C Linked list error: taking address of rvalue [-fpermissive]-CodePudding

I'm getting the error:

taking address of rvalue [-fpermissive]
   31 |     ListNode l = ListNode(2, &ListNode(4));

when executing the following code:

#include<iostream>
class ListNode {
public:
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};


int main(){
    ListNode l = ListNode(2, &ListNode(4));
    return 0;
}

I don't really know how to use classes in C but I would like to use a Class here for the LinkedList instead of a struct.

CodePudding user response：

You would get exactly the same error with a struct. The problem is that you are taking the address of a temporary object, here &ListNode(4) and that's bad because the address will live longer than the object, and you end up with a pointer to an object which no longer exists.

To fix, turn the temporary object into a variable

ListNode m = ListNode(4);
ListNode l = ListNode(2, &m);

Now the ListNode(4) object is held by a variable, so it's safe to take the address of it.

But usually in a linked list class you solve this problem by using dynamic memory allocation with new.