struct member pointer still null after allocating memory

Question

I have two structs that represent a node of a binary search tree and a bst. Calling the add method the first time the output is as expected: tree is null, but why is it still: tree is null after I call it the second time?

If I send as parameter to the add method the whole bst, it works.

#include <iostream>

using namespace std;

struct node
{
    int data;
    node* right = NULL;
    node* left  = NULL;
};

struct bst
{
    node* root = NULL;
};



void add(node* tree) 
{

    if (tree == NULL)
    {
        cout << "Tree is NULL "<<endl;
        tree = new node;
    }
    else cout << "Not NULL"<<endl;
}

int main()
{
    bst* tree = new bst;
    add(tree->root);
    add(tree->root);
    system("pause");
    return 0;

}

This is not C. And in C++, you should not use the macro `NULL`, but the `nullptr` keyword. — too honest for this site, May 03 '16 at 12:01
You're not passing `tree` pointer by reference, working on a duplicate... — LogicStuff, May 03 '16 at 12:02

score 1 · Answer 1 · answered May 03 '16 at 12:06

You can't pass tree back out of the function unless you declare it as the address of the pointer like this:

    void add(node** tree) {
        if ( tree == NULL ) {
          return;
        }
        if (*tree == NULL) {
          cout << "Tree is NULL " << endl;
          *tree = new node;
        } else {
          cout << "Not NULL" << endl;
        }       
    }

score 0 · Answer 2 · answered May 03 '16 at 12:04

0

You're passing the node itself instead of its address. Try

void add(node** tree)
{
  if (*tree == NULL)
  {
      cout << "Tree is NULL "<<endl;
      *tree = new node;
  }
  else cout << "Not NULL"<<endl;
}

int main()
{
  bst* tree = new bst;
  add(&(tree->root));
  add(&(tree->root));
  system("pause");
  return 0;
}

answered May 03 '16 at 12:04

Robert Kock

5,795
1
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20

he is actually passing the adress of the node. – 463035818_is_not_an_ai May 03 '16 at 12:26

struct member pointer still null after allocating memory

2 Answers2