delete a double pointer

Question

How to properly delete a double-pointer array? when I tried this code, memcheck told me that "Use of the uninitialized value of size 8" and "Invalid write of size 4". I couldn't figure out where I did wrong.

struct Node
{
    int value;
    Node* next;
};


int main()
{

    Node** doublePtrNode= new Node* [10];
     
     for (unsigned int i = 0; i < 10; i++)
    {
        
        doublePtrNode[i]->value=i;
        
    }

    for (unsigned int i = 0; i < 10; i++)
    {
        delete doublePtrNode[i];
    }
  
    delete[] doublePtrNode;

    return 0;
}

Your problem is allocation of the second dimension not deletion. This question should help: [https://stackoverflow.com/questions/936687/how-do-i-declare-a-2d-array-in-c-using-new](https://stackoverflow.com/questions/936687/how-do-i-declare-a-2d-array-in-c-using-new) — drescherjm, Nov 18 '21 at 16:16
You never assign a value to the elements of the array `doublePtrNode` (which are pointers). — , Nov 18 '21 at 16:17
Side note: explicit use of `new` and `delete` since C++11 is considered as a bad practice. You should use `std::vector` which will do proper memory management in your behalf. In other cases there is `std::unique_ptr` and `std::shared_ptr`. — Marek R, Nov 18 '21 at 17:07

Ted Lyngmo · Accepted Answer · 2021-11-18T16:27:12.983

You are already deallocating what you have allocated but doublePtrNode[i]->value=i; assumes that you've allocated a Node there, but you haven't so the program has undefined behavior.

If you are going to use raw owning pointers, you could fix it like this:

Node** doublePtrNode = new Node*[10];
// now allocate the actual `Node`s too:
for(unsigned i = 0; i < 10; ++i) doublePtrNode[i] = new Node;

// your current assigning loop goes here

// deallocate the `Node`s:
for(unsigned i = 0; i < 10; ++i) delete doublePtrNode[i];
delete[] doublePtrNode;

A much simpler and safer option is to use a std::vector<Node>. That way you do not need any manual memory management. It does it for you.

#include <vector>

int main() {
    std::vector<Node> nodes(10);

    // your current assigning loop goes here

} // all `Node`s are deleted when `nodes` goes out of scope

Thank you, Ted, for your explanation. so when I created the double-pointer array, I just created 10 pointers of Node, Since they are just pointers. I still need to allocate memory to each Node so that they can be used right? — Steven, Nov 18 '21 at 16:32

arutar · Answer 2 · 2021-11-18T16:27:48.977

1

you need create every node doublePtrNode[i] before assign on it value

struct Node
{
    int value;
    Node* next;
};


int main()
{

    Node** doublePtrNode = new Node * [10];

    for (int i = 0; i < 10; i++)
    {

        doublePtrNode[i] = new Node{ i,nullptr };

    }

    for (unsigned int i = 0; i < 10; i++)
    {
        delete doublePtrNode[i];
    }

    delete[] doublePtrNode;

    return 0;
}

edited Nov 18 '21 at 16:27

answered Nov 18 '21 at 16:22

arutar

1,015
3
9

Thank you @arutar, I have a better understanding of pointers now. – Steven Nov 18 '21 at 16:36

delete a double pointer

2 Answers2