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Date: Fri, 31 May 2013 14:11:40 +0200
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To: Anthony Ferrara <ircmaxell@gmail.com>
Cc: Richard Lynch <ceo@l-i-e.com>, "internals@lists.php.net" <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Cannot call constructor
From: linepogl@gmail.com (Lazare Inepologlou)

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2013/5/30 Anthony Ferrara <ircmaxell@gmail.com>

> Richard et al,
>
>
> On Thu, May 23, 2013 at 5:24 PM, Richard Lynch <ceo@l-i-e.com> wrote:
>
> > Consider this common scenario:
> >
> > I use some OOP library, that is a "black box" and I like it that way.
> >
> > As part of the integration, I need to extend one of the library's
> > classes:
> >
>
> The problem with this entire thread (I've been thinking about this for a
> while) is that it's setup from the very beginning with a failed assertion=
.
> You want to treat a library as a black box. Great! However, you also want
> to extend one of the library's classes. That's not ok. Because you cannot
> ever treat inheritance as a black box. It goes against just about every
> principle out there. And it's provably not possible to do.
>
>

I cannot agree with you here. This is exactly why the "protected" keyword
exists: to provide a black-box interface for class usage through
inheritance. Yes, this is not automatic and, yes it needs designing, but it
is certainly not against the principles. If it were, we should remove the
"protected" keyword as well.





> Treating it like a black box will cause LSP violations (which PHP is so
> keen on enforcing). Why? Because if you treat the parent as a black-box,
> you can't guarantee that the pre-conditions, post-conditions
> and invariants are not enforced (which LSP requires). So without looking =
at
> the parent, you can't have a child that adheres to LSP.
>
> But let's get less "rule" and more practical. Imagine the parent has no
> constructor. So you, in future hindsight, decide to do this:
>
> if (method_exists(get_parent_class(), '__construct')) {
>     call_user_func_array(array(get_parent_class(), '__construct'),
> func_get_args());
> }
>
> Now, what happens if the parent constructor now takes a parameter by
> reference? The reference will be lost. The conceptual implementation is n=
ow
> gone. Not to mention that the parameters may (and will likely) be differe=
nt
> from what you were expecting.
>
> So basically, we can't ever develop completely in a black box. So if the
> parent has a constructor, we need to call it (or not). But the point is,
> it's never a conditional in the code. It requires you to look at, and mak=
e
> a decision.
>
> So that brings us to the case when you override a class that doesn't have=
 a
> constructor, which then gets one later on down the road. This is really
> just the same as before and boils down to: 1. Test your code, as a test
> should catch that. 2. Don't blindly update and expect code to work. 3. Ev=
en
> if you did have a conditional, you're likely to get the arguments wrong (=
or
> worse)...
>
> So realistically, while I can see the appeal to having the ability to
> always call parent::__construct(), I think it's actually a red-herring to
> the actual problem. And it's not really necessary in the first place. In
> fact, using it is likely to be a source of *more* bugs, as the object sti=
ll
> won't be initialized properly (but you think it is)...
>
> My $0.02 at least,
>
> Anthony
>



Lazare INEPOLOGLOU
Ing=C3=A9nieur Logiciel

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