Solving multiple inheritance in PHP without composition

Question

I'm working on a web app with Yii2 and PHP and am facing a typical multiple inheritance situation.

I have the classes A and B extend Yii2's ActiveRecord class since they represent relational data stored in a DB. But then I have class C, which doesn't (and shouldn't) extend ActiveRecord but shares common behaviors and member variables with A and B. In other words, the three classes genuinely share a common "is a" relation with a real-world entity, but only A and B are storable in a DB.

The way I've got things somewhat working until now is by using traits :

abstract class AbstractMotherClass extends ActiveRecord {
    use MyTrait;
}

class A extends AbstractMotherClass {}

class B extends AbstractMotherClass {}

class C {
    use MyTrait;
}

trait MyTrait {
    public $someVariableInherentToAllThreeClasses;

    public function someMethodInherentToAllThreeClasses() {
        // do something
    }
}

Then I have a method which can take any of the three classes (A, B or C) and work with it. Until now, I only had to throw A or B at it so I just wrote

public fonction someMethod(AbstractMotherClass $entity) {}

so I could get type hinting and other things in my IDE. But now I have to pass C as well and the app crashes since the method doesn't get its expected AbstractMotherClass instance if I call someMethod(new C());. To solve this, I would need a common class that all A, B, AND C could extend, so that I could type hint that class in my method. But that would be multiple inheritance since A and B must also extend ActiveRecord but C can't.

I've found a lot of multiple inheritance problems, but they all have been solved by changing the object structure, splitting responsibilities, using composition over inheritance, and so on. I couldn't manage to apply those solutions here as they didn't seem suitable nor practical, but I might be wrong.

What would be the best way to do this ?

Also if anyone has a better title suggestion, I'd be happy to change it (I couldn't find a good one).

Can you define an interface which MyTrait implements, and then someMethod can take an object of that type? — Greg Schmidt
– Greg Schmidt, Commented Apr 18, 2018 at 19:51
I'm pretty sure that Traits can't implement interfaces and that you can't type hint a Trait. But maybe I didn't understand your comment well and you meant something like like @zzarbi below. — Scentle5S
– Scentle5S, Commented Apr 19, 2018 at 2:59

Scentle5S · Accepted Answer · 2018-04-19 03:59:52Z

2

As Greg Schmidt mentioned as well, you could use interfaces

class ActiveRecord {

}

interface SameInterface {
    public function someMethodInherentToAllThreeClasses();
}

abstract class AbstractMotherClass extends ActiveRecord implements SameInterface{
    use MyTrait;
}

class A extends AbstractMotherClass {}

class B extends AbstractMotherClass {}

class C implements SameInterface{
    use MyTrait;
}

trait MyTrait {
    public $someVariableInherentToAllThreeClasses;

    public function someMethodInherentToAllThreeClasses() {
        return 'bar';
    }
}

function foo(SameInterface $o) {
    return $o->someMethodInherentToAllThreeClasses().PHP_EOL;
}

echo foo(new C());

Granted you have to copy paste someMethodInherentToAllThreeClasses in the interface. Interfaces are usually used for solving some multiple inheritance problem.

edited Apr 19, 2018 at 3:59

Scentle5S

7701 gold badge6 silver badges13 bronze badges

answered Apr 18, 2018 at 20:08

zzarbi

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5 Comments

Scentle5S Over a year ago

That would do most of what I want, indeed. Thanks for the detailed exemple. However, I believe this won't give me autocompletion on the public variable defined in the Trait, would it ? This might not be that big of a deal, but I think it shows a design problem, since even the IDE isn't able to figure out what's being used. Not only must I go back to the Trait to check the exact syntax of the variable when I want to use it, but if I later want to rename it through refactoring tools, I will run into issues since the IDE won't be able to rename the call in your foo method.

zzarbi Over a year ago

Correct it won't give you autocompletion on the variable. But does the variable has to be public?

Scentle5S Over a year ago

Well it's a variable that gets computed in another class since it's not really the classes' (A, B or C) job, then set in the instance, and then read from other places. So if I don't set the variable public, I guess I could write accessors, but I don't see the point if all I do is read and write the variable. Maybe there's another design problem here, but I didn't think so and I thought there was a way to achieve what I need while keeping the variable public.

rob006 Over a year ago

@Scentle5S Then create setter and getter for this attribute and make them part of the interface?

Scentle5S Over a year ago

Ok I'll go with that and accept the answer, thanks !

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