Self Referential Structures

Self Referential structures are those structures that have one or more pointers which point to the same type of structure, as their member.

In other words, structures pointing to the same type of structures are self-referential in nature

Example:

CPP

struct node {
    int data1;
    char data2;
    struct node* link;
};

int main()
{
    struct node ob;
    return 0;
}

Python3

class node:
    def __init__(self):
        self.data1=0
        self.data2=''
        self.link=None

if __name__ == '__main__':
    ob=node()

Javascript

// Define the 'node' object
class node {
    constructor(data1, data2) {
        this.data1 = data1;
        this.data2 = data2;
        this.link = null;
    }
}
// Create an instance of the 'Node' object
let ob = new Node();

In the above example ‘link’ is a pointer to a structure of type ‘node’. Hence, the structure ‘node’ is a self-referential structure with ‘link’ as the referencing pointer. 
An important point to consider is that the pointer should be initialized properly before accessing, as by default it contains garbage value.

Types of Self Referential Structures

1. Self Referential Structure with Single Link
2. Self Referential Structure with Multiple Links

Self Referential Structure with Single Link: These structures can have only one self-pointer as their member. The following example will show us how to connect the objects of a self-referential structure with the single link and access the corresponding data members. The connection formed is shown in the following figure. 

Implementation:

C++

#include <stdio.h>

struct node {
    int data1;
    char data2;
    struct node* link;
};

int main()
{
    struct node ob1; // Node1

    // Initialization
    ob1.link = NULL;
    ob1.data1 = 10;
    ob1.data2 = 20;

    struct node ob2; // Node2

    // Initialization
    ob2.link = NULL;
    ob2.data1 = 30;
    ob2.data2 = 40;

    // Linking ob1 and ob2
    ob1.link = &ob2;

    // Accessing data members of  ob2 using ob1
    printf("%d", ob1.link->data1);
    printf("\n%d", ob1.link->data2);
    return 0;
}

Java

// java implementation of above approach
public class Main {
    static class Node {
        int data1;
        int data2;
        Node link;
    }

    public static void main(String[] args)
    {
        Node ob1 = new Node(); // Node1

        // Initialization
        ob1.link = null;
        ob1.data1 = 10;
        ob1.data2 = 20;

        Node ob2 = new Node(); // Node2

        // Initialization
        ob2.link = null;
        ob2.data1 = 30;
        ob2.data2 = 40;

        // Linking ob1 and ob2
        ob1.link = ob2;

        // Accessing data members of  ob2 using ob1
        System.out.println(ob1.link.data1);
        System.out.println(ob1.link.data2);
    }
}
// This code is implemented by Chetan Bargal

Python3

class node:
    def __init__(self):
        self.data1=0
        self.data2=0
        self.link=None

if __name__ == '__main__':
    ob1=node() # Node1

    # Initialization
    ob1.link = None
    ob1.data1 = 10
    ob1.data2 = 20

    ob2=node() # Node2

    # Initialization
    ob2.link = None
    ob2.data1 = 30
    ob2.data2 = 40

    # Linking ob1 and ob2
    ob1.link = ob2

    # Accessing data members of  ob2 using ob1
    print(ob1.link.data1)
    print(ob1.link.data2)

C#

using System;

public class MainClass {
    public class Node {
        public int data1;
        public int data2;
        public Node link;
    }

    public static void Main(string[] args)
    {
        Node ob1 = new Node(); // Node1

        // Initialization
        ob1.link = null;
        ob1.data1 = 10;
        ob1.data2 = 20;

        Node ob2 = new Node(); // Node2

        // Initialization
        ob2.link = null;
        ob2.data1 = 30;
        ob2.data2 = 40;

        // Linking ob1 and ob2
        ob1.link = ob2;

        // Accessing data members of  ob2 using ob1
        Console.WriteLine(ob1.link.data1);
        Console.WriteLine(ob1.link.data2);
    }
}

Javascript

class node {
  constructor() {
    this.data1 = 0;
    this.data2 = 0;
    this.link = null;
  }
}

