Property setters and getters - final solution

PHP currently supports property overloading provided by magic methods __get() and __set(). Those methods are called when accessed property is not declared in the class. Actually this overloading is a bit frustrating.

This is my solution how to simulate properties with accessor methods in a Delphi way.

So…

// define new "like-keyword"
// value is short and unusual string
define('property',   "\0\0");

Maybe _property is better choice. It doesn't matter now.

And the Initializator:

function _property($obj)
{
  // use cache (analyze object only once)
  static $cache;
  $props = & $cache[ get_class($obj) ];

  if ($props === null) {
	// build list of properties
	// 1) get_object_vars prevents ObjectIteration
	// 2) outside of objects returns only public members
	$props = array_flip(array_keys(get_object_vars($obj), property, true));
  }

  foreach ($props as $name => $foo) {
	unset($obj->$name); // unset property to pass control to __set() and __get()
  }

  return $props;
}

See documented source code of class:

class Rectangle
{
  // list of properties
  private $_props;

  // shadow properties
  private $_width;
  private $_height;


  /**
   * @var int  phpDoc for virtual property!
   * getter: $_width  setter: setWidth
   */
  public $width  = property;
  public $height = property;
  public $area   = property;


  function __construct()
  {
	// property initialization
	$this->_props = _property($this);
  }



  /**
   * Universal setter.
   *
   * If property 'abc' is declared,
   * there must be setAbc() setter,
   * otherwise property is read-only
   */
  final function __set($name, $value)
  {
	if (!isset($this->_props[$name])) {
	  throw new Exception("Undefined property '$name'.");
	} elseif (method_exists($this, 'set'.$name)) {
	  $this->{'set'.$name}($value);
	} else {
	  throw new Exception("Property '$name' is read-only.");
	}
  }


  /**
   * Universal getter.
   *
   * If property 'abc' is declared,
   * there must be getAbc() getter,
   * otherwise variable $_abc
   */
  final function &__get($name)
  {
	if (!isset($this->_props[$name])) {
	  throw new Exception("Undefined property '$name'.");
	} elseif (method_exists($this, 'get'.$name)) {
	  return $this->{'get'.$name}();
	} else {
	  return $this->{'_'.$name};
	}
  }


  // getters and setters:

  protected function setWidth($value)
  {
	$this->_width = abs($value);
  }

  protected function setHeight($value)
  {
	$this->_height = abs($value);
  }

  protected function getArea()
  {
	return $this->_height * $this->_width;
  }

}


// usage:

$rect = new Rectangle();
$rect->width = 10;
$rect->height = 5;

echo $rect->area;
// prints 50

// try receive private member
echo $rect->props;
// throw exception

Pros:

• universal __set & __get, no overriden is needed
• magic methods know if specific property exists
• so this mechanism cannot be misused to get private members
• properties are case-sensitive, like other variables
• these __set & __get works very fast (no switches or ifs)
• virtual properties may be documented with any of the existing tools (phpDoc, doxygen)
• IDE autocomplete based on phpDoc will work
• easy adaptive for future PHP6 (I hope)

Cons:

• overloading is still not safe (see bug #36484, fixed in PHP 5.2.1)

Since PHP 5.1.0 it is also possible to overload the isset() and unset():

	final private function __isset($name)
	{
		return isset($this->_props[$name]);
	}

	final private function __unset($name)
	{
		unset($this->_props[$name]);
	}

Some links:

• PHP 5 → 4 converter
• How to emulate PHP5 object model in PHP4