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Date: Mon, 19 Sep 2011 09:40:51 +0100
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Subject: Re: [PHP-DEV] __constructor parameter limitations.
From: glopes@nebm.ist.utl.pt ("Gustavo Lopes")

Em Sun, 18 Sep 2011 20:06:31 +0100, Stas Malyshev <smalyshev@sugarcrm.com>  
escreveu:

> On 9/18/11 11:23 AM, Nikita Popov wrote:
>> to tell you: I need this and that method accepting these and these
>> arguments to work correctly. If the signature isn't enforced, this
>> doesn't make sense anymore. You could just as well drop the signature
>> from abstract method definitions, as it's pointless then.
>
> No it is not. The signature tells "this method would accept certain  
> arguments". If you call it with these arguments, it would work. However,  
> there's no promise to never extend the cases where it works - it goes  
> contrary to the whole point of OOP to say "I will never loosen  
> preconditions on my methods".

While this is true, we should be a bit pragmatic here.

If the subclass method specifies less parameters, it is very likely a  
mistake. It's not usual (certainly it's not more frequent than a mistake  
being the case) for arguments to be simply intentionally ignored. In fact,  
I don't know of any popular language that implements this type of  
contravariance. Plus, consider the case in which the superclass takes an  
argument by reference:

function increment(&$foo) { $foo = $foo + 1; }

It is obvious that in this case a subclass override that takes no  
arguments cannot possibly satisfy the contract of the original method.  
That is to say, by ref arguments are also used for postconditions.

That said, I think the only rules worth implementing are:

* Allow arguments with a supertype. For instance, this should give no  
warning:

<?php
class A {}
class B extends A {}
class C { function g(B $b) {} }
class D extends C { function g(A $a) { } }

* Allow extra parameters with a default value (implemented):

<?php
class C { function g($a) {} }
class D extends C { function g($a,$b=true) { } }

-- 
Gustavo Lopes