类与对象
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对象继承

继承已为大家所熟知的一个程序设计特性，PHP 的对象模型也使用了继承。继承将会影响到类与类，对象与对象之间的关系。

比如，当扩展一个类，子类就会继承父类的所有公有和保护方法。但是子类的方法会覆盖父类的方法。

继承对于功能的设计和抽象是非常有用的，而且对于类似的对象增加新功能就无须重新再写这些公用的功能。

Example #1 继承代码示例

<?php

class foo
{
    public function printItem($string) 
    {
        echo 'Foo: ' . $string . PHP_EOL;
    }
    
    public function printPHP()
    {
        echo 'PHP is great.' . PHP_EOL;
    }
}

class bar extends foo
{
    public function printItem($string)
    {
        echo 'Bar: ' . $string . PHP_EOL;
    }
}

$foo = new foo();
$bar = new bar();
$foo->printItem('baz'); // Output: 'Foo: baz'
$foo->printPHP();       // Output: 'PHP is great' 
$bar->printItem('baz'); // Output: 'Bar: baz'
$bar->printPHP();       // Output: 'PHP is great'

?>
类与对象
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PHP手册 - N: 对象继承

用户评论:

msg2maciej at aol dot com (29-Jan-2011 12:08)

PHP supports single class inheritance. My bare idea on accessing protected methods with power of abstracts and sort of "multi-class inheritance SIMULATION":

<?php
error_reporting(E_ALL);

abstract class Base {
    abstract protected function __construct ();
    abstract protected function hello_left ();
    abstract protected function hello_right ();
}

abstract class NotImplemented_Left extends Base {
protected function hello_right () {
echo 'well, wont see that'; }}

abstract class NotImplemented_Right extends Base {
protected function hello_left () {
echo 'well, wont see that'; }}

class Left extends NotImplemented_Left {
protected function __construct () {        # limited visibility, no access from "outside"
echo __CLASS__.'::protected __construct'. "\n"; }
protected function hello_left () {        # limited visibility, no access from "outside"
echo 'protected hello_left in ' . __CLASS__ . "\n"; }}

class Right extends NotImplemented_Right {
protected function __construct () {        # limited visibility, no access from "outside"
echo __CLASS__.'::protected __construct'. "\n"; }
protected function hello_right () {        # limited visibility, no access from "outside"
echo 'protected hello_right in ' . __CLASS__ . "\n"; }
protected function hello_left () {
echo "wont see that, and easy to get rid of it from here\n"; }}

class Center extends Base {
private $left;
private $right;
public function __construct () {
echo 'welcome in ' . __CLASS__ . "\n";
echo 'Center::'; $this->left = new Left;
echo 'Center::'; $this->right = new Right;
echo " oh and\n";
$this->hello_left();
$this->hello_right();
}
public function hello_left () {            # calling class Left
echo __CLASS__.'::'; $this->left->hello_left(); }
public function hello_right () {        # calling class Right
echo __CLASS__.'::'; $this->right->hello_right(); }
}

$c = new Center;
?>

Produces:

welcome in Center
Center::Left::protected __construct
Center::Right::protected __construct
 oh and
Center::protected hello_left in Left
Center::protected hello_right in Right

OZ (14-Nov-2010 11:24)

Model for Mixins pattern:

<?php
interface IMixinsCaller
{
    public function __mixin_get_property($property);

    public function __mixin_set_property($property, $value);

    public function __mixin_call($method, $value);
}

abstract class MixinsCaller implements IMixinsCaller
{
    protected $mixins = array();

    public function __call($name, $arguments)
    {
        if (!empty($this->mixins))
        {
            foreach ($this->mixins as $mixin)
            {
                if (method_exists($mixin, $name))
                {
                    return call_user_func_array(array($mixin, $name), $arguments);
                }
            }
        }
        trigger_error('Non-existent method was called in class '.__CLASS__.': '.$name, E_USER_WARNING);
    }

    public function __mixin_get_property($property)
    {
        if (property_exists($this, $property))
        {
            return $this->$property;
        }
        trigger_error('Non-existent property was get in class '.__CLASS__.': '.$property, E_USER_WARNING);
    }

    public function __mixin_set_property($property, $value)
    {
        if (property_exists($this, $property))
        {
            return $this->$property = $value;
        }
        trigger_error('Non-existent property was set in class '.__CLASS__.': '.$property, E_USER_WARNING);
    }

    public function __mixin_call($method, $value)
    {
        if (method_exists($this, $method))
        {
            return call_user_func_array(array($this, $method), $value);
        }
        trigger_error('Non-existent method was called in class '.__CLASS__.': '.$method, E_USER_WARNING);
    }

    public function AddMixin($mixin)
    {
        $this->mixins[] = $mixin;
    }
}

abstract class Mixin
{
    /** @var IMixinsCaller $parent_object */
    private $parent_object;

    public function __construct(IMixinsCaller $parent_object)
    {
        $this->parent_object = $parent_object;
    }

    public function __get($property)
    {
        return $this->parent_object->__mixin_get_property($property);
    }

    public function __set($property, $value)
    {
        return $this->parent_object->__mixin_set_property($property, $value);
    }

    public function __call($method, $value)
    {
        return $this->parent_object->__mixin_call($method, $value);
    }
}
?>

janturon at email dot cz (23-Sep-2010 06:11)

