PHP 手册
后退章 18. 类与对象（PHP 4）前进


继承

通常需要这样一些类，这些类与其它现有的类拥有相同变量和函数。实际上，定义一个通用类用于所有的项目，并且不断丰富这个类以适应每个具体项目将是一个不错的练习。为了使这一点变得更加容易，类可以从其它的类中扩展出来。扩展或派生出来的类拥有其基类（这称为“继承”，只不过没人死）的所有变量和函数，并包含所有派生类中定义的部分。类中的元素不可能减少，就是说，不可以注销任何存在的函数或者变量。一个扩充类总是依赖一个单独的基类，也就是说，不支持多继承。使用关键字“extends”来扩展一个类。

<?php
class Named_Cart extends Cart {
    var $owner;

    function set_owner ($name) {
        $this->owner = $name;
    }
}
?>

上述示例定义了名为 Named_Cart 的类，该类拥有 Cart 类的所有变量和函数，加上附加的变量 $owner 和一个附加函数 set_owner()。现在，以正常的方式创建了一个有名字的购物车，并且可以设置并取得该购物车的主人。而正常的购物车类的函数依旧可以在有名字的购物车类中使用：

<?php
$ncart = new Named_Cart;    // 新建一个有名字的购物车
$ncart->set_owner("kris");  // 给该购物车命名
print $ncart->owner;        // 输出该购物车主人的名字
$ncart->add_item("10", 1);  // （从购物车类中继承来的功能）
?>

这个也可以叫做“父－子”关系。创建一个类，父类，并使用 extends 来创建一个基于父类的新类：子类。甚至可以使用这个新的子类来创建另外一个基于这个子类的类。

注: 类只有在定义后才可以使用！如果需要类 Named_Cart 继承类 Cart，必须首先定义 Cart 类。如果需要创建另一个基于 Named_Cart 类的 Yellow_named_cart 类，必须首先定义 Named_Cart 类。简捷的说：类定义的顺序是非常重要的。


add a note add a note User Contributed Notes
Neelam Bhatt
10-May-2006 05:44
Tilman I think you are little bit confused about get_parent_class() function.

Actually this function return the 'parent class' of the class, object belong to; with which it is called.

In your example one_get_parent_class_name() and two_get_parent_class_name() are similar functions and are located in different classes. Their behaviour will be same if will be called with same object. Just add the following two line at the end. You will get the better understanding.

$two=new two();
$two->two_get_parent_class_name();
Edward_nl
03-Mar-2006 06:54
If you are using a child-class. Remember to call the constructor of the parent class aswell before you start using it. Otherwise you might get different results then you expected. It is stated in this document, but I got confused by the given example. So, here is my example:

<?php
error_reporting(E_ALL);

class test {
  var $var;

  function test() {
   $this->var = 3;
  }
}

class testing extends test {
   function testing() {
     parent::test();
   }

   function My_test() {
     return $this->var;
   }
}

$p = new testing();
echo $p->My_test();
doedje
13-Feb-2006 05:37
tilman's note is not quite true. He states the fact that php gives wrong information about the parent class. But this is not the case. Php does give correct information about the parent class, to prove that I give the following code:

<?php
class a
{
   function showInfo()
   {
     print "I am class " . get_class($this) . ", my parent is " .get_parent_class($this);
   }
}

class b extends a {}

class c extends b {}

$myInstanceB = new b();
$myInstanceB->showInfo();
print "<br />";
$myInstanceC = new c();
$myInstanceC->showInfo();

/*
output:
I am class b, my parent is a
I am class c, my parent is b
*/
?>

The thing is the function showInfo as they were called by my script, are not a member of class a, but are a member of class b and c. They respond accordingly. Showing correct output in the case of get_class and get_parent_class. So don't go blaming php! =]
tilman dot schroeder at gmx dot de
20-Jan-2006 03:24
Hi...
as alan hogan pointed out it is possible to inherit a class from multiple other classes but notice that php does not always give you the right name of the parent class if you are deriving from multiple classes (tested on php 4.4.0):
 
