Type Hinting

PHP 5 introduces Type Hinting. Functions are now able to force parameters to be objects by specifying the name of the class in the function prototype.

Example 19-39. Type Hinting example


<?php

// An example class

class MyClass

{

    /**

     * A test function

     *

     * First parameter must be an object of type OtherClass

     */

    public function test(OtherClass $otherclass) {

        echo $otherclass->var;

    }

}



// Another example class

class OtherClass {

    public $var = 'Hello World';

}

?>

Failing to satisfy the type hint results in a fatal error.


<?php

// An instance of each class

$myclass = new MyClass;

$otherclass = new OtherClass;



// Fatal Error: Argument 1 must be an object of class OtherClass

$myclass->test('hello');



// Fatal Error: Argument 1 must be an instance of OtherClass

$foo = new stdClass;

$myclass->test($foo);



// Fatal Error: Argument 1 must not be null

$myclass->test(null);



// Works: Prints Hello World

$myclass->test($otherclass);

?>

Type hinting also works with functions:


<?php

// An example class

class MyClass {

    public $var = 'Hello World';

}



/**

 * A test function

 *

 * First parameter must be an object of type MyClass

 */

function MyFunction (MyClass $foo) {

    echo $foo->var;

}



// Works

$myclass = new MyClass;

MyFunction($myclass);

?>

Type Hints can only be of the object type. Traditional type hinting with int and string are not supported.

User Contributed Notes
Type Hinting

10-May-2005 10:41


If wrong object is given, error points to method definition instead of place where method has been called. Because of that type hinting is not really useful. Instead use:



<?php



function($foo)

{

  assert('$foo instanceof ClassRequired');

  // ...

}



?>



and use your custom error handler that dumps debug_backtrace() -- this will allow you to find caller that passes wrong object. Works in PHP4 as well.

ryo at shadowlair dot info
28-Feb-2005 01:53


Although type-hinting prevents null values from being passed, setting a default null value is not a parse error.

However, calling the function with a null parameter, or calling it without the parameter (thus using the default value) will result in a fatal error:



<?php



function foo(MyClass $object = null) {

    // this is a valid function definition

}



$o = new MyClass();

foo($o);

foo(null); // fatal error

foo(); // fatal error



?>

caliban at darklock dot com
23-Feb-2005 10:34


In case you're worried, type hinting does allow descendants. Extending the documentation example:



<?php

   // Example class

   class MyClass

   {

      public function test(OtherClass $otherclass) 

      {

          if(is_callable(array($otherclass,$otherclass->var)))

          {

             $otherclass->{$otherclass->var}();

          }

          else

          {

             echo $otherclass->var;

          }

      }

   }



   // Another example class

   class OtherClass 

   {

      public $var = 'Hello World';

   }



   // Yet another example class

   class DerivedClass extends OtherClass 

   {

      function __construct() 

      { 

         $this->var="Planet"; 

      }



      public function Planet()

      {

         echo "Hello ".$this->var;

      }

   }



   $myclass = new MyClass;

   $otherclass = new OtherClass;

   $derivedclass = new DerivedClass;



   // Works - prints "Hello World"

   $myclass->test($otherclass);



   // Works - calls DerivedClass::Planet()

   //    which prints "Hello Planet"

   $myclass->test($derivedclass);

?>

Tim Ansell
10-Feb-2005 08:28


Type hinting also prevents null from been given.

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