Here is an easy way how to implement multiple inheritance in PHP using magic methods. Let's have this class:

class Extender {
  private $objs = array();
  public function __construct() {
    $drones = func_get_args();
    foreach($drones as $drone) $this->objs[$drone] = new $drone();
    foreach($this->objs as $obj) $obj->hive = $this;
  }
  public function __get($attr) {
    foreach($this->objs as $obj)
      if(isset($obj->$attr))
        return $obj->$attr;
  }
  public function __set($attr,$val) {
    if($attr=="hive") return;
    foreach($this->objs as $obj)
      if(isset($obj->$attr))
        $obj->$attr = $val;
  }
  public function __call($meth,$args) {
    foreach($this->objs as $obj)
      if(method_exists($obj,$meth))
        return call_user_func_array(array($obj,$meth),$args);
  }
}

Now multiple inheritance can be written this way ($hive is reference to child classes)

class c1 {
  public $p1 = "x";
  function f1() { echo $this->p1; }
}

class c2 {
  public $p2 = "x";
  function f2() { echo $this->p2; }
}

class c_extends_c1_c2 {
  static $hive;
  function test() {
    $this->hive->p1 = "hello,";
    $this->hive->p2 = "world!";
    return $this->hive->f1() . $this->hive->f2();
  }
}

And here is how to use it:

$obj = new Extender("c1","c2","c_extends_c1_c2");
echo $obj->test(); //"hello,world!"

strata_ranger at hotmail dot com (19-Sep-2010 05:37)

I was recently extending a PEAR class when I encountered a situation where I wanted to call a constructor two levels up the class hierarchy, ignoring the immediate parent.  In such a case, you need to explicitly reference the class name using the :: operator.

Fortunately, just like using the 'parent' keyword PHP correctly recognizes that you are calling the function from a protected context inside the object's class hierarchy.

E.g:

<?php
class foo
{
  public function something()
  {
    echo __CLASS__; // foo
    var_dump($this);
  }
}

class foo_bar extends foo
{
  public function something()
  {
    echo __CLASS__; // foo_bar
    var_dump($this);
  }
}

class foo_bar_baz extends foo_bar
{
  public function something()
  {
    echo __CLASS__; // foo_bar_baz
    var_dump($this);
  }

  public function call()
  {
    echo self::something(); // self
    echo parent::something(); // parent
    echo foo::something(); // grandparent
  }
}

error_reporting(-1);

$obj = new foo_bar_baz();
$obj->call();

// Output similar to:
// foo_bar_baz
// object(foo_bar_baz)[1]
// foo_bar
// object(foo_bar_baz)[1]
// foo
// object(foo_bar_baz)[1]

?>

jackdracona at msn dot com (15-Apr-2010 04:53)

Here is some clarification about PHP inheritance – there is a lot of bad information on the net.  PHP does support Multi-level inheritance.  (I tested it using version 5.2.9).  It does not support multiple inheritance.
 
This means that you cannot have one class extend 2 other classes (see the extends keyword).  However, you can have one class extend another, which extends another, and so on.
 
Example:
 
<?php
class A {
        // more code here
}
 
class B extends A {
        // more code here
}
 
class C extends B {
        // more code here
}
 
 
$someObj = new A();  // no problems
$someOtherObj = new B(); // no problems
$lastObj = new C(); // still no problems
 
?>

php at sleep is the enemy dot co dot uk (07-Apr-2010 10:36)

Here's fun, an attempt to make some degree of multiple inheritance work in PHP using mixins. It's not particularly pretty, doesn't support method visibility modifiers and, if put to any meaningful purpose, could well make your call stack balloon to Ruby-on-Rails-esque proportions, but it does work.

