why after the first push head is still null? - c++

Question

I am trying to create singly-linked list. After the first push, head is still null. Why is the head not updated after the first push?
using namespace std;
typedef struct node {
int data; // will store information
node *next; // the reference to the next node
};
void push(node*,int);
void print(node*);
int main()
{
node* head = NULL; //empty linked list
push(head, 2);
if (head == NULL) {
cout << "vrvrvr";
}
push(head, 3);
push(head, 5);
push(head, 2);
//print(head);
getchar();
return 0;
}
void push(node* x, int y){
node *temp = new node();
if (x == NULL) { // check linked list is empty
temp->next = x;
temp->data = y;
x = temp;
}
else {
node *temp1 = new node();
temp1 = x;
while (temp1->next != NULL) { // go to the last node
temp1 = temp1->next;
}
temp1->next = temp;
temp->data = y;
temp->next = NULL;
delete temp1; // 'temp' node will be the last node
}
}
void print(node* x){
node *temp1 = new node();
temp1 = x;
while (temp1->next != NULL) {
cout << temp1->data << endl;
temp1 = temp1->next;
}
}

Answer 1

The main problem with push is that the changes made to x in the function are local to the function. It does not change the value of head in main.
You can fix that by changing the argument type to node*&.
void push(node*& x, int y) {
...
}
The other problems I see are in the block:
else {
node *temp1 = new node();
temp1 = x;
// Problem 1:
// After this, the memory returned by the previous line is lost.
// It is a memory leak.
while (temp1->next != NULL) { // go to the last node
temp1 = temp1->next;
}
temp1->next = temp;
temp->data = y;
temp->next = NULL;
delete temp1; // 'temp' node will be the last node
// Problem 2:
// You are deleting a node from the linked list.
// The linked list now has a dangling pointer.
}
You can correct those problems by using:
node *temp1 = x;
while (temp1->next != NULL) { // go to the last node
temp1 = temp1->next;
}
temp1->next = temp;
temp->data = y;
temp->next = NULL;
}
Suggested improvements
Remove typedef from definition of node. It is a dangling typedef in your posted code. Also, you can use node without a typedef in C++.
struct node {
int data;
node *next;
};
Add a constructor to node.
struct node {
node(int d) : data(d), next(nullptr) {}
int data;
node *next;
};
That will simplify your code in push.
void push(node*& x, int y){
node *temp = new node(y);
if (x == NULL) { // check linked list is empty
x = temp;
}
else {
node *temp1 = x;
while (temp1->next != NULL) { // go to the last node
temp1 = temp1->next;
}
temp1->next = temp;
}
}