Please help, don't know why program is crashing...???

[SMA01]

I am reading in values from a file, and am not sure what's wrong with my code? My main issue is within the area in bold, can anyone tell me what's wrong?

Thanks.

void createList(intNode*& intList)
{
    intNode* lastInt; //points to last integer in file
    lastInt = NULL;
    int fileInt; //int read from input file
    ifstream intInputFile;

    intInputFile.open("intInput.txt");
    if (intInputFile.is_open())
    {
        cout << "intInput.txt open successful" << endl;
    }
    else
    {
        cout << "intInput.txt open unsuccessful" << endl;
    }
    intInputFile >> fileInt;
    while(!intInputFile.eof())
    {
        intNode* anotherInt;
        anotherInt = new intNode;
        if(intList==NULL)
        {
            intList = anotherInt;
            lastInt = anotherInt;
        }
        else
        {
            lastInt->nextNode = anotherInt;
        }
        lastInt = lastInt->nextNode;
        lastInt->intValue = fileInt;
        lastInt->nextNode = NULL;
        intInputFile >> fileInt;
    }
    intInputFile.close();
    cout << "List created from input file" << endl;
}

[helios]

On the first iteration you set lastInt to anotherInt. lastInt->nextNode is presumably a null or invalid pointer. Then, rather than storing fileInt (which by the way the loop doesn't even touch, nor does it touch intInputFile) in lastInt->intValue, you move lastInt to the next node, which isn't guaranteed to exist, and assign to that the integer.

[Disch]

The first time the loop runs:

- anotherInt gets created. (line 21)
- intLast gets set to anotherInt (line 24)
- lastInt gets set to anotherInt (line 25)
- lastInt gets set to anotherInt->nextNode (line 31 -- PROBLEM)

This is a problem because you never initialized anotherInt->nextNode. Depending on what you're doing in intNode's constructor, you're either dereferencing a null pointer (bad), or dereferencing an uninitialized pointer (worse). Each have potential to crash.


It seems to me that line 31 should be inside the else clause.

[SMA01]

Thanks. I'm confused as to how to fix it, I've tried intializing lastInt->nextNode to null but it still doesn't work. Can you explain how I would fix the problem?