linked list and pointer compile error

[bluefrog]

I'm attempting to create a doubly linked list, and have come up with the following:

#ifndef LINK_LIST_H_
#define LINK_LIST_H_

#include <utility>

typedef std::pair <int, int> Item;

class Node {
  public:
    Item   data;
    Node*  next;
    Node*  prev;
    Node(Item & node_data, Node* next_node = nullptr, Node* prev_node = nullptr)
      : data(node_data), next(next_node), prev(prev_node) { }
};

class Linked {
  public:
    Node*   head;
    Node*   tail;

    // Create a head & tail as part of the constructor
    Linked(Node & node) { // Constructor
      head->next = tail;
      tail->prev = head;
      tail->next = nullptr;
      head->prev = nullptr;
    }

    ~Linked() { // destructor
      Node* tmp;
      for(;head;head = tmp) {
        tmp = head->next;
        delete head;
      }
    }

    void append(Node & last) {
      Node * tmp = this->tail->prev;
      last->prev = tmp->prev;
      last->next = tmp->next;
      tmp->next = last;
      tail->prev = last;
    }

//    void Linked::ins
};

#endif 

I get the following compile error though:

g++ -Wall -c -std=gnu++11 "linked_list.h" (in directory: /linked_list)
linked_list.h: In member function ‘void Linked::append(Node&)’:
linked_list.h:40:11: error: base operand of ‘->’ has non-pointer type ‘Node’
       last->prev = tmp->prev;
           ^
linked_list.h:41:11: error: base operand of ‘->’ has non-pointer type ‘Node’
       last->next = tmp->next;
           ^
linked_list.h:42:17: error: cannot convert ‘Node’ to ‘Node*’ in assignment
       tmp->next = last;
                 ^
linked_list.h:43:18: error: cannot convert ‘Node’ to ‘Node*’ in assignment
       tail->prev = last;
                  ^
Compilation failed.

I assumed last->prev is a pointer and so is tmp, so an assignment should be feasible.
Any clues would be appreciated?

[AbstractionAnon]

Lines 40-41: last is passed by reference it is not a pointer. last->member is not valid. Use last.member.

Lines 42-43: Again, last is passed by reference. It is not a pointer. tmp->next and tail->prev are pointers. You can't assign a non-pointer type to a pointer. Use &last.

[bluefrog]

thanks for the help, I'll stick to pointers as arithmetic is possible.

so this solves the above compile errors

void append(Node * last) {
      Node * tmp = this->tail;
      last->prev = tmp->prev;
      last->next = tmp->next;
      tail->next = nullptr;
      tail->prev = last;
    }

[cire]

thanks for the help, I'll stick to pointers as arithmetic is possible.

Pointer arithmetic for most implementations of a linked list, yours included, would be a horrible mistake. Nevertheless, I'd stick to pointers for consistency internally. Client code, of course, should have no need to know about Nodes, pointers or not.

[bluefrog]

Hi

thanks for the tips, I've made my linked list somewhat more generic by making the data within Node a type. When I attempt to append to the end of the list I get the following compile error:

g++ -c -Wall -std=gnu++11 track_score.cpp
track_score.cpp: In function ‘int main()’:
track_score.cpp:12:15: error: request for member ‘append’ in ‘score_sheet’, which is of non-class type ‘LinkedList<int>(Node<int>)’
   score_sheet.append(Node<Item>);
               ^
track_score.cpp:12:32: error: expected primary-expression before ‘)’ token
   score_sheet.append(Node<Item>);
                                ^
makefile:12: recipe for target 'track_score.o' failed
make: *** [track_score.o] Error 1

Modified code:

#ifndef LINK_LIST_H_
#define LINK_LIST_H_

template <class T>
class Node {
  public:
    T data;
    Node*  next;
    Node*  prev;
    Node(T & node_data, Node* next_node = nullptr, Node* prev_node = nullptr)
      : data(node_data), next(next_node), prev(prev_node) { }
    ~Node() {}
};

template <class T>
class LinkedList {
  private:
    Node<T>* head;
    Node<T>* tail;

  public:
    LinkedList(Node<T> * node) : head(node) {
        head->prev = tail;
        head->next = nullptr;
        tail->next = head;
        tail->prev = nullptr;
    } // Create a head & tail as part of the constructor

    ~LinkedList() { }
    void append(Node<T> * last) {
      Node<T> * tmp = this->tail;
      last->prev = tmp->prev;
      last->next = tmp->next;
      this->tail->next = nullptr;
      this->tail->prev = last;
    }
};
#endif 

#include <iostream>
#include <utility>
#include "linked_list.h"

int main() {
  typedef int Item;
  Item * i;
  *i = 6;
  LinkedList<Item> score_sheet(Node<Item>);

  *i = 7;
  score_sheet.append(Node<Item>);
  return 0;
}

Any suggestions would be appreciated. Thanks.

[AbstractionAnon]

Problems I see:

linked_list.h
Line 23,25,26: tail is an uninitialized pointer (garbage).

main.cpp
Line 7: i is an uninitialized pointer.

Line 8. You're trying to dereference an uninitialized pointer.

Line 9,12: You're trying to pass a specialized class name. LinkedList's constructor is expecting a pointer.