Assuming everyone knows what generics are, let's get down to business right away. PHP does not support generics or something similar, though it could be very useful in PHP development.  Luckily, using some standard OO-practises, a semi-generic array can easily be created, even in multiple ways! Here's the road to PHP generics. 

The hard way...

One of the roads to PHP generics is some simple inheritance and type hinting. Let's have a look at PHP's ArrayObject. This class has 2 interesting methods, namely offsetSet() and append(). This would mean I can simply create a new class which inherits from ArrayObject, and uses type hinting to restrict some additions:

[code:c#]

// Example class
class Example {
  public $SomeProperty;
}

// Example class generic ArrayObject
class ExampleArrayObject extends ArrayObject {
  public function append(Example $value) {
    parent::append($value);
  }

  public function offsetSet($index, Example $value) {
    parent::offsetSet($index, $value);
  }
}


// Example additions
$myArray = new ExampleArrayObject();
$myArray->append( new Example() ); // Works fine
$myArray->append( "Some data..." ); // Will throw an Exception!

[/code]

The flexible way

There are some disadvantages to the above solution. For a start, you can't create a generic "string" array unless you encapsulate strings in a specific object type. Same goes for other primitive types. Let's counter this problem! Here's the same code as above using a "GenericArrayObject":

[code:c#]

// Example class
class Example {
  public $SomeProperty;
}

// Validation function
function is_class_example($value) {
  return $value instanceof Example;
}

/**
 * Class GenericArrayObject
 *
 * Contains overrides for ArrayObject methods providing generics-like functionality.
 *
 * @author    Maarten Balliauw
 */
class GenericArrayObject extends ArrayObject {
    /**
     * Validation function
     *
     * @var     string
     * @access    private
     */
    private $_validationFunction = '';
       
    /**
     * Set validation function
     *
     * @param     string    $functionName    Validation function
     * @throws     Exception
     * @access    public
     */
    public function setValidationFunction($functionName = 'is_string') {
        if ($this->_validationFunction == '') {
            $this->_validationFunction = $functionName;
            return;
        }
       
        $iterator = $this->getIterator();
        while ($iterator->valid()) {
            if (!call_user_func_array($functionName, array($iterator->current()))) {
                throw new Exception("Switching from " . $this->_validationFunction . " to " . $functionName . " is not possible for all elements.");
            }
           
            $iterator->next();
        }
       
        $this->_validationFunction = $functionName;
    }
   
    /**
     * Append
     *
     * @param     mixed    $value
     * @throws     Exception
     * @access    public
     */
    public function append($value) {
        if ($this->_validationFunction == '') {
            throw new Exception("No validation function has been set.");
        }
       
        if (call_user_func_array($this->_validationFunction, array($value))) {
            parent::append($value);
        } else {
            throw new Exception("Appended type does not meet constraint " . $this->_validationFunction);
        }
    }
   
    /**
     * offsetSet
     *
     * @param     mixed    $index
     * @param     string    $newval
     * @throws     Exception
     * @access    public
     */
    public function offsetSet($index, $newval) {
        if ($this->_validationFunction == '') {
            throw new Exception("No validation function has been set.");
        }
       
        if (call_user_func_array($this->_validationFunction, array($newval))) {
            parent::offsetSet($index, $newval);
        } else {
            throw new Exception("Appended type does not meet constraint " . $this->_validationFunction);
        }
    }
}

// Example additions
$myArray = new GenericArrayObject();
$myArray->setValidationFunction('is_class_example');
$myArray->append( new Example() ); // Works fine
$myArray->append( "Some data..." ); // Will throw an Exception!

[/code]

Using this flexible class, you can simply set a validation function on the GenericArrayObject, which enabels you to use PHP's built-in functions like is_string (string-only ArrayObject), is_int, ... You can even write a small validation function which matches a string against a regular expression and for example create an e-mail address ArrayObject rejecting any string that does not match this regular expression.

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