how to delete a binary tree throught another name

Question

I have the following code:

typedef struct TreeNode
{
    int data;
    TreeNode *lchild, *rchild;
} TreeNode;
TreeNode *CreateBTNode(TreeNode *bt) // pre-build
{
    char ch;
    cin >> ch;
    if (ch == '#')
        bt = nullptr;
    else
    {
        bt = (TreeNode *)malloc(sizeof(TreeNode));
        bt->data = ch;
        bt->lchild = CreateBTNode(bt->lchild);
        bt->rchild = CreateBTNode(bt->rchild);
    }
    return bt;
}
void DeleteBiTree(TreeNode *&bt) 
{
    if (!bt)
        return;
    DeleteBiTree(bt->lchild);
    DeleteBiTree(bt->rchild);
    free(bt);
    bt = nullptr;
}
void PreOrder(TreeNode *bt)
{
    if (bt)
    {
        cout << bt->data << " ";
        PreOrder(bt->lchild);
        PreOrder(bt->rchild);
    }
}
int main()
{
    TreeNode *tree = CreateBTNode();
    TreeNode *tmp = tree;

    DeleteBiTree(tree);       // line 1

    DeleteBiTree(tmp);        // line 2

    PreOrder(tree);           // line 3
    
    return 0;
}

In this code the tree and tmp have the same address. when i delete this tree using line 1 and output the tree, there is no output and it's right, but when i try to delete the tree using line 2, there comes to a mess, why? Thanks a lot.

DeleteBiTree takes a reference to the pointer argument and modifies it. If you pass different pointers into the function, you'll modify different pointers. — cigien
– cigien, Commented Sep 18, 2024 at 8:44
Same situation: void f(int& a) { a = 1; } int main() { int x = 0; int y = x; f(y); cout << x; } prints '0', not '1'. — molbdnilo
– molbdnilo, Commented Sep 18, 2024 at 8:47
This is C with cout, not C++. Please don't write code like this and don't learn this. Don't use malloc in C++. — user16540390
– user16540390, Commented Sep 18, 2024 at 8:52
As a side note, a C++ compiler should not compile this code because you call CreateBTNode() without a parameter (which you do not need, anyway). — DYZ
– DYZ, Commented Sep 18, 2024 at 8:53

Caleth · Accepted Answer · 2024-09-18 10:36:03Z

0

You have two TreeNode * objects pointing to one TreeNode object. When you call DeleteBiTree(tmp);, you destroy the TreeNode, then set one of those pointers to null. The other pointer now has an invalid value. Once a pointer has an invalid value, the only valid operation on it is assigning it a new value. Even comparing it to nullptr has undefined behaviour.

answered Sep 18, 2024 at 10:36

Caleth

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