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Date: Fri, 10 Oct 2003 13:55:18 -0400
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To: Wez Furlong <wez@thebrainroom.com>
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Subject: Re: [PHP-DEV] Accessing child constants from base class
From: dan@wep.net (Dan Cox)

Hi Wez-

Wez Furlong wrote:

>This sounds like you're doing something wrong (no offense!).
>
>You want to access a *constant* of a descendant class, when
>your ancestor doesn't even know if it exists.
>Well, that sounds more than a little odd (backwards even).
>
>  
>

>Why not just use a property with a known name, and set the
>value of that property in you descendant class constructor?
>
>  
>
Yes, this is how I'm doing things now. It just seems like I
shouldn't be forced to always have to use
parent::__construct(_MY_CONSTANT_)

>Performance wise, its not going to make much difference,
>because no matter what you are doing, to dynamically resolve
>the value of a constant will involve hash lookups.
>
>The other alternative, and this is the official POV of the
>Zend guys IIRC, is that you can use eval() to look up
>the value:
>
>$node = $doc->createElement(eval(get_class($this) . "::ElementName"));
>
>  
>
Yes. Normally, at least with most other programming languages,
using eval() is a major performance hit (as much as 10x slower),
so it shouldn't be used unless there is absolutely no other way.
Maybe this isn't the case with PHP?

>If you think about it, what exactly does child:: refer to anyway?
>A child class of the current object? But which one?  What if
>the child doesn't have the constant?  What if there are interfaces
>involved?
>
>  
>
A deriving class of the current *instance* .. so childInstance::
then :) Perhaps you could use interfaces to enforce children
having the needed constants.

>The engine doesn't know about descendant classes either (the
>inheritance tree works in the other direction), and it shouldn't
>have to second-guess what your code is doing - its much clearer to
>explicitly write code like that eval above.
>
>Hope that helps!
>
>--Wez.
>
>  
>
I suppose. It just seems odd that a class can talk to it(self::)
and it's parent:: but not its child:: even though the child::
instance started the conversation in the first place :)

Dan Cox