linked-list: why extend a single-node list by adding the new node to head->next, but not head=head->next and then adding to head

Question

A simple linked-list code:

#include<iostream>

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

int main() {
  node *head, *head1;
  head = new node;
  head1 = head; //we'll use head1 to print the entire list at the end

  head->data =  2;

the following block of code, commented out, doesn't add new node to the list

  /* head = head->next;
  head = new node;
  head -> data = 20;*/

but the (seemingly similar) below method works, and we print the entire list in the end

  head->next = new node; 
  head = head->next; 
  head->data = 20;
  
  while (head1 != nullptr) {
    std::cout<<head1->data<<std::endl;
    head1 = head1->next;
  }
}

Since both head and head->next are of type node*, how to understand that head->next = new node; works, but not head = head->next; head=new node;

Answer 1

The code shown works.

If you try head = head->next; head=new node; it will fail because head->next at this point is null, so now head is null. You can assign a new node to it, but that does not affect the next member of the object head was previously pointing to.

Here's how things start out:

+----------+
| *head    |
+----------+
     |
     |
     V
+----------+               +--------+
| node     |<--------------| *head1 |
| -------- |               +--------+
| data = 2 |       ??????
| *next ---------->?????? 
+----------+       ??????

Now, if we try what you're asking about, we end up with:

+----------+               +--------+
| node     |<--------------| *head1 |
| -------- |               +--------+
| data = 2 |       ??????
| *next ---------->?????? 
+----------+       ??????

+----------+
| *head    |
+----------+
     |
     |
     V
+-----------+             
| node      |
| --------- |           
| data = 20 |      ??????
| *next ---------->?????? 
+-----------+      ??????

Note that head1 still points to the original node containing the value 2, but there is nothing linking that node to the node containing the value 20.