PHP attributes : is it possible to use static function for a attribute parameter value?

Question

I have a project that use attributes :

    #[OperationQueryParam('id', IntegerOperation::class, requirements: new Numeric(min: 1, max: 2_147_483_647))]

Everything works fine but there are multiple attribute that have the same requirement argument new Numeric(min: 1, max: 2_147_483_647) so i wanted to create a static function in my numeric class :

    final public static function createPostiveInt(): self {
       return new self(min:1, max:2_147_483_647);
    }

then call the static function instead of the direct instanciation of the Numeric object to avoid repeating the constructor parameters.

    #[OperationQueryParam('id', IntegerOperation::class, requirements: Numeric::createPostiveInt())]

but i have this error : Compile Error: Constant expression contains invalid operations

Am i missing something ? Why does a direct instanciation works but not the static function ?

EDIT :

using define(POSITIVE_INT,Numeric::createPostiveInt()); works but it seems dirty.

It looks like calling a function is not allowed. new is a keyword. That is different. — Markus Zeller
– Markus Zeller, Commented Jun 17, 2024 at 11:15

Alex Howansky · Accepted Answer · 2024-06-17 13:34:46Z

2

From https://www.php.net/manual/en/functions.arguments.php:

Default parameter values may be scalar values, arrays, the special type null, and as of PHP 8.1.0, objects using the new ClassName() syntax.

In other words, you can't use a run-time construct like a function call as a default value for a parameter. You could extend the Numeric class here, to build-in your common defaults:

class MyNumeric extends Numeric
{
    public function __construct(int $min = 1, int $max = 2_147_483_647) {
        parent::__construct(min: $min, max: $max);
    }
}

Or even just remove the args completely:

class MyNumeric extends Numeric
{
    public function __construct() {
        parent::__construct(min: 1, max: 2_147_483_647);
    }
}

And then your definition becomes:

#[OperationQueryParam(
    'id',
    IntegerOperation::class,
    requirements: new MyNumeric()
)]

answered Jun 17, 2024 at 13:34

Alex Howansky

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1 Comment

Chris Haas Over a year ago

I like this one better than mine, much simpler and should work regardless of how the project uses reflection.

Chris Haas · Accepted Answer · 2024-06-17 13:28:27Z

Depending on how the project uses reflection to get attribute values, it might be possible for you to subclass OperationQueryParam and include your constant values via parent::__construct(). To do this, the reflecting code must not check for simple equality on class name, but check for it being an instance of it. There's a couple of ways to do this, but one option is to use the second parameter to getAttributes() which is ReflectionAttribute::IS_INSTANCEOF

Below is a quick PoC. We have a base attribute which requires a $name parameter, and then our custom attribute which doesn't, and instead includes that in the parent constructor.

#[Attribute]
class BaseAttribute
{
    public function __construct(public readonly string $name)
    {
    }
}

#[Attribute]
class ConstantlyNameAttribute extends BaseAttribute
{
    public function __construct()
    {
        parent::__construct('Chris');
    }
}

#[ConstantlyNameAttribute]
class Thing{}

$reflectionClass = new ReflectionClass(Thing::class);

// Doesn't work
var_dump(getName($reflectionClass->getAttributes(BaseAttribute::class)));

// Works
var_dump(getName($reflectionClass->getAttributes(BaseAttribute::class, ReflectionAttribute::IS_INSTANCEOF)));

// Helper function
function getName(array $reflectionAttributes) : ?string
{
    if(!count($reflectionAttributes)){
        return null;
    }
    
    $attribute = array_pop($reflectionAttributes);
    return $attribute->newInstance()->name;
}

Demo: https://3v4l.org/SWTa6#v8.3.8

If the project is a third-party and you can't modify things, it might be worth it kicking this request upstream to see if they are willing to modify it.

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