Traits and static methods

Hi Simas:

The first thing that comes to my mind is a Singleton
implementation - this is where I found myself literally "copying" most of
the code in the past. As far as I know there is no way to implement this
pattern properly in PHP without duplicate code (due to lack of multiple
inheritance/templating).

Unfortunately, the same RFC document mentions this for trait methods:

The static modifier is not supported, because it would change the methods

semantics and references to $this would break.

.. and that pretty much defeats Singleton-Trait implementation. Is there any
particular technical/design reasons why traits can't handle $this inside
static methods just like regular static methods do (with a fatal error)?
Hm, good question. I don't recall any implementation details. Haven't touched the code for too long.
From the top of my head, I don't see why it should not work.

If I would have elaborated a bit more what I was thinking when I wrote that comment...

Following the motto 'it is just compiler assisted copy and paste' I think it should be implemented.

Best regards
Stefan

--
Stefan Marr
Software Languages Lab
Vrije Universiteit Brussel
Pleinlaan 2 / B-1050 Brussels / Belgium
http://soft.vub.ac.be/~smarr
Phone: +32 2 629 2974
Fax: +32 2 629 3525

Hello,

I was just skimming through Traits RFC document on the wiki trying to find
a
practical use for them. The first thing that comes to my mind is a
Singleton
implementation - this is where I found myself literally "copying" most of
the code in the past. As far as I know there is no way to implement this
pattern properly in PHP without duplicate code (due to lack of multiple
inheritance/templating).

Unfortunately, the same RFC document mentions this for trait methods:

The static modifier is not supported, because it would change the methods

semantics and references to $this would break.

.. and that pretty much defeats Singleton-Trait implementation. Is there
any
particular technical/design reasons why traits can't handle $this inside
static methods just like regular static methods do (with a fatal error)?

p.s. a Singleton-Trait implementation could look like:

trait Singleton {
public static function getInstance() { ... }
}

class Child extends Parent {
use Singleton;
}

Child::getInstance();

for reusable singleton pattern with php 5.3
http://www.slideshare.net/fabpot/design-patternrevisitedphp53
slide 11 and 12

Tyrael

Am 16.11.2010 18:46, schrieb Simas Toleikis:

p.s. a Singleton-Trait implementation could look like:

trait Singleton {
public static function getInstance() { ... }
}

class Child extends Parent {
use Singleton;
}

Child::getInstance();

This is not really a good example as the getInstance() method alone is
not sufficient for a reusable implementation of the Singleton pattern.

A trait for a reusable implementation of the Singleton pattern should
look more like

 trait Singleton
 {
     private static $uniqueInstance = NULL;

     protected function __construct() {}
     private final function __clone() {}

     public static function getInstance()
     {
         if (self::$uniqueInstance === NULL) {
             self::$uniqueInstance = new Singleton;
         }

         return self::$uniqueInstance;
     }
 }

which of course does not work as traits are stateless and

 class MySingleton
 {
     use Singleton;
 }

 MySingleton::getInstance();

results in

 Access to undeclared static property: MySingleton::$uniqueInstance

And rightfully so. Nobody should need a mechanism to make it as easy as
pie to clutter the code base with singletons ;-)

--
Sebastian Bergmann Co-Founder and Principal Consultant
http://sebastian-bergmann.de/ http://thePHP.cc/