the else section of the Build function. However, the first time Build is called, head should be null and the if condition should hold true. I am not sure what am I missing here. — wateva
– wateva, Commented Feb 17, 2016 at 7:58
Thanks Ulrich. I have updated the question based on your comments. — wateva
– wateva, Commented Feb 17, 2016 at 8:01
How do we know that head should be a null pointer? You don't show it being initialized anywhere. Please create a Minimal, Complete, and Verifiable Example and show us. — Some programmer dude
– Some programmer dude, Commented Feb 17, 2016 at 8:06

wateva · Accepted Answer · 2016-02-17 09:19:58Z

1

The issue was the missing constructor in the class VW,

VW(){ head=NULL;}

Thanks all!

answered Feb 17, 2016 at 9:19

wateva

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Some programmer dude · Accepted Answer · 2016-02-17 07:28:38Z

0

Here is a big problem:

tmp = this->head;

...

while (tmp->NextObj() != NULL)
{
    tmp = tmp->NextObj();
}
tmp->SetNext(newObj);

...

delete tmp;

When the loop ends and after you do tmp->SetNext(newObj) then tmp will point to the second to last node in the list. Which you then promptly destroy by deleting it.

That means your list will now somewhere contain a node that no longer exist, and attempting to dereference it will lead to undefined behavior and most likely a crash.

The solution is simply to not delete tmp.

answered Feb 17, 2016 at 7:28

Some programmer dude

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1 Comment

wateva Over a year ago

Thanks Joachim for the quick response. I removed the delete statement. However, I am still facing my earlier issue.

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