<?php
header("Content-Type: text/plain");
class three {
   function three() {
       echo("constructor of three\n");
   } // end function three
} // end class three
  
class two extends three {
   function two() {
       echo("constructor of two\n");
   } // end function two
   function two_get_parent_class_name() {
       echo("i am two and my parent is: ".get_parent_class($this)."\n");
   } // end one_function get_parent_class_name()
} // end class two

class one extends two {
   function one() {
       echo("constructor of one\n");
   } // end function one
   function one_get_parent_class_name() {
       echo("i am one and my parent is: ".get_parent_class($this)."\n");
   } // end one_function get_parent_class_name()
} // end class one

$one=new one();
$one->two();
$one->three();
$one->one_get_parent_class_name();
$one->two_get_parent_class_name();

/*
 *    will print out:
 *
 *        constructor of one
 *        constructor of two
 *        constructor of three
 *        i am one and my parent is: two
 *        i am two and my parent is: two // should print out "my parent is: three"
 */
?>

So be careful when dealing with parents within classes ...
However, if you are using parent:: it will give you access to BOTH parent classes:

<?php
header("Content-Type: text/plain");
class three {
   function three() {
       echo("three\n");
   }
}
class two extends three {
   function two() {
       echo("two\n");
   }
}
class one extends two {
   function one() {
       echo("one\n");
       parent::two();
       parent::three();
   }
}
one::one();
/*
 *    will print out:
 *        one
 *        two
 *        three
 */
?>
alan hogan
27-Nov-2005 09:48
Just a note:  It is possible to have a class inherit from multiple other classes, but only in a one-at-a-time linear hierarchy.

So this works, and C gets A and B functions:

<?php
class A {
  public function af() { print 'a';}
  public function bark() {print ' arf!';}
}
class B extends A {
  public function bf() { print 'b';}
}
class C extends B {
  public function cf() { print 'c';}
  public function bark() {print ' ahem...'; parent::bark();}
}

$c = new C;
$c->af(); $c->bf(); $c->cf();
print "<br />";
$c->bark();
/**results:**/
//abc
//ahem... arf!
?>

This does NOT work:

<?php
class A {
  public function af() { print 'a';}
  public function bark() {print ' arf!';}
}
class B {
  public function bf() { print 'b';}
}
class C extends B, A /*illegal*/ {
  public function cf() { print 'c';}
  public function bark() {print ' ahem...'; parent::bark();}
}

$c = new C;
$c->af(); $c->bf(); $c->cf();
print "<br />";
$c->bark();
//Parse Error
?>
Bash I.
19-Nov-2005 01:43
Here is a simple idea that I use when I need my abstract classes (the inherited classes) implemented before my functional classes.

<?php
  
   $_CLASSES = array_merge (
       glob ("classes/*/*.abstract.php"),
       glob ("classes/*/*.class.php")
   );
  
   foreach ($_CLASSES AS $_CLASS) {
       require ($_CLASS);
   }
  
?>
mga at nrd dot lt
18-Apr-2005 10:03
[Editor's note: For an alternative to multiple inheritance, see the dynamic binding via object aggregation in the corresponding section of the manual.]

Multiple Inheritance is not supported but it is easy to emulate it

The example listed above does not show the main advantages of multiple inheritance. I mean, that multiple inheritance is used not for different outputs, but to combain properties ant functionality from different classes.

If you will try to create complex structure based on this example, I am sure, will be big problems regarding member variables values and etc.

In example, each time when you use "inherited" class it is temporarily created!

The main advantage of inheritance is direct incapsulation of member variables and functions. And here are no possibility to use this advantage :(
volte6 at nowhere dot com
01-Apr-2005 02:11
When declaring a class that relies upon another file ( because it extends the class defined in that file ), you should ALWAYS require_once() that file at the top.
This applies even when planning on looping through and including everything in the folder. Use require_once() in your loop, and at the top of the file that NEEDS the include.
tomnezvigin at comcast dot net
07-Mar-2005 09:19
This may seem obvious, but check this scenario. You have a class folder:

+ class
--classA.php
--classB.php
--classC.php
--mainClass.php

Here... classA, classB, classC all extend the mainClass.

If you try to create a function that automatically includes all of the classes in a folder, normally, they are included alphabetically.

When you try to instantiate classC, for example, you will get an error:

"Cannot inherit from undefined class mainClass"

EVEN IF you instantiate the mainClass before you instantiate all of the other classes.

