运算符重载+ =表示LinkedList无法编译

Question

除非我使运算符重载函数const，否则它将不会编译，但是如果我这样做，则LinkedList1的成员变量不会像应有的那样更改。

LinkedList LinkedList::operator+=(const LinkedList& object)
{
    Node::data_type temp;
    LinkedList n1 = *this;
    Node* ptr;
    ptr = object.head;
    while (ptr != nullptr)
    {       
        temp.set_name(ptr->get_data().get_name());
        temp.set_score(ptr->get_data().get_score());
        n1.addToTail(temp);
        ptr = ptr->get_linkN();     
    }
    return n1;
}

int main()
{
    LinkedList firstList;
    LinkedList secondList;

    initialize(firstList, secondList);

    firstList += secondList;
}