PHP superclass calling subclass methods without knowing about them?

oop, php, subclass, superclass

Solution

Now, to my mind, the base class should not be able to make any assumptions about the subclass at all, except for the ones it enforces for the subclass by declaring (or inheriting) abstract methods, or by implementing an interface.

That's how things work in statically typed languages. In PHP (and many others) you are free to call any method on any value (it need not even be an object). If the call does not make sense, you get a runtime error.

Of course it's a good idea to declare the method if you expect derived classes to implement it; this is simply good practice. Additionally, many well-known PHP IDEs will detect such an omission and flag the call to bring it to your attention exactly because the compiler cannot.

This seems like broken behaviour to me. Unless the base class declares `abstract protected function childMethod();`, it should not be able to call it, should it?

It should, as explained above. If you find this behavior undesirable, PHP is not the language you should be using.

Aside: In a closely related note, you might also find it counter-intuitive that a base class can access `protected` members defined in a derived class without any trouble. This is documented. In fact it is exactly what happens in your example, but I mention it specifically because it's a different kind of counter-intuitive (your example would also be as puzzling if the method were `public`).

Consider the endless list of constructs that PHP allows which also "should not be possible" from a statically typed perspective, which include:

// #1
$varName = 'foo';

// How do you know $object has a property named "foo"?
// How do you know that "foo" is a valid property name in the first place?
// How do you know that $object is an object to begin with?
echo $object->$varName;

// #2
$object = new SomeObject;
$methodName = 'someMethod';

// it's practically impossible to reason about this before runtime
call_user_func(array($object, $methodName));

Problem

Original source