Is this a bad way of initializing a vector?

Question

vector<vector<int>> levelOrder(TreeNode* root) {
    vector<vector<int>> result;
    queue<TreeNode *> que;

    if (root != nullptr) {
        que.emplace(root);
    }

    while (!que.empty()) {
        vector<int> level;
        int size = que.size();
        for (int i = 0; i < size; i++) {
            auto *front = que.front();
            que.pop();
            level.emplace_back(front->val);
            if (front->left != nullptr) {
                que.emplace(front->left);
            }
            if (front->right != nullptr) {
                que.emplace(front->right);
            }
        }
        result.emplace_back(move(level));
    }

    return result;
}

Problem: https://leetcode.com/problems/binary-tree-level-order-traversal/description/

Above is the function that returns a vector>.

However, since I initialized the vector as a local variable vector<vector<int>> result; does it mean that it is a code smell for me to return it?

Since the vector is a local variable, it is allocated on the stack and the vector will get destroyed when this function call is over.

Should I have done this instead auto results = new vector<vector<int>>

I would not recommend the `new vector>` version. Returning by value is the best way imo. — Galik, Nov 26 '17 at 23:07
I'd recommend the syntax `if (root != NULL) { ... }`. Or, even better, `if (root != 0) { ... }` — paulsm4, Nov 26 '17 at 23:14
@paulsm4 Bah, `if (root)`, clearly. Or OP's version, but definitely not what you suggest. — juanchopanza, Nov 26 '17 at 23:15

Jive Dadson · Accepted Answer · 2017-11-26T23:30:36.660

4

No.

Since the software will not allow me to stop at that, I will elaborate. The "No" applies to the question only. I have not checked the code for errors. So, no, it is not wrong to return the result as written. There is no cost involved. Any modern compiler will use return-value optimization to return the result.

It would be very bad form to use operator new. Doing so would put the burden of deleting the object upon the calling function. Search for "C++ RAII".

edited Nov 26 '17 at 23:30

answered Nov 26 '17 at 23:16

Jive Dadson

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Why the down-vote? If this is wrong, please tell us why. – Jive Dadson Nov 26 '17 at 23:18
"Doing so would put the burden of deleting the object upon the calling function." - which is why we have smart pointers. – Nov 26 '17 at 23:26
2

@Neil - True. But there is no reason to use a pointer here, smart, stupid, or of average IQ. – Jive Dadson Nov 26 '17 at 23:28

Is this a bad way of initializing a vector?

1 Answers1