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Thread: Templates: Generic Linked List, I stuck at returning type in template functions.

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    Question Templates: Generic Linked List, I stuck at returning type in template functions.

    Hello.

    Until now, I used to do things the old way (As I learned in the java course). For example, in order to create a generic linked list, I would create a Node class which the user can inherit to define what data this node will contain and a List class which the user can inherit in order to override things like the search method etc.

    I started learning templates and for learning purposes, I'm trying to make a linked list. This is where I stuck:

    Code:
            //Search.
            template<typename U>
            T *search(bool (*compare)(const T other, U custom), U custom)
            {
                //Current node starts from the head.
                LinkedListNode<T> *current = m_Head;
    
                //GO though each node until the end.
                while (current != NULL)
                {   
                    //Do the comparison based on the user's custom function.
                    if ( compare(current->m_Data, custom) )
                        return current->m_Data;
    
                    //Next node.
                    current = current->m_Next;
                }
          
                //??????????????????????????????????????????????????????
                //Not found.
                return null; //WHAT SHOULD I RETURN HERE????????????????????
               //??????????????????????????????????????????????????????
            }
    If the comparison (as defined by the user) returns true, then I'm returning current->m_Data which is type T. But if the search fails what should I return? I don't know what type T is yet.

    I solved this by returning the entire list node to the user as a pointer. This solves the problem but the following is very confusing:
    Code:
    //Search.
            template<typename U>
            LinkedListNode<T> *search(bool (*compare)(const T other, U custom), U custom)
            {
                //Current node starts from the head.
                LinkedListNode<T> *current = m_Head;
    
                //GO though each node until the end.
                while (current != NULL)
                {   
                    //Do the comparison based on the user's custom function.
                    if ( compare(current->m_Data, custom) )
                        return current;
    
                    //Next node.
                    current = current->m_Next;
                }
    
                //Not found.
                return NULL;
            }
    
    
            //Remove.
            template<typename U>
            bool remove(bool (*compare)(const T other, U custom), U custom)
            {
                LinkedListNode<T> *found = this->search(compare, custom);
    
                if (found != NULL)
                {
                    delete found;
                    return true;
                }
    
                return false;
            }
    Remove() uses the search method to find and remove a node from the list. As you can see it returns a boolean. The thing is that I want to return the actual data T which is stored in found->m_Data. The reason is that if that data is an allocated object I want to give the user that object back to him because the deletion of the node won't handle the deletion of the data too. Also, I can't return the node as I did in the search method because I am deleting it.

    Should I create another wrapper class to hold the data? But this is not good. After returning that wrapper object, who is going to delete it afterward, the user?

    What should I do?

    Should I ask the user through a constructor to provide me with a T value that will be returned in cases such as the above? For example, if the linked list stores int's the user might give me the value -1 in case of a search not found error. If he stores pointers, he might give me NULL.
    Last edited by babaliaris; September 7th, 2019 at 11:35 AM.

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