New to classes and Objects in c++

I'm new to classes and Objects in C++..I cannot understand why the linked list is not being created. It just prompts the first value then crashes. I cannot figure out where the problem is and already wasted too many hours on this. Finally decided to get some help. Thank You for your time.

#include <iostream>
using namespace std;

class Node
private:
int data;
Node* next;
Node* previous;

public:

Node(int value)
data = value;
next = NULL;
previous = NULL;


void setValue(int value)

data = value;


int getValue()

return data;


Node* getNext()

return next;


void setNext(Node* address)

next = address;


Node* getPrevious()
return previous;


void setPrevious(Node* address)
previous = address;


;


class LinkedList
private:
Node* head;
Node* tail;

public:

LinkedList()
Node* head = NULL;
Node* tail = NULL;




void createLinklist()

int n;
cout << "Enter the number of nodes = ";
cin >> n;



for(int i=0;i<n;i++)

int value;
cout << "Enter the value at " << i <<"=";
cin >> value;

Node* node = new Node(value);

if(head == NULL)

head = node;
tail = node;

else
insertAtEnd(node,tail);







void insertAtEnd(Node* newNode,Node* lastNode)

lastNode->setNext(newNode);
newNode->setPrevious(lastNode);
newNode->setNext(NULL);
tail = newNode;







void display()

Node* start = head;

while(start!=NULL)

cout << "Address=" << start << endl;
cout << "value = " << start->getValue() << endl;
cout << "Next = " << start->getNext() << endl;
start = start->getNext();






;


int main()

LinkedList newLink;
newLink.createLinklist();
newLink.display();




3 Answers
3

Problem

In

LinkedList()
Node* head = NULL;
Node* tail = NULL;



Node* head = NULL; tells the compiler to make a new Automatic variable named head that is a pointer to a Node and set this new variable to NULL. This new head shadows LinkedList::head, replacing it for the rest of the constructor. The result is head a variable that only exists within the body of the constructor, gets the initialization (assignment really) intended for LinkedList::head.

Node* head = NULL;

head

Node

NULL

head

LinkedList::head

head

LinkedList::head

That means when you get to

if(head == NULL)


in createLinklist, LinkedList::head probably isn't NULL and instead points off into the wild blue yonder, so the program executes

createLinklist

LinkedList::head

NULL

insertAtEnd(node,tail);


and LinkedList::tail suffered the same fate as LinkedList::head and likely points somewhere you cannot safely write. The program probably crashes at this point, but it could overwrite something else important and cause the program to crash later, hiding the true location of the bug.

LinkedList::tail

LinkedList::head

Solution

LinkedList()
head = NULL;
tail = NULL;



assigns NULL toheadandtail`. A more idiomatic approach is to use the Member Initializer List

NULL to

and

LinkedList(): head(NULL), tail(NULL)

// does nothing.



Sidenote

a good compiler with the warning level turned up will warn you that

Node* head = NULL;


doesn't do anything useful. Never ignore compiler warnings. A compiler warning means that while your program may be syntactically correct, it probably doesn't do what you want it to do. Warnings are your first line of defense against logic errors. Always try to understand and resolve what the compiler is telling you. It may save you hours of debugging later.

Issue is your constructor:

class LinkedList{
private:
Node* head;
Node* tail;

public:

LinkedList()
Node* head = NULL;
Node* tail = NULL;



In the constructor, you are declaring two LOCAL variables to NULL rather than the class ones. This means that the class ones point to wherever, but in all likelihood are not NULL.

Suggestions: Learn C++11 or later.

Basically C++11 allows you to do this:

class LinkedList{
private:
Node* head = nullptr;
Node* tail = nullptr;


and you won't need a constructor in your case, though it is good practice to add:

LinkedList() = default;


if you want to use the default one.

if(head == NULL)

unique_ptr

next

prev

unique_pr

next

prev

shared_ptr

weak_ptr

std::vector

change your code to

LinkedList()
head = NULL;
tail = NULL;



because you have already defined head and tail.

head

tail

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