// Create node1
let ob1 = new node();

// Initialization
ob1.link = null;
ob1.data1 = 10;
ob1.data2 = 20;

// Create node2
let ob2 = new node();

// Initialization
ob2.link = null;
ob2.data1 = 30;
ob2.data2 = 40;

// Linking ob1 and ob2
ob1.link = ob2;

// Accessing data members of ob2 using ob1
console.log(ob1.link.data1);
console.log(ob1.link.data2);

Output

30
40

Self Referential Structure with Multiple Links: Self referential structures with multiple links can have more than one self-pointers. Many complicated data structures can be easily constructed using these structures. Such structures can easily connect to more than one nodes at a time. The following example shows one such structure with more than one links.
The connections made in the above example can be understood using the following figure. 

Implementation:

CPP

#include <stdio.h>

struct node {
    int data;
    struct node* prev_link;
    struct node* next_link;
};

int main()
{
    struct node ob1; // Node1

    // Initialization
    ob1.prev_link = NULL;
    ob1.next_link = NULL;
    ob1.data = 10;

    struct node ob2; // Node2

    // Initialization
    ob2.prev_link = NULL;
    ob2.next_link = NULL;
    ob2.data = 20;

    struct node ob3; // Node3

    // Initialization
    ob3.prev_link = NULL;
    ob3.next_link = NULL;
    ob3.data = 30;

    // Forward links
    ob1.next_link = &ob2;
    ob2.next_link = &ob3;

    // Backward links
    ob2.prev_link = &ob1;
    ob3.prev_link = &ob2;

    // Accessing  data of ob1, ob2 and ob3 by ob1
    printf("%d\t", ob1.data);
    printf("%d\t", ob1.next_link->data);
    printf("%d\n", ob1.next_link->next_link->data);

    // Accessing data of ob1, ob2 and ob3 by ob2
    printf("%d\t", ob2.prev_link->data);
    printf("%d\t", ob2.data);
    printf("%d\n", ob2.next_link->data);

    // Accessing data of ob1, ob2 and ob3 by ob3
    printf("%d\t", ob3.prev_link->prev_link->data);
    printf("%d\t", ob3.prev_link->data);
    printf("%d", ob3.data);
    return 0;
}

Python3

class node:
    def __init__(self):
        self.data=0
        self.prev_link=None
        self.next_link=None

if __name__ == '__main__':
    ob1=node() #Node1

    # Initialization
    ob1.prev_link = None
    ob1.next_link = None
    ob1.data = 10

    ob2=node() #Node2

    # Initialization
    ob2.prev_link = None
    ob2.next_link = None
    ob2.data = 20

    ob3=node() # Node3

    # Initialization
    ob3.prev_link = None
    ob3.next_link = None
    ob3.data = 30

    # Forward links
    ob1.next_link = ob2
    ob2.next_link = ob3

    # Backward links
    ob2.prev_link = ob1
    ob3.prev_link = ob2

    # Accessing  data of ob1, ob2 and ob3 by ob1
    print(ob1.data,end='\t')
    print(ob1.next_link.data,end='\t')
    print(ob1.next_link.next_link.data)

    # Accessing data of ob1, ob2 and ob3 by ob2
    print(ob2.prev_link.data,end='\t')
    print(ob2.data,end='\t')
    print(ob2.next_link.data)

    # Accessing data of ob1, ob2 and ob3 by ob3
    print(ob3.prev_link.prev_link.data,end='\t')
    print(ob3.prev_link.data,end='\t')
    print(ob3.data)

Output

10    20    30
10    20    30
10    20    30

In the above example we can see that ‘ob1’, ‘ob2’ and ‘ob3’ are three objects of the self referential structure ‘node’. And they are connected using their links in such a way that any of them can easily access each other’s data. This is the beauty of the self referential structures. The connections can be manipulated according to the requirements of the programmer.

Applications: Self-referential structures are very useful in creation of other complex data structures like: 

• Linked Lists
• Stacks
• Queues
• Trees
• Graphs etc.