c++) Problems that are automatically deleted and not returned when returning an object in the operator function

I'm writing a linked list.

I wrote +=, = operator and checked that it works properly.

After that, write the + operator.

unvlist unvlist::operator+(const unvlist &ul){
    unvlist *list = new unvlist();
    *list = *this;
    *list += ul;
    return *list;
}

I wrote it like this, so I don't know the reason for the leak (12byte) Detected memory leaks! Dumping objects -> {216} normal block at 0x0122B298, 12 bytes long. Data: < 8 " " > 05 00 00 00 38 B3 22 01 D0 B4 22 01 Object dump complete. It happens, but I think it's because there's no process of generating and deleting *list in the function.

unvlist unnvlist::operator+(constunvlist &ul) {
    unvlist list = unvlist();
    list = *this;
    list += ul;
    return list;
}

I wrote it like this.

unvlist=unvlist(), ll=unvlist();
l = *l1;
l += *l2;
ll = *l1 + *l2;

If you run l, it works fine, but operator= fails the moment you run l = *l1 + *l2

The reason is that *l1 + *l2 goes in size normally, but none of the remaining nodes go in. (NULL)

As I checked within the operator+ function, it works fine until the return.

Below is my destructor function

unvlist::~unvlist(){
    clear();
    //  //  free(this);
}
void unvlist::clear(){
    NodeBase* T = Tail;
    NodeBase* tmp = Head;
    NodeBase* ttmp;
    while (tmp != NULL){
        ttmp = tmp->nextNode;
        delete tmp;
        tmp = ttmp;
    }
    Tail = NULL;
    Size = 0;
}

The unvlist class is as follows:

class unvlist {
private:
    int Size;
    NodeBase* Head;
    NodeBase* Tail;
public:
    unvlist operator+(const unvlist &ul);
    unvlist& operator=(const unvlist &ul);
    unvlist& operator+=(const unvlist &ul);
    bool operator==(const unvlist &ul);
    bool operator!=(const unvlist &ul);
    ....
}

That is, to summarize,

The value of *l1+*l2 is automatically deleted before it is returned (from operator+), so it is not returned normally.

What's the cause?

https://kldp.org/node/132271

I think it's a similar case to me, so I looked into it, but I don't know why there's an error even though I just set the return type as a type, so I'm asking you a question.