Implimenting Doubly Linked List-CodePudding

I have been given the framework to implement a doubly linked list. I am getting stumped on the PushFront() method. The method should add the provided element to the front of the linked list and it should return the address to the new head node. I am confused about how to access the current head of the list so I can assign it to the pNext pointer. The PushFront() method looks like this so far:

Element* Element::PushFront(Element* d){
   Element* newElement = new Element(Data());    // Allocate space for new element
   newElement->ElementData = d->ElementData;    // Assign ElementData to the new  element                
   newElement->pNext = // Head address
   newElement->pPrev = NULL;
return nullptr;
}

Element Class Constructor:

Element::Element(Data d){
   ElementData = d;
   pNext = NULL;
   pPrev = NULL;
}

Data Class:

Data::Data(){
Name = "Unknown";
SN = 0;
Program = "Unknown";
}

Data::Data(string NameStr, unsigned int sNumber, string Prog) :
Name(NameStr), SN(sNumber), Program(Prog) {};

Main:

Element* pList = new Element(Data("Cam", 12345, "Testing1"));   
Element newE(Data("Bob", 335567, "Testing2"));
pList = pList->PushFront(&newE);

My understanding is that you would typically provide the address of the head when calling PushFront(), however since I am not provided that I am unsure of how I can access it.

CodePudding user response：

Using your approach to the list definition the function can look the following way

Element * Element::PushFront( Element *d )
{
    Element *newElement = new Element( d->ElementData );

    newElement->pNext = this;
    newElement->pPrev = this->pPrev;
    this->pPrev = newElement;

    return newElement;
}

Though such a declaration of the function does not make a great sense. The function should be declared at least like

Element * Element::PushFront( const Data &d );

instead of

Element * Element::PushFront( Element *d );

Or even like

Element * Element::PushFront( const char *Name, unsigned int SN, const char *Program );

Pay attention to that it is much better to declare one more data structure that indeed will represent the doubly-linked list and that will contain pointers to the first (head) and the last (tail) objects of the type Element in the list.

CodePudding user response：

I am confused about how to access the current head of the list

As you call the PushFront method on the pList instance, the this reference will be pList, which is assumed to be the head of the list. The main program is responsible for maintaining a reference to the head.

Note that sometimes another class is created for representing the list, which then has a head member. However, in this case, you seem not to have such a class, and the main program must manage the head itself, i.e. pList.

The PushFront method must then "help" the main program in doing this, and return the reference to the new head. So don't return nullptr.

I don't really understand why in PushFront you create a new Element, since you already get an element passed as argument. I would think the purpose is to add that element to the list, and not a copy of it.

So:

Element* Element::PushFront(Element* d){
   d->pNext = this;
   d->pPrev = NULL;
   return d;
}

In the main program I would suggest creating both Element instances with new, so that their memory management is the same:

Element* pList = new Element(Data("Cam", 12345, "Testing1"));   
Element* newE = new Element(Data("Bob", 335567, "Testing2"));
pList = pList->PushFront(newE);