<?php
abstract class Mix {
   
    protected $_mixMap = array();
   
    public function __construct(){
       
         $this->_mixMap = $this->collectMixins($this);
    }
   
    public function __call($method, $args){
       
        // doesn't pass scope
        //return call_user_func_array(array($className, $method), $args);
       
        // Error: Given object is not an instance of the class this method was declared in
        //$method = new ReflectionMethod($className, $method);
        //return $method->invokeArgs($this, $args);
       
        $payload = $this->buildMixinPayload($this->_mixMap, $method, $args);
        if(!$payload) throw new Exception('Method ' . $method . ' not found');
       
        list($mixinMethod, list($method, $args)) = $payload;
       
        return $this->$mixinMethod($method, $args);
       
    }
   
    protected function collectMixins($class){
       
        static $found = array();
        static $branch = array();
       
        if(empty($branch)) $branch[] = get_class($this);
        $mixins = array();
       
        foreach(array_reverse(get_class_methods($class)) as $method){
            if(preg_match('/^mixin(\w+)$/', $method, $matches)){
               
                $className = $matches[1];
               
                if(in_array($className, $branch))
                    throw new Exception('Circular reference detected ' . implode(' > ', $branch) . ' > ' . $className);
                   
                if(!in_array($className, $found)){
               
                    if(!class_exists($className)) throw new Exception('Class ' . $className . ' not found');
           
                    // populate props from mixin class
                    foreach(get_class_vars($className) as $key => $value){       
                        if(!property_exists($this, $key)) $this->$key = $value;
                    }
                   
                    $found[] = $branch[] = $className;               
                    $mixins[$className] = $this->collectMixins($className);
                }
               
                $branch = array(get_class($this));
            }
        }
       
        return $mixins;
    }
   
    protected function buildMixinPayload($mixins, $method, $args){
       
        foreach($mixins as $className => $parents){
           
            $mixinMethod = 'mixin' . $className;
           
            if(method_exists($className, $method)) return array($mixinMethod, array($method, $args));
           
            if(!empty($parents) && $return = $this->buildMixinPayload($parents, $method, $args)){
                return array($mixinMethod, $return);
            }
        }
       
        return false;
    }
   
}
?>

php at sleep is the enemy dot co dot uk (31-Mar-2010 03:47)

Here's some example usage of the mixin class.

<?php

class Lunch extends Mix {
   
    public $edible = true;
   
    /*
     * Circular references are, of course, illegal and will be detected
     */
    /*
    public function mixinSteakAndKidneyPie($method, $args){
        return SteakAndKidneyPie::$method(@$args[0], @$args[1], @$args[2], @$args[3], @$args[4], @$args[5]);
    }
    //*/
   
    public function isEdible($affirm, $negate){
        return $this->edible ? $affirm : $negate;
    }
}

class Pie extends Mix {
   
    /*
     * class tokens are bound at compile time so need to be explicitly declared
     * Need to make sure there are enough argument placeholders to cover all mixed in methods of Lunch
     * Late static binding may improve this situation
     */
    public function mixinLunch($method, $args){
        return Lunch::$method(@$args[0], @$args[1], @$args[2], @$args[3], @$args[4], @$args[5]);
    }
   
    public function buildPie($sep = ','){
        return 'Crust' . $sep . $this->getFilling() . $sep . 'More Crust';
    }
   
    public function getFilling(){
        $this->edible = false;
        return 'Baking beans';
    }
}

class SteakAndKidney extends Mix {
   
    public function mixinLunch($method, $args){
        return Lunch::$method(@$args[0], @$args[1], @$args[2], @$args[3], @$args[4], @$args[5]);
    }
   
    /*
     * everything to be mixed in must be public
     * protected/private methods called from within mixed in methods will fail
     */
    public $filling = 'Steak and Kidney';
   
    public function getFilling(){
        $this->edible = true;
        return $this->filling;
    }
}

class SteakAndKidneyPie extends Mix {
   
   
    /*
     * order of mixin declaration significant
     * later declarations override earlier ones
     */
    public function mixinSteakAndKidney($method, $args){
        return SteakAndKidney::$method(@$args[0], @$args[1], @$args[2], @$args[3], @$args[4], @$args[5]);
    }
   
    public function mixinPie($method, $args){
        return Pie::$method(@$args[0], @$args[1], @$args[2], @$args[3], @$args[4], @$args[5]);
    }
   
    /*
     * Pick specific methods like so:
     */
    //*
    public function getFilling(){
        return SteakAndKidney::getFilling();
    }
    //*/
   
}

$pie = new SteakAndKidneyPie();
echo $pie->buildPie(' | ');
echo '<br/>Pie ' . $pie->isEdible('is', 'is not') . ' Edible';

/*
OUTPUTS:
Crust | Steak and Kidney | More Crust
Pie is Edible
*/

?>

jarrod at squarecrow dot com (27-Oct-2009 02:01)

You can force a class to be strictly an inheritable class by using the "abstract" keyword. When you define a class with abstract, any attempt to instantiate a separate instance of it will result in a fatal error. This is useful for situations like a base class where it would be inherited by multiple child classes yet you want to restrict the ability to instantiate it by itself.

Example........

<?php

abstract class Cheese
{
      //can ONLY be inherited by another class
}

class Cheddar extends Cheese
{
}

$dinner = new Cheese; //fatal error
$lunch = new Cheddar; //works!

?>