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interacting with elements of a templated container

Posted on 2006-11-08
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Last Modified: 2010-04-02
Hi,

I made a templated container class for testing out templates. It is a simple doubly linked list. Previously I had specified the exact type the container used so there were class specific operations in there, one of which was dumping the contents of the container (just pseudo code):

    class CLinkedList {

         CSpecificClass *m_pLinkedList;

         void DumpToScreen()
         {
              while (m_pLinkedList) {
                   next->PrintContents();
              }
    };

Now CLinkedList uses templates for the data it stores and so what can I do for an operation to dump all the contents of the list since I don't know what the user is going to stick in there? I can't do anything really, can I? I'd have to do it externally now like:

     int main()
     {
          CLinkedList<CWhatever> list;
          list.AddNode(...);
          list.AddNode(...);
          list.AddNode(...);

          // Iterate through each element somehow out here since I know the type only outside, not internally:
          while (somehow iterate externally over the list's elements) {
              *it->Print();
          }
     }

I guess this is why stl implemented the iterators, I'd have to do the same thing?

Thanks
   
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Question by:DJ_AM_Juicebox
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jkr earned 500 total points
Comment Utility
Implementing iterators on your own might be some overkill for startng out, but you could do something like

    template<typename T>
    class CLinkedList {

        public:

        CLinkedList () { // default constructor

            m_pNodes = NULL;
            m_pCurrent = NULL;
        }

        CNode<T>* First() const { m_pCurrent = m_pNodes; return m_pCurrent;}
        CNode<T>* Next() const {

            if (!m_pCurrent) return NULL;

            m_pCurrent = m_pCurrent->m_pNext;

            return m_pCurrent;
        }

        private:
        CNode<T> *m_pNodes;
        CNode<T> *m_pCurrent;
    };

     int main()
     {
          CLinkedList<CWhatever> list;
          list.AddNode(...);
          list.AddNode(...);
          list.AddNode(...);

          // Iterate through each element somehow out here since I know the type only outside, not internally:
         CWhatever<CSomeType>* it = list.First();;
          while (it) {
              it->Print();

              it = list.Next();
          }
     }
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by:jkr
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Ooops, remove the 'const' in the above, that should just be

        CNode<T>* First() { m_pCurrent = m_pNodes; return m_pCurrent;}
        CNode<T>* Next() {

            if (!m_pCurrent) return NULL;

            m_pCurrent = m_pCurrent->m_pNext;

            return m_pCurrent;
        }
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Author Comment

by:DJ_AM_Juicebox
Comment Utility
Yep that's perfect, thanks as always.
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by:Axter
Comment Utility
Hi DJ_AM_Juicebox,
> I guess this is why stl implemented the iterators, I'd have to do the same thing?
>>I guess this is why stl implemented the iterators, I'd have to do the same thing?

Are you sure you need a link list.  Rarely is a link list the correct container type, and for most requirements, a std::vector type contianer is much more efficient and more appropriate.
Moreover, with a std::vector, you can use operator[] instead of iterators.

David Maisonave (Axter)
Cheers!
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