pointer to template problem
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  1. #1
    Join Date
    Oct 2007
    Posts
    36

    pointer to template problem

    Hi,
    I wrote a template for a double linked list and i try to dynamically allocate an array where each element is a list from that template, but when i try to insert an element i get a compilation error and i don't understand the reason for getting this.
    Here's the code:

    template array definition:
    Code:
    List <gift> *buckets;

    inserting the element:
    Code:
    gift * newGift;
    ...
    ...
    buckets[pos].insertNode (*newGift);
    element insertion code
    Code:
    template <class contain> 
    int List<contain>::insertNode (contain &element)
    {
      ListNode  <contain> *temp, *newNode;
      
      // if first element allocate first pointer.
      if (!size) {
        list = new ListNode <contain> (element);
        lastNode = list;
        return ++size;
      }
      
      //else traverse the list and insert element at the end.
      temp = list;
      while (temp -> getNext())
        temp = temp -> getNext();
      
        
      //if memory allocation error return 0
      if (!(newNode = new ListNode <contain> (element))) return 0;
      
      temp -> setNext (newNode);
      lastNode = newNode;
      return ++size; // else return the size of the list
    }
    and compiler error:
    Code:
    list.cpp: In member function ‘int List<contain>::insertNode(contain&) [with contain = gift]’:
    santa.cpp:41:   instantiated from here
    list.cpp:48: error: could not convert ‘temp->ListNode<contain>::getNext [with contain = gift]()’ to ‘bool’
    list.cpp:49: error: cannot convert ‘ListNode<gift>’ to ‘ListNode<gift>*’ in assignment
    If any additional information is needed please let me know.
    Thanks in advance.

  2. #2
    Join Date
    Jan 2004
    Location
    Düsseldorf, Germany
    Posts
    2,401

    Re: pointer to template problem

    The error references the line
    Code:
      while (temp -> getNext())
    so you should show us your definition of ListNode and ListNode::getNext().
    More computing sins are committed in the name of efficiency (without necessarily achieving it) than for any other single reason - including blind stupidity. --W.A.Wulf

    Premature optimization is the root of all evil --Donald E. Knuth


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  3. #3
    Join Date
    Oct 2007
    Posts
    36

    Re: pointer to template problem

    These are all the definitions for the list template, the first class is the list node and the second class is the header of the list:

    Code:
    template <class contain> class ListNode {
    private:
      contain Data;
      ListNode *next;
      ListNode *previous;
    public:
      ListNode() { next = NULL; }
      ListNode (contain &);
      ~ListNode() {}
      void setData (contain &element) { Data = element; }   
      contain &getData () { return Data; }
      void setNext (ListNode * newNext) { next = newNext; }
      ListNode &getNext () { return *next; }
    };
    
    template <class contain> class List {
    private:
      int size;
      ListNode <contain> *list;
      ListNode <contain> *lastNode;
    public:
      List();
      ~List();
      int getSize () { return size; }
      ListNode <contain> * getList () { return list; }
      int insertNode (contain &);
      int popNode ();
    };

  4. #4
    Join Date
    Mar 2002
    Location
    NY, USA
    Posts
    12,097

    Re: pointer to template problem

    getNext returns a REFERENCE, as such it must ALWAYS be a valid object. Therefore the if statement is illegal. Ift is the same as:

    Code:
    class A {};
    A a;
    if (a) {...}
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  5. #5
    Join Date
    Jan 2004
    Location
    Düsseldorf, Germany
    Posts
    2,401

    Re: pointer to template problem

    In your constructor of ListNode you set next to 0. Calling getNext() on such an object will result in dereferencing the null pointer, which will probably crash your program.

    Simply change the class to have getNext() to return a pointer (possibly the null pointer).
    More computing sins are committed in the name of efficiency (without necessarily achieving it) than for any other single reason - including blind stupidity. --W.A.Wulf

    Premature optimization is the root of all evil --Donald E. Knuth


    Please read Information on posting before posting, especially the info on using [code] tags.

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