Re: [RFC][Discussion] Why can constructors violate LSP?

> On Nov 23, 2023, at 4:50 PM, Robert Landers <landers.robert@gmail.com> wrote: > > On Thu, Nov 23, 2023 at 10:30 PM Deleu <deleugyn@gmail.com> wrote: >> >> Constructors are an implementation detail of a specialized class and as such they're >> not subject to LSP because the goal of LSP is to be able to make sure that any object of a given >> type hierarchy can be used to accomplish a certain behavior. If you take a step back from PHP's >> dynamic nature and think about LSP from a more pure type system, the fact you're expecting an >> object of type C, but then you completely disregard everything about the object itself and dive into >> it's metadata to build another object, that's the moment you're no longer playing by >> the rules of OOP. It's like those mathematical equations that prove that 1 = 2, they all have >> one thing in common: they end up dividing by 0 at some point. >> >> OOP here dictates that you should reach for patterns like Builder, Abstract Factory or >> similar. That way you constraint yourself to the rules of OOP and you won't get weird outcomes. >> >> From another point of view, when a type is expected by a function or method, all we can >> expect from it is whatever was defined as the blueprint (class/interface) of that object and the >> __construct() is a special method that is not assumed to be part of that blueprint because it's >> not reasonable to do $object->__construct(); after receiving an >> object. As such, a constructor cannot break LSP because the constructor is not part of the >> object's API from a "receptor" point of view. >> >> I don't have a vote so take my opinion with a bucket of salt, but if I could I would >> definitely vote against such RFC. >> >> >> -- >> Marco Deleu > > Thanks Marco, > > That's an interesting perspective and one I would agree with for the > most part, especially if you take my illustration at face value. Where > it gets weird/breaks down is when you have a class-string, that you > assert is the correct type, and then try to instantiate it: > > // from somewhere > $class = "C"; > > if(is_subclass_of($class, P::class)) { > $example = new $class("world"); > } > > If PHP didn't offer these built-in methods, then I would fully agree > with you, but it does, which puts it into a weird position where > sometimes a class is substitutable, and in this one special case, it > is not. What Marco said pretty much mirrors the answer in Software Engineering SE: https://softwareengineering.stackexchange.com/a/270738/9114 <https://softwareengineering.stackexchange.com/a/270738/9114> As if probably obvious now, when you are using a variable containing a class name to instantiate a class you are actually not using OOP, you are using the text expansion capabilities of PHP. The fact that it appears to be similar to a method call is just likely coincidence, Now if classes were first class objects in PHP, then there might be an argument, but alas, they are not. Anyway, I always addressed the problem you are running into by defining a make_new() static method. Here's what your example might look after an initial refactor: class P { public $msg; function __construct($msg){ $this->msg = $msg; } } class C extends P { static function make_new($props){ return new C($props["msg"]); } } $class = "C"; if(is_subclass_of($class, P::class)) { $example = $class::make_new( ["msg"=>"world"] ); print_r($example); } Of course, getting it to production code will take a lot more, like you can see here (although this code has now been update for probably five years since I am no longer coding in PHP professionally): https://github.com/search?q=repo%3Awplib%2Fwplib%20make_new&type=code <https://github.com/search?q=repo:wplib/wplib%20make_new&type=code> And if you are dealing with 3rd party classes, you'll need to write wrapper classes. Hope this helps. -Mike