free(): double free detected in tcache 2 on calling assignment operator

Question

I've just asked a very similar question, but I've been over the answers to that one and I just can't see what I'm doing wrong this time around. I'm working on a university project implementing the c++ list<int> using a linked list. I'm working on the assignment operator, and here's what I have so far.

Linkedlist &Linkedlist::operator=(const Linkedlist &l) {
    clear();
    if(l.empty())
    {
        return *this;
    }
    Node *iterRight = l.head;
    head = new Node;
    Node *iterLeft = head;
    while(iterRight != NULL)
    {
        iterLeft -> data = iterRight -> data;
        iterLeft -> next = new Node;
        iterLeft -> next -> prev = iterLeft;
        iterLeft = iterLeft -> next;
        iterRight = iterRight -> next;
    }
    return *this;
}

When I run, the assingment operator DOES work to copy the data from one list to another, but I get this after the run -

free(): double free detected in tcache 2

I don't understand how I used my pointers inappropriately. Can someone show me what I'm doing wrong?

EDIT: The destructor might be useful too.

```
Linkedlist::~Linkedlist() {
Node *del;
while(head != NULL)
{
    del = head;
    head = head -> next;
    delete del;
}
}

EDIT: My apologies, I'm very new to Stack Overflow. If I understand a MRE correctly, here's the minimum amount of code to reproduce (Everything above with the addition of my main program and my constructor).

int main() {
Linkedlist a(20);
Linkedlist b = a;
return 0;
}

Linkedlist::Linkedlist(unsigned int n) {
    size = n;
    tail = NULL;
    head = new Node;
    Node *iter = head;
    for(int i = 0; i < n; i++)
    {
        iter -> data = i;
        iter -> next = new Node;
        iter -> next -> prev = iter;
        iter = iter -> next;
    }
    tail = iter -> prev;
    tail -> next = NULL;
}

Calling the constructor for the Linkedlist a does not crash, it only crashes once I call the assignment. I ran my program in valgrind, but since I'm rather new at memory management, I'm not quite sure what I'm looking at. It shows me and invalid free() in my destructor, but I can't find where it was already free'd prior.

Please provide a [mcve]; it's possible the code is somewhere else. If you're stuck, run your program through valgrind or build with address sanitizer turned on (they'll both tell you where the data was first `delete`'d). — Stephen Newell, Nov 05 '21 at 16:57
This code alone does not produce a double-free. Without a [mre], you are only guessing what code to show, and we can only guess what code is not shown. My current guess? [Rule of three](https://stackoverflow.com/questions/4172722/what-is-the-rule-of-three). — Drew Dormann, Nov 05 '21 at 17:07
Sorry @DrewDormann, I think I edited it with a minimal reproducible example. — Drake Ford, Nov 05 '21 at 17:24
The code you show here [does not compile](https://godbolt.org/z/5nf5Weqhe), so it can not reproduce the message you describe. I am very confident that there is more code that you are still not showing us. Your main function does suggest my earlier guess that you are violating the [Rule of Three](https://stackoverflow.com/questions/4172722/what-is-the-rule-of-three) in the code that we have not been shown. — Drew Dormann, Nov 05 '21 at 17:34

score 0 · Accepted Answer · answered Nov 05 '21 at 17:38

The line:

Linkedlist b = a;

calls copy constructor, not assignment operator. If you didn't provide copy constructor, then the compiler-generated one will just copy head pointer. Then, during destruction the same head will be deleted both from list a and list b leading to "double free".

free(): double free detected in tcache 2 on calling assignment operator

1 Answers1