Static in php, why it use one cache for both class

Question

I create parent class for my models, it look like this

class Model {

   protected static $cache = [];

   public static function load($id)
   {
       return static::$cache[$id];
   }

   public static function findById($id)
   {
       $model = static::find($id);

       static::$cache[$id] = model;

       return model;
   }

   ...
}

And i have two child class, examples

class A extend Model {}
class B extend Model {}

then, i write

A::findById(1);
B::findById(1);

$a = A::load(1);
$b = B::load(1);

I think type of class $b is class B, but not, it is class A, because function "load" use static cache and load result from function A::findById(1). Why?

Possible duplicate of Inheritance of static members in PHP

tyteen4a03
– tyteen4a03

2017-05-05 11:10:24 +00:00
Commented May 5, 2017 at 11:10 — tyteen4a03
– tyteen4a03, Commented May 5, 2017 at 11:10

axiac · Accepted Answer · 2017-05-05 11:21:01Z

3

By accessing static class properties as static::$cache you tell the interpreter to use the $cache property of the class used to call the method and not the one of the class where the method is defined.

In plain English, when you call A::findById() you expect that findById() uses the property $cache of A and not the one of Model. The same for A::load() and similar for B::findById() and B::load().

The question now is: do the A and B classes declare a static property $cache on their own? Because if they do not declare this property, the static:: in front of $cache in the function A::findById() cannot find A::$cache and uses Model::$cache instead. The same for B::findById().

The easy solution to your problem is to declare the protected static $cache property in all classes that extend Model.

The correct solution is to not use static properties and methods. Static properties and methods are not OOP. They are procedural programming (and thinking) disguised as OOP code. Code that is difficult to understand and test.

answered May 5, 2017 at 11:21

axiac

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1 Comment

ConorHolt Over a year ago

Thank you, i understood!

Peter · Accepted Answer · 2017-05-05 11:36:42Z

3

static properties are inherited, if you want to have them separately, redefine them

class A extend Model { static $cache; }
class B extend Model { static $cache; }

also what you are trying to do is bad OOP practice.

this is how you can do it proper way

class CacheFactory
{
    static public function createModelCache($model)
    {
        static $instances = [];
        if (!isset($instances[$model])) {
            $instances[$model] = new Cache($model);
        }
        return $instances[$model];
    }
}

class Cache
{
    protected $name;

    protected $data = [];

    public function __construct($name)
    {
        $this->name = $name;
    }

    public function get($key)
    {
        return isset($this->data[$key]) ? $this->data[$key] : null;
    }

    public function set($key, $value)
    {
        $this->data[$key] = $value;
    }
}

abstract class Model
{
    abstract protected $name;

    /**
     * @type Cache
     */
    protected $cache;

    public function __construct()
    {
        $this->cache = CacheFactory::createModelCache($this->name);
    }

    public function findById($id)
    {
        if (($data = $this->cache->get($id)) !== null) {
            return $data;
        }
        $data = ...;
        $this->cache->put($id, $data);
    }
}

class A extends Model
{
    protected $name = "modelA";
}


class B extends Model
{
    protected $name = "modelB";
}

this just an example and you can change few bits like

you can move CacheFactory::createModelCache to Cache
you can use get_class instead using abstract protected $name
you can just create static array instead Cache class

but thing is - you just don't use static everywhere, this is not how you code in OOP

from my example you can see the only thing what is static is an array with Cache objects and factory method. this is only thing you need to store globally.

edited May 5, 2017 at 11:36

answered May 5, 2017 at 11:15

Peter

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3 Comments

ConorHolt Over a year ago

Thank you, i already create like in your answers, but not understand, why php not create protected static $cache = []; in each classes with empty value

axiac Over a year ago

@ConorHolt why should PHP create $cache in the derived classes? Static properties are not class members, they are global variables (possible with limited visibility) with funky names.

Peter Over a year ago

@ConorHolt static properties are inherited

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