Need help in finding the bug (related to pointers to variable on stack)

Question

Can some one please tell me what is wrong in my program. This is basically a program to insert a new node in a binary search tree.

The thing is that my insert function is working correctly and the node is getting inserted which i am verifying in the main program by the line

     cout<<a.left->right->right->data;

The output of which is coming correctly ie 5

but when i try to print a level order traversal of the binary tree some junk value is getting passed in place of the new node and the program is crashing

Can some one please have a look and explain to me what i am doing wrong and how can in the main program the correct value is getting displayed.

#include<iostream>
#include<string>
#include<conio.h>
#include<array>
#include<stack>
#include<sstream>
#include<algorithm>
#include<vector>
#include<ctype.h>//isdigit
#include<deque>
#include<queue>
#include<map>
using namespace::std;
struct BST
{
    int data;
    BST *left;
    BST *right;
    BST(int d,struct BST* l,BST *r):data(d) , left(l) ,right(r)
    {
    }
};

void levelOrder(struct BST *root)
{
    struct BST *temp=NULL;
    int count =0;
    deque<struct BST*> dq;
    if(!root)
    {
        return;
    }
    dq.push_back(root);
    count=dq.size();
    while(!dq.empty())
    {
        temp=dq.front();
        cout<<temp->data<<" ";
        if(temp->left)
        {
            dq.push_back(temp->left);
        }
        if(temp->right)
        {
            dq.push_back(temp->right);
        }
        dq.pop_front();
        if(--count==0)
        {
            cout<<endl;
            count=dq.size();
        }
    }
}
void Insert(struct BST*root,int data)
{
    struct BST temp(data,NULL,NULL);
    if(!root)
    {
        return;
    }
    while(root)
    {
        if((root)->data >data)
        {
            (root)=(root)->left;
            if(!(root)->left)
            {
                (root)->left=&temp;
                break;
            }
        }
        else
        {
            (root)=(root)->right;
            if(!(root)->right)
            {
                (root)->right=&temp;
                break;
            }
        }
    }
}
int main()
{
    deque<struct BST> dq1,dq2;
    BST e(4,NULL,NULL);
    BST f(3,NULL,NULL);
    BST d(1,&f,NULL);
    //BST g(4,NULL,NULL);
    BST b(2,&d,&e);
    BST c(8,NULL,NULL);
    BST a(6,&b,&c);
    levelOrder(&a);
    Insert(&a,5);
    cout<<a.left->right->right->data;
    cout<<endl;
    levelOrder(&a);
    _getch();
    return 0;
}

Answer 1

The reason is because you are putting in pointers to a local variable in the tree in the Insert function.

When a function returns, all its local variables are no longer "alive", and accessing a pointer to one of those local variables is undefined behavior . In fact, the memory those variables once occupied may be overwritten by the next function call, no matter what function you call.

If you want to add a new node, you need to allocate it on the heap using eg new . However, due to your design of the tree, this will cause a memory leak as you don't free any sub-nodes. In fact, as you use pointers to local variables in the main function (that's okay, as the lifetime of those variables are for the duration of main which is the whole program) you can't just simply delete the pointers Willy-nilly.