Strange behavior while using queue that contains pointers

Question

I am trying to create a binary tree that when you try to add a new node to that, it adds the node to first position that is nullptr.

actually making a complete binary tree.

Look at the code below:

#include <iostream>
#include <queue>
using namespace std;

class node{
private:
    char data;
    node* right;
    node* left;
public:
    node(char n){
        data = n;
        left = nullptr;
        right = nullptr;
    }
    char getdata(){
        return data;
    }
friend class binTree;
};

class binTree{
private:
    node *root;
public:
    binTree(){
        root = nullptr;
    }
    binTree(node *root){
    this->root = root;
    }

    node* getRoot(){
    return this->root;
    }


    void addNode(char data){
        cout << "adding " << data << endl;

        if(root == nullptr) {
            root = new node(data);
            return;
        }


        queue<node*> Q;
        Q.push(root);
        node* toadd;
        while(true) {
            node* toadd = Q.front();
            Q.pop();
            Q.push(toadd->left);
            Q.push(toadd->right);

            if(toadd->left == nullptr)  break;
            if(toadd->right == nullptr) break;
        }

        if((toadd->left) == nullptr)
        {
            cout << "add data to the left of " << toadd -> data << endl;
            toadd->left = new node(data);
        } else if((toadd -> right) == nullptr){
            cout << "add data to the right of " << toadd -> data << endl;
            toadd->right = new node(data);
        } else {
            cout << "toadd left and right are not nullptr" << endl;
        }

    }
};

int main()
{
    binTree bin;
    string s = "abcdefg";
    cout << s << endl << endl;
    for(int i = 0; i < s.size(); i++)
    {
        bin.addNode(s[i]);
    }
}

when I run this code the output is:

abcdefg

adding a
adding b
toadd left and right are not nullptr
adding c
toadd left and right are not nullptr
adding d
toadd left and right are not nullptr
adding e
toadd left and right are not nullptr
adding f
toadd left and right are not nullptr
adding g
toadd left and right are not nullptr

the strange part is when printing "toadd left and right are not nullptr" because there is a while(true) and the only exit conditions are:

            if(toadd->left == nullptr)  break;
            if(toadd->right == nullptr) break;

so one of these conditions were true that we could break the loop; so we should enter one of if or else if part of the following code (after the while) but surprisingly we enter the else part and end of printing "toadd left and right are not nullptr". can anyone explain this behavior?

Mark Ransom · Accepted Answer · 2018-12-04 14:05:42Z

7

You've defined toadd twice, once before the loop and once inside. Easy mistake to make.

answered Dec 4, 2018 at 14:05

Mark Ransom

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1 Comment

nwp Over a year ago

-Werror=shadow on gcc and clang avoids this category of error.

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