I'm writing a linked list, and using my main function to test it. Here's my code:

#include <iostream>
using namespace std;

class LinkedList {
  int value;
  LinkedList* next;

  public:

    LinkedList(int valueIn, LinkedList* nextIn) {
      value = valueIn;
      next = nextIn;
    }
    LinkedList(int valueIn) {
      value = valueIn;
    }

    int getValue() {
      return value;
    }

    void addNode(LinkedList* node) {
      next = node;
    }

    LinkedList& getNext() {
      return *next;
    }
};

int main() {
  cout << "starting..." << std::endl;
  LinkedList list1(1);
  LinkedList list2(2, &list1);
  cout << list1.getValue() << " --> " << list1.getNext().getValue() << std::endl;
  return 0;
}

I expect the output to be 1 --> 2, but I get 1 -->. As far as I understand, getNext() should return a reference to another list (list2 in this case), but something seems to be going wrong. My debugging efforts show me that list2 does have the correct value 2 when it's initialized, but when it's referenced for the final output, it doesn't seem to have anything for value. I can't for the life of me figure out why this is. Could someone help me to understand?

CodePudding user response：

You are insertin list1(which is actually a node) to the end of list2, not the other way around, yet you call getNext() on list1. You should change the code in main to the below:

int main() {
  std::cout << "starting..." << std::endl;
  LinkedList list1(1);
  LinkedList list2(2, &list1);
  std::cout << list2.getValue() << " --> " << list2.getNext().getValue() << std::endl;
  return 0;
}

Please note that there are a couple of other things which would be better to change:

1. Create a list class and a Node class woud make things clearer
2. Initializing the pointer to be NULL(or nullptr from C 11) in the LinkedList(int valueIn) constructor
3. Return the pointer to the node in getNext() rather than copy the node

CodePudding user response：

You are not getting a blank value. Actually your program is crashing when you are trying to call list1.getNext().getValue() as getNext() is returning reference to a NULL.

You are doing the opposite of what you want to do. Your list2 is pointing to list1 and list1 is pointing to NULL.

You should change your code with this:

LinkedList list2(2);
LinkedList list1(1, &list2);
cout << list1.getValue() << " --> " << list1.getNext().getValue() << std::endl;