C++ Linked List Assignment Operator Problem

Question

I am writing an Ordered Linked List class definition (OLList). I have written the assignment operator function, but when I try to test it by chaining assignment operations, the program gets caught in the while loop of the OLList::copy function. I know this because I tested using console prints.

//OLList.h
struct Node {
  ListItem item;
  Node *next;
};

class OLList {
  public:
  OLList& OLList::operator =(const OLList& rhs)
  {
    if (this != &rhs) {
        destroy();
        copy(rhs);
    }
    return *this;
  }

  void OLList::destroy()
  {
    Node *current_node = this->headM;
    Node *next_node;
    while(current_node->next != nullptr)
    {
        next_node = current_node->next;
        delete(current_node);
        current_node = next_node;
    }


    return;
  }

void OLList::copy(const OLList& source)
  {
    Node *new_node, *current_node;
    Node *current_source_node = source.headM;
    this->headM->item = source.headM->item;
    current_node = this->headM;

    while(current_source_node->next != nullptr)
    {
        new_node = new(Node);
        current_node->next = new_node;
        current_node = current_node->next;
        current_source_node = current_source_node->next;
        current_node->item = current_source_node->item;
    }

    return;
  }
}

Below is the code used to test the class. I have made sure that the print() function works fine so that's definitely not an issue.

//main.cpp
int main()
{
    OLList the_list;
    the_list.insert(1);
    the_list.insert(2);

    OLList second_list;
    second_list.insert(3);
    second_list.insert(4);

    OLList third_list;
    third_list.insert(5);
    third_list.insert(6);

    third_list = second_list = the_list;
    third_list.print();
}

When it is compiled and run, the program never terminates as it is caught in the loop mentioned above.

Answer 1

Your destroy() method will fail if headM is nullptr. You should be using while(current_node != nullptr) instead of while(current_node->next != nullptr). But more importantly, it doesn't reset headM to nullptr after destroying the list. So after operator= calls destroy(), headM is no longer in a valid state for copy() to use.

Your copy() method is similarly not checking if either source or target headM are nullptr. But more importantly, it assumes the target list is empty beforehand, otherwise it leaks memory, if it does not crash outright (per above). And frankly, it simply is not coded correctly in general to copy one list to another.

So, your code is invoking undefined behavior, this anything could happen.

Like @PaulMcKenzie stated in comments, you really should be using a proper copy constructor instead (and a destructor - and since you are clearly using C++11 or later, a move constructor and move assignment operator, too - see the Rule of 5). Your assignment operator can then be implemented using your copy constructor (and likewise for move assignment).

Try something more like this:

struct Node {
    ListItem item;
    Node *next = nullptr;

    Node(const ListItem &value) : item(value) {}
};

class OLList {
private:
    Node *headM = nullptr;

public:
    OLList() = default;

    OLList(const OLList &src)
    {
        Node *current_source_node = src.headM;
        Node **current_node = &headM;

        while (current_source_node)
        {
            *current_node = new Node(current_source_node->item);
            current_node = &((*current_node)->next);
            current_source_node = current_source_node->next;
        }

        /* alternatively:
        Node *current_source_node = src.headM;
        while (current_source_node) {
            insert(current_source_node->item);
        }
        */
    }

    OLList(OLList&& src)
    {
        src.swap(*this);
    }

    ~OLList()
    {
        Node *next_node;    

        while (headM)
        {
            next_node = headM->next;
            delete headM;
            headM = next_node;
        }
    }

    void clear() {
        OLList().swap(*this);
    }

    OLList& operator=(const OLList& rhs)
    {
        if (this != &rhs) {
            OLList(rhs).swap(*this);
        }
        return *this;
    }

    OLList& OLList::operator=(OLList&& rhs)
    {
        OLList(std::move(rhs)).swap(*this);
        return *this;
    }

    void swap(OLList &other) {
        std::swap(headM, other.headM);
    }

    void insert(const ListItem &value) {
        ...
    }

    void print() const {
        ...
    }
    ...
};