In other words, make sure your primary class is included before all others.
Roy Yong
28-Jan-2005 12:37
I'm not sure if i'm outdated but I think the current PHP version still does not support extension of methods (something that stands between polymorphism and overriding). Hopefully, this could be included in the next release version of PHP!
jo at durchholz dot org
18-Dec-2004 12:54
Multiple inheritance, if designed properly into the language, most definitely is worth the trouble. Solutions exist for all associated problems (including the name clash issue mentioned above - identify a function by adding the name of the class where is was first defined, and all ambiguities vanish; make the class identification optional if nonambiguous, and it's even backwards-compatible).

The rub is that it's difficult to get a language design that's simple to use, efficiently implementable, and applicable to a dynamically-typed language like PHP.
Msquared
19-Nov-2004 10:48
Multiple inheritence is often more trouble than it's worth.  For example, you have a class foo that inherits from both class bar and class baz.  Classes bar and baz both have a fubar() method.  When you create a foo object and call its fubar() method, which fubar() method is called: bar's or baz's?

It seems to me that using aggregate to glue one class's methods and data to another object is a bit like Ruby's fixins, but I could be wrong...

[[Editor's note:
The aggregate_* functions have been dropped, as of PHP 5
-S
]]
Dave Fancella
14-Mar-2004 11:09
I recommend against using the examples here given for multiple inheritance.

The essential problem is having readable code.  If you want to be able to read your code in a few months, or if you want others to be able to read your code, you're better off using member variables to store class instances than using hacked on multiple inheritance.

One of the big advantages to multiple inheritance in a language that properly supports it like C++ is that you can see, in the class declaration, what it inherits from.  In the example where you extend some class called MultipleInheritance, that only obfuscates your class's ancestry.

So, use inheritance, it's good.  But it's not the be all end all of solutions.  If it were, we wouldn't have templates and various other things in C++.  So don't depend on it.  Instead of asking yourself "How do I use inheritance to solve this problem?" you should ask yourself "Is inheritance the right tool to solve this problem?"  If the language doesn't support multiple inheritance and you're trying to use it anyway, then you have certainly chosen the wrong tool to solve your problem.
efredin at redtempest dot com
03-Mar-2004 09:35
It is possible to override a method innherited from a parent class by simply re-defining the method (for those of us who enjoy using abstract classes).

<?
class A
{
   var $foo;

   function A()
   {
       $this->foo = "asdf";
   }
  
   function bar()
   {
       echo $this->foo." : Running in A";
   }
}

class B extends A
{
   function bar()
   {
       echo $this->foo." : Running in B";
   }
}

$myClass = new B;
$myClass->bar();
?>
robertvanderlinden78 at hotmail dot com
19-Oct-2003 02:50
the model as in the example of "quinton" is what you want to have. Not the method of "kimh216" with function callClass

when calling

   function C()
   {
       //global $a,$b;
       $this->a = parent::callClass('A');//no effect
       $this->b = parent::callClass('B');//no effect
   }

you will not have 1 data object.If i wanted to add a function to class A and i wanted to call it , I can only do it from Class C .. otherwise if I were to do
$c = new C();
$c->someExtraFunctionThatIsInClassA(); it will give an error stating "Fatal error:  Call to undefined function:  functionName() in f:\ibserver\www\meuk\classvbs\test.php.php on line 34"
------------
If i add a new function in Person class of the first example of quinton:
  function boe(){
     echo "some text";
  }

then i can call it by :
$family->members[0]->boe();
since the members array has Personobjects in it. Now you have 1 Object that can access every function as a whole.
regards rml
mazsolt at yahoo dot com
04-Jul-2003 11:49
Just a simple example about inheritance:

class a1{
  var $a=10;
  function a1($a){
     $this->a=$a;
  }
}

class a2 extends a1{
  var $x=11;
  function a2($x,$y){
     $this->x=$x;
     parent::a1($y); // or a1::a1($y) or $this->a1($y)
  }
}

class a3 extends a2{
  var $q=999;
}

$x=new a3(99,9);
echo $x->a,"<br>",$x->x,"<br> ",$x->q;

The output will be:

9
99
999
calimero at creatixnet dot com
23-Jun-2003 11:58
Just a quick note to make things more clear : while multiple inheritance is not allowed, several levels of single inheritance  ARE ALLOWED indeed. Just test this example :
<?
class A {
   var $name='A';

   function disp() {
       echo $this->name;
   }
}

class B extends A {
   var $name='B';
}

class C extends B {
   var $name='C';
}

$truc = new C() ;
$truc->disp(); // Will output C
?>

This is especially important to keep in mind while building a huge object hierarchy. for example :

