Question

What is the difference between a doubly and singly linked list and what would this code look like if it were a doubly linked list?
template <typename Object>
class List
{
private:   
    struct Node
    {
        Object data;
        Node   *prev;
        Node   *next;

        Node( const Object & d = Object{ }, Node * p = nullptr, Node * n = nullptr )
          : data{ d }, prev{ p }, next{ n } { }
       
        Node( Object && d, Node * p = nullptr, Node * n = nullptr )
          : data{ std::move( d ) }, prev{ p }, next{ n } { }
    };

Answer 1

Singly linked list	Doubly linked list
A singly linked list is a linked list where the node contains the data and a pointer to the next node in the list	A doubly linked list is type of linked list where the node contains the data and a pointer to the next node as well as the previous node in the list
It allows traversal only in one direction means its head pointer till its ends with NULL	It allows a traversal in both direction means its can go from its head to forward(through next pointer) and backward (through previous pointer)
It uses less memory per node (single pointer)	It uses more memory per node(two pointers)

Structure of single link list is as follow:

It can be seen above, it can go in only forward direction through next pointer.

Structure of doubly link list

It can be seen above, it can go in forward and backward direction.

The given code has following information:

1. A class name List.

2. List contains a structure Node which is private.

3. Structure Node contains a Data of Object type and two pointers prev and next, which are pointing to node itself. Basically, it is declaration of doubly link list.

4.Two constructors are given, both have given default parameters.

5. 1st constructor has initialized the structure members with parameters given in constructor call, if parameter is not given, then it initialize with default parameter given in declaration i,e a new object is created and assigned to data member, prev and next pointer is assigned to NULL.

6. 2nd constructor also the same as 1st constructor, but it has 1st parameter has not default values, so it is necessary to give 1st parameter value. Here it copies the data values of 'd' to data member of structure.