Covariance (OOP) in PHP - how does it work

Question

class Param() {
}
class Subparam extends Param {
}

class Base {

  function mymethod(Param a) {
  }

}

class Sub extends Base {

 function mymethod(Subparam a) {
 }

}

In PHP, this leads to a warning: Declaration should be compatible with Base->mymethod(a : \Param)

What can I do to prevent that, other than using annotations only?

za-ek · Answer 1 · 2017-07-06T14:12:29.917

-1

You can utilize interface:

interface Test {
}
class Param implements Test  {
}
class Subparam extends Param implements Test {
}

class Base {

  function mymethod(Test $a) {
  }

}

class Sub extends Base {

 function mymethod(Test $a) {
 }

}

edited Jul 06 '17 at 14:12

answered Jul 06 '17 at 13:22

za-ek

467
5
11

How does this specify that `Sub::mymethod()` should only be called with a `Param` object? – Barmar Jul 06 '17 at 13:27
There was an error. I changed it :) – za-ek Jul 06 '17 at 14:13
I still don't see how this makes the method covariant. – Barmar Jul 06 '17 at 16:38
1

Since `Param` implements `Test`, this will allow `$sub->mymethod(new Param)`, which isn't supposed to be allowed. – Barmar Jul 06 '17 at 16:40
This is not covariance. – The Onin Feb 19 '18 at 19:07

score -1 · Answer 2 · answered Jul 06 '17 at 13:27

class Param() {
}
class Subparam extends Param {
}

class Base {

  function mymethod(a) {
    // Remove the parameter type from the method declaration.
    if (a !instanceof Param) {
      throw new \Exception('Paramater type should be of 'Param.');
    }

}

class Sub extends Base {

 function mymethod(a) {
 }

}

This should get you close.

Covariance (OOP) in PHP - how does it work

2 Answers2