php-doc-en/language/oop5/typehinting.xml

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 <sect1 xml:id="language.oop5.typehinting" xmlns="http://docbook.org/ns/docbook">
  <title>Type Hinting</title>
  <para>
   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) or arrays (since PHP 5.1). However, if <type>NULL</type> is used
   as the default parameter value, it will be allowed as an argument for any 
   later call.
  </para>
  
  <example>
   <title>Type Hinting examples</title>
    <programlisting role="php">
<![CDATA[
<?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 test function
     *
     * First parameter must be an array
     */
    public function test_array(array $input_array) {
        print_r($input_array);
    }
}

// Another example class
class OtherClass {
    public $var = 'Hello World';
}
?>
]]>
   </programlisting>
   <para>
    Failing to satisfy the type hint results in a catchable fatal error.
   </para>
    <programlisting role="php">
<![CDATA[
<?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);

// Fatal Error: Argument 1 must be an array
$myclass->test_array('a string');

// Works: Prints the array
$myclass->test_array(array('a', 'b', 'c'));
?>
]]>
   </programlisting>
   <para>
    Type hinting also works with functions:
   </para>
    <programlisting role="php">
<![CDATA[
<?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);
?>
]]>
   </programlisting>
   <para>
    Type hinting allowing NULL value:
   </para>
    <programlisting role="php">
<![CDATA[
<?php

/* Accepting NULL value */
function test(stdClass $obj = NULL) {

}

test(NULL);
test(new stdClass);

?>
]]>
   </programlisting>
  </example>
  <para>
   Type Hints can only be of the <type>object</type> and <type>array</type>
   (since PHP 5.1) type. Traditional type hinting with <type>int</type> and
   <type>string</type> isn't supported.
  </para>
 </sect1>
 
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