WeakReference type in PHP8.0

WeakReference type in PHP8.0

With the official release of PHP8.0, we have ushered in many new features and improvements, one of the important improvements is the addition of the WeakReference type. For some PHP developers, the WeakReference type may still be an unfamiliar concept. This article will introduce the usage and advantages of this new type.

What is the WeakReference type?

First of all, we need to know what a reference is. In PHP, we can assign or pass variables in the form of $var or &$var, which is a variable reference. Reference is different from copying. It points to the same piece of data in memory, that is, the same variable can have multiple references.

For example, the $bar variable in the following code points to the $foo variable by reference:

$foo = 'Hello, world!';
$bar = &$foo;

If you modify the value of $bar at this time, you will find that the value of $foo also changes. :

$bar = 'Goodbye, world!';
echo $foo; // 输出Goodbye, world!

In PHP8.0, a new type - WeakReference is introduced. The difference between WeakReference and ordinary reference is that it is a weak reference and does not prevent the object from being recycled by the garbage collector. That is to say, when using WeakReference, even if the object has been recycled, WeakReference will not throw an exception, but return null or an empty object.

How to use WeakReference type?

Weak references are usually used in scenarios such as caching and event management. Below we use an example to illustrate its use.

First, we create a User class to save the user's id and name information:

class User
{
    public $id;
    public $name;
    
    public function __construct($id, $name)
    {
        $this->id = $id;
        $this->name = $name;
    }
}

Then, we create a UserCache class to cache user information. Among them, we use the SplObjectStorage class to save the WeakReference of the User object:

class UserCache
{
    private SplObjectStorage $users;
    
    public function __construct()
    {
        $this->users = new SplObjectStorage();
    }
    
    public function addUser(User $user)
    {
        $this->users->attach(new WeakReference($user));
    }
    
    public function getUserById($id)
    {
        foreach ($this->users as $user) {
            if ($user->get()->id === $id) {
                return $user->get();
            }
        }
        
        return null;
    }
}

In the constructor, we create a SplObjectStorage object through new SplObjectStorage() to save the WeakReference of the User object. In the addUser method, we convert the User object into a WeakReference object through $newUser = new WeakReference($user) and add it to SplObjectStorage. In the getUserById method, we traverse all WeakReference objects in SplObjectStorage and use the get() method to obtain the corresponding User object to determine whether it is equal to the target id.

Finally, let’s test the usage of the UserCache class:

$cache = new UserCache();

$user1 = new User(1, 'Alice');
$user2 = new User(2, 'Bob');

$cache->addUser($user1);
$cache->addUser($user2);

var_dump($cache->getUserById(1));
var_dump($cache->getUserById(2));

unset($user2);

var_dump($cache->getUserById(1));
var_dump($cache->getUserById(2));

Run the above code and you will find that after the $user2 object is unset, the getUserById method can still work normally without throwing an exception. , instead returns a null value.

Summary

In PHP8.0, the WeakReference type was added, which is a weak reference that will not prevent the object from being recycled by the garbage collector. By using the WeakReference type, we can better manage the life cycle of objects and improve the reliability and robustness of the code. If you want to learn more about the WeakReference type, you can check out the detailed description in the official PHP documentation.

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