Implementing Iterator pattern of a single Linked List

4.1

STL is one of the pillars of C++. It makes life lot easier, especially when your focus is on problem solving and you don’t want to spend time in implementing something that is already available which guarantees a robust solution. One of the key aspects of Software Engineering is to avoid reinventing the wheel. Reusability is always preferred.

While relying on library functions directly impacts our efficiency, without having a proper understanding of how it works sometimes loses meaning of the engineering efficiency we keep on talking. A wrongly chosen data structure may come back sometime in future to haunt us. The solution is simple. Use library methods, but know how does it handles operations under the hood.

Enough said! Today we will look on how we can implement our own Iterator pattern of a single Linked List. So, here is how an STL implementation of Linked List looks like:

#include <iostream>
#include <vector>
using namespace std;

int main()
{
    // creating  a list
    vector<int> list;

    // elements to be added at the end.
    // in the above created list.
    list.push_back(1);
    list.push_back(2);
    list.push_back(3);

    // elements of list are retrieved through iterator.
    for (vector<int>::iterator it = list.begin();
                                it != list.end(); ++it)
        cout << *it << " ";

    return 0;
}

Output

1 2 3 

One of the beauty of cin and cout is that they don’t demand format specifiers to work with the type of data. This combined with templates make the code much cleaner and readable. Although I prefer naming method in C++ start with caps, this implementation follows STL rules to mimic exact set of method calls, viz push_back, begin, end.

Here is our own implementation of LinkedList and its Iterator pattern:

// C++ program to implement Custom Linked List and
// iterator pattern.
#include <iostream>
using namespace std;

// Custom class to handle Linked List operations
// Operations like push_back, push_front, pop_back,
// pop_front, erase, size can be added here
template <typename T>
class LinkedList
{
    // Forward declaration
    class Node;

public:
    LinkedList<T>() noexcept
    {
        // caution: static members can't be
        // initialized by initializer list
        m_spRoot = nullptr;
    }

    // Forward declaration must be done
    // in the same access scope
    class Iterator;

    // Root of LinkedList wrapped in Iterator type
    Iterator begin()
    {
        return Iterator(m_spRoot);
    }

    // End of LInkedList wrapped in Iterator type
    Iterator end()
    {
        return Iterator(nullptr);
    }

    // Adds data to the end of list
    void push_back(T data);

    void Traverse();

    // Iterator class can be used to
    // sequentially access nodes of linked list
    class Iterator
    {
    public:
    Iterator() noexcept :
        m_pCurrentNode (m_spRoot) { }

    Iterator(const Node* pNode) noexcept :
        m_pCurrentNode (pNode) { }

        Iterator& operator=(Node* pNode)
        {
            this->m_pCurrentNode = pNode;
            return *this;
        }

        // Prefix ++ overload
        Iterator& operator++()
        {
            if (m_pCurrentNode)
                m_pCurrentNode = m_pCurrentNode->pNext;
            return *this;
        }

        // Postfix ++ overload
        Iterator operator++(int)
        {
            Iterator iterator = *this;
            ++*this;
            return iterator;
        }

        bool operator!=(const Iterator& iterator)
        {
            return m_pCurrentNode != iterator.m_pCurrentNode;
        }

        int operator*()
        {
            return m_pCurrentNode->data;
        }

    private:
        const Node* m_pCurrentNode;
    };

private:

    class Node
    {
        T data;
        Node* pNext;

        // LinkedList class methods need
        // to access Node information
        friend class LinkedList;
    };

    // Create a new Node
    Node* GetNode(T data)
    {
        Node* pNewNode = new Node;
        pNewNode->data = data;
        pNewNode->pNext = nullptr;

        return pNewNode;
    }

    // Return by reference so that it can be used in
    // left hand side of the assignment expression
    Node*& GetRootNode()
    {
        return m_spRoot;
    }

    static Node* m_spRoot;
};

template <typename T>
/*static*/ typename LinkedList<T>::Node* LinkedList<T>::m_spRoot = nullptr;

template <typename T>
void LinkedList<T>::push_back(T data)
{
    Node* pTemp = GetNode(data);
    if (!GetRootNode())
    {
        GetRootNode() = pTemp;
    }
    else
    {
        Node* pCrawler = GetRootNode();
        while (pCrawler->pNext)
        {
            pCrawler = pCrawler->pNext;
        }

        pCrawler->pNext = pTemp;
    }
}

template <typename T>
void LinkedList<T>::Traverse()
{
    Node* pCrawler = GetRootNode();

    while (pCrawler)
    {
        cout << pCrawler->data << " ";
        pCrawler = pCrawler->pNext;
    }

    cout << endl;
}

//Driver program
int main()
{
    LinkedList<int> list;

    // Add few items to the end of LinkedList
    list.push_back(1);
    list.push_back(2);
    list.push_back(3);

    cout << "Traversing LinkedList through method" << endl;
    list.Traverse();

    cout << "Traversing LinkedList through Iterator" << endl;
    for ( LinkedList<int>::Iterator iterator = list.begin();
            iterator != list.end(); iterator++)
    {
        cout << *iterator << " ";
    }

    cout << endl;

    return 0;
}

Output:

Traversing LinkedList through method
1 2 3 
Traversing LinkedList through Iterator
1 2 3 

Exercise:
The above implementation works well when we have one data. Extend this code to work for set of data wrapped in a class.

This article is contributed by Aashish Barnwal. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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