deletion of pointers do not "nullify" values.

This is a discussion on deletion of pointers do not "nullify" values. within the C++ Programming forums, part of the General Programming Boards category; I'm doing a binary search tree deletion algorithm but having problems with deletions. The constructors for my BinaryTreeNode class has ...

  1. #1
    Ethernal Noob
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    deletion of pointers do not "nullify" values.

    I'm doing a binary search tree deletion algorithm but having problems with deletions. The constructors for my BinaryTreeNode class has to nullify the elements of the BinaryTreeNode[2] arrays because by default they are not null, otherwise it would mess up my insertions and searches. But when I delete, they do not revert to a null status. So if I delete one by using a pointer then the deleted one reains a value other than 0x0. Anyone know what's going on?

    Here's the incomplete function

    Code:
    void BSTree::Delete(int x)
    {
        if(root == NULL)
        {
            std::cout << "-- Tree is empty --" << std::endl;
            return;
        }
    
        BinaryTreeNode* rt = root;
        BinaryTreeNode* advance = NULL;
    
        while(true)
        {
            //delete based on deletion rules
            if(rt->data == x)
            {
                //if both children are empty, delete and return
                if(rt->child[0] == NULL && rt->child[1] == NULL)
                {
                    delete rt;
                    std::cout << "-- " << x << " has been deleted --" << std::endl;
                    return;
                }
                
                //..other deletion rules.
            }
            else
            {
                size_t dir = x < rt->data ? 0 : 1;
    
                if(rt->child[dir] == NULL)
                {
                    std::cout << "-- "<< x
                              <<" not found, cannot delete --" << std::endl;
                    return;
                }
                else
                    rt = rt->child[dir];
    
            }
    
        }
    }
    Last edited by indigo0086; 10-02-2007 at 11:55 AM.

  2. #2
    Deathray Engineer MacGyver's Avatar
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    Uh, I'm confused. Is delete supposed to set memory to 0? I thought it was just supposed to free the memory, in a way similar to free(), and then call the destructor (if applicable).... am I missing anything?

  3. #3
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    I thought by freeing the memory, any reference to it had a null value ???

  4. #4
    Deathray Engineer MacGyver's Avatar
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    Accessing memory after it has been deallocated has undefined behavior, I'm pretty sure. If you're deleting an object, you can do something in your deconstructor for that object (set something to null or whatever).

  5. #5
    and the hat of int overfl Salem's Avatar
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    If you have, in various parts of the code, the equivalent of
    p = new int;
    q = p;
    delete q;

    Then there is nothing you can do which will affect what p is pointing to. It's obvious to see here that p is now pointing to deallocated memory, and dereferencing it would be "a bad thing".

    You need to add to the code a clear idea of who is responsible for tracking each block of memory. Because if you end up with multiple aliases, then you're always going to be stuck between a crash (any use after free) or a memory leak (not freeing it at all).
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  6. #6
    Algorithm Dissector iMalc's Avatar
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    Advice: Take only as directed - If symptoms persist, please see your debugger

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  7. #7
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    Ok guys, I guess I'm doing too much C# that I mixed the two up. I would have known this a few months ago.

  8. #8
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    OK I got it to work. I just had to keep a front and back pointer too keep track of the nodes so when I deleted the forward, I could set any references to it to null appropriately.

  9. #9
    Cat without Hat CornedBee's Avatar
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    I have no idea how you might get the idea that this would work from C#. C# doesn't even have explicit deletion in the first place.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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