What wrong with this overload of the operator+?

Question

I have a class called List which implements a linked list.

I'm trying to overload the '+' operator of a linked list class so I can do this kind of thing:

List l1;
/* add a and b to l1 */
List l2;
/* add c and d to l2 */
List l3 = l1 + l2;
/* l3 contains a, b, c, d and l1 and l2 are unchanged */

I've implemented operator+= like so, and it appears to work fine.

List& List::operator+=(List& otherList) {
  Node* currNode = otherList.getHead();
  while (currNode) {
    this->add(currNode->getData());
    currNode = currNode->getNext();
  }
  return *this;
}

Here's my attempt at implementing operator+ but it doesn't appear to work.

List List::operator+(List& otherList) {
  List* l = new List();
  l += *this;
  l += otherList;
  return *l;
}

When I try it like so:

List l1;
List l2;
List l3;
l3 = l1 + l2;

I get this error:

Main.cc:25:13: error: no match for ‘operator=’ in ‘l3 = List::operator+(List&)((* & l2))’

Any ideas what I'm doing wrong?

UPDATE: I also have an operator= that looks like this and appears to work fine

List& List::operator=(List& otherList);

Answer 1

Your operator+ is not at fault; you are apparently missing a copy assignment operator.

Also, stop dynamically allocating everything; you're leaking like a seive.

Answer 2

If operator+= is correct then just do this (outside of your class definition):

List operator+(List x, List const &y) { return x += y; }

As others have said, since you are copying your objects, you must have a copy-assignment operator, and a copy-constructor and a destructor. (Unless the default ones work, which we can't be sure about without seeing the definition of List ).

Answer 3

Looks like I found a solution that will suffice ... this worked for me:

List& List::operator+(List& otherList) {
  List* l = new List();
  *l += *this;
  *l += otherList;
  return *l;
}

Thanks for the help, everyone.