Missing downcast argument solution in PHP

Question

Imagine a generic library defines a class:

class A_CLASS {
    public function foo(B_CLASS $b) {
        // Use $b as B_CLASS
    }
}
class B_CLASS {
}

And I need to override this one, but with my own child B_CLASS, so I would declare

class A_CHILD_CLASS extends A_CLASS {
    public function foo(B_CHILD_CLASS $b) {
        parent::foo($b);
        // Use $b as B_CHILD_CLASS
    }
}
class B_CHILD_CLASS extends B_CLASS {
}

This is not working on PHP 5.6. In other languages, this will work, and we could also use a generic interface. But in PHP, this is not available and there is no generic interface.

So, how could I implement it in PHP ? This is really a problem in my project.

If i use this source, I get "Declaration of A_CHILD_CLASS::foo() should be compatible with A_CLASS::foo(B_CLASS $b)" — Loenix, Nov 19 '17 at 09:55
For what class ? B_CLASS does not implements required methods. — Loenix, Nov 19 '17 at 13:24
But as class `B_CHILD_CLASS` extends `B_CLASS` it can also be considered as argument of type `B_CLASS`. — u_mulder, Nov 19 '17 at 14:04

score 2 · Answer 1 · answered Nov 19 '17 at 10:14

The reason for this error:

Declaration of A_CHILD_CLASS::foo() should be compatible with A_CLASS::foo(B_CLASS $b)

is that you are trying to override the foo() function but with different arguments. As far as I know this is called overloading which isn't supported by PHP. (while there are some patches to bypass it)

Therefore, the solution should be to modify this function:

public function foo(B_CHILD_CLASS $b)

To:

public function foo(B_CLASS $b)

And to find a different way to achieve what's you're looking for. Consider removing the declaration of the argument's type.

public function foo($b)

Further reading:

Yes, I know but I hoped for a better solution :D – Loenix Nov 20 '17 at 21:16 — Loenix, Nov 20 '17 at 21:16

Missing downcast argument solution in PHP

1 Answers1