Question

I’m confused about the following output in this linked list

class ListNode{
    public $next = NULL;
    public $data = NULL;
    public function __construct($data){
        $this->data = $data;
    }

}

class LinkedList{
    private $firstNode = NULL;
    private $lastNode = NULL;


    public function insertFirst($data){
        $link = new ListNode($data);
        $link->next = $this->firstNode;
        $this->firstNode = &$link;
        if($this->lastNode == NULL){
            $this->lastNode = &$link;
        }

    }

    public function readList(){
        while($this->firstNode != NULL){
            echo $this->firstNode->data;
            $this->firstNode = $this->firstNode->next;
        }
    }

    public function assessList(){
        $copy = $this->firstNode;
        echo $copy->data;
        echo $this->firstNode->data;
        $copy->data = 'm';
        echo $copy->data;
        echo $this->firstNode->data;        
    }

}

$linkedList = new LinkedList();
$linkedList->insertFirst('c');
$linkedList->insertFirst('b');
$linkedList->insertFirst('a');
//$linkedList->readList();  //output a b c
$linkedList->assessList();  //outputs a a m m

I would expect the output to be a a m a. I thought $copy is just a copy of the value stored in $this->firstNode.

Isn’t this line of code $copy = $this->firstNode an assignment by value? I would expect the output to be a a m m if it was an assignment by reference $copy = &$this->firstNode but not if it was an assignment by value.

Can someone please clarify?

EDIT (additional example)

public function assessList(){
    $copy = $this->firstNode->data;
    echo $copy. "<br/>";
    echo $this->firstNode->data. "<br/>";
    $copy = 'm';
    echo $copy. "<br/>";
    echo $this->firstNode->data. "<br/>";       
}

Answer 1

This:

$copy = $this->firstNode;

Is not a copy of the object, it’s a copy of the “pointer” to the original object, so when you modify it, you modify the underlying object. You need to use the clone keyword to get a true copy:

$copy = clone $this->firstNode;

From the PHP docs (emphasis mine):

When assigning an already created instance of a class to a new
variable, the new variable will access the same instance as the object
that was assigned.

You can see from this example that your code snippet now outputs:

aama