当我打印单链接列表时，为什么会有一个虚拟节点？

Question

My code prints an extra node(garbage value) why? Is there is any issue with my code? just let me how to fix it.

void push(node **head_ref,int value)  //function to insert a new node on front of the list
{
    node *new_node=(node*)malloc(sizeof(node));
    new_node->data=value;
    new_node->next=*head_ref;
    *head_ref=new_node;
}

void printll(node *head)    //function to print the list
{
    node *temp = head;
    while(temp!=NULL)
    {
        printf("%d ",temp->data);
        temp=temp->next;
    }
}

Actual Output :
45 88 24 34 77 0

Expected Output :
45 88 24 34 77

---

Full code:

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cassert>

using namespace std;

struct node
{
    int data;
    node *next;
};

void push(node **head_ref,int value)
{
    node *new_node=(node*)malloc(sizeof(node));
    new_node->data=value;
    new_node->next=*head_ref;
    *head_ref=new_node;
}

void printll(node *head)
{
    node *temp = head;
    while(temp!=NULL)
    {
        printf("%d ",temp->data);
        temp=temp->next;
    }

}

int main()
{
    node *head= (node*)malloc(sizeof(node));
    push(&head,77);
    push(&head,34);
    push(&head,24);
    push(&head,88);
    push(&head,45);
    printll(head);
    printf("\n");
    return 0;
}

Answer 1

When you allocate memory using malloc the memory is not initialized in any way, it's content is indeterminate . That means that when you allocate the first node (which is a dummy extra node that's not needed) it's next pointer is not null, and dereferencing this indeterminate pointer leads to undefined behavior .

The simplest solution? Considering your code is closer to C than C++, don't allocate memory initially at all, instead just create a pointer and initialize it to NULL :

node *head = NULL;

---

One proper way to do it in C++ is to not use malloc at all, instead use the C++ operator new and add a constructor to the node structure which initializes it:

struct node
{
    node() : data(0), next(nullptr) {}
    node(int d, node* n) : data(d), next(n) {}

    int data;
    node* next;
};

void push(node** head_ref, int value)
{
    *head_ref = new node(value, *head_ref);
}

...

int main()
{
    node* head = nullptr;
    ...
}

Now you can create a new node and it will have an initial value of 0 , and its next pointer will be a null-pointer. You can also, as shown above, create and initialize a new node with specific value and next .

_{[Substitute nullptr for 0 if your compiler doesn't support the C++11 nullptr value]}

Answer 2

Instead of this definition

node *head= (node*)malloc(sizeof(node));

you should write simply

node *head = NULL;

or

node *head = nullptr; // C++

Otherwise your program has undefined behaviour because the allocated node for the head was not initialized.

Also if it is a C++ program you should use operator new instead of C function malloc . For example function push will look like

void push( node * &head_ref, int value )
{
    head_ref = new node { value, head_ref };
}

and called like

push( head, 77 );

Take into account that you have also to write a function that will free the all allocated memory for the list.

Answer 3

You have a dummy node (head itself) in your design. So the print function needs to skip that dummy node.