+GenericObject
->+ Person
->->Employee
->+Computer
->->+WorkStation
->->-> PPC
->->-> Intel
->->+Server
->->->LDAPServer
->->->IntranetWebServer

.. and so on. Multiple level hierarchy relationship are possible in a tree-like structure (each child has one and only one parent, except for the root object).
quinton at free dot fr
11-Jun-2003 04:07
a nice example using extends and multiple classes  and constructors.

<?

class CoreObject {
  var $name;
 
  function CoreObject($name){
   $this->_constructor($name);
  }
 
  function _constructor($name){
   $this->name = $name;
  }

  function show(){
   printf("%s::%s\n", $this->get_class(), $this->name);
  }
 
  function get_class(){
     return get_class($this);
  }
}

class Container extends CoreObject{
 var $members;
 function Container($name){
   $this->_constructor($name);
 }
 
 function &add(&$ref){
   $this->members[] = $ref;
   return ($ref);
 }
 
  function show(){
   parent::show();
   foreach($this->members as $item){
     $item->show();
   }
 }
 function apply(){
 }
}

class Person extends CoreObject{
  function Person($name){
   $this->_constructor($name);
  }
}

class Family extends Container {

 var $members;
 function Family($name){
   $this->_constructor($name);
 }
}

echo "<pre>\n";

$family = new Family('my family');
$family->add(new Person('father'));
$family->add(new Person('mother'));
$family->add(new Person('girl'));
$family->add(new Person('boy'));

$family->show();

print_r($family);

?>
j237fl at hotmail dot com
26-Feb-2003 10:28
This code fragment does NOT seem to work.  It seems as though, the PHP engine when loading a class dosen't set the member vars FIRST, it seems as though it loads the BasicLife member vars before the Super Class methods.

class BasicLife extends Benefit{
   //MEMBER VAR simply REFERS TO METHOD OF SUPER CLASS
   var $simply = Benefit::get_simply();
   //MEMBER VAR me REFERS TO METHOD OF THIS->CLASS
   var $me = $this->get_me();

   function BasicLife(){//CONSTRUCTOR}

   function get_me(){}
  
}
kimh216 at hotmail dot com
29-Oct-2002 09:19
I made some change...
If you don't want to call parent::callClass('A') on the every function which have to call it's parent's method.

//multipleInheritance
class multipleInheritance
{
   function callClass($class_to_call)
   {
       return new $class_to_call();
   }
}

class A
{
   function insideA()
   {
       echo "I'm inside A!<br />";
   }
}

class B
{
   function insideB()
   {
       echo "I'm inside B!<br />";
   }
}

class C extends multipleInheritance
{
   var $a;
   var $b;

   function C()
   {
       //global $a,$b;
       $this->a = parent::callClass('A');//no effect
       $this->b = parent::callClass('B');//no effect
   }
   function insideA()
   {
       // global $a,$b;
       $this->a->insideA();
   }
   function insideB()
   {
       //global $a,$b;
       $this->b->insideB();
   }
   function insideC()
   {
       echo("I'm inside C!<BR>");
   }
}

$c = new C();
$c->insideA();
$c->insideB();
$c->insideC();
novaki at dcs dot vein dot hu
21-Oct-2002 01:13
inheritance from multiple parent classes:

// won't work
class myChildClass extends pClass1, pClass2 {

}

PHP uses aggregation for inheriting from multiple parents, which I think is a bit more complex. See the aggregate function.

(my personal oppinion is that the first method of multiple inheritance should also be imlemented, but I got the answer that it won't be fixed)
"inerte" is my hotmail.com username
28-Sep-2002 04:36
[Editor's note: For an alternative to multiple inheritance, see the dynamic binding via object aggregation in the corresponding section of the manual.]

Multiple Inheritance is not supported but it is easy to emulate it:

class multipleInheritance
{
   function callClass($class_to_call)
   {
       return new $class_to_call();
   }
}

class A
{
   function insideA()
   {
       echo "I'm inside A!<br />";
   }
}

class B
{

   function insideB()
   {
       echo "I'm inside B!<br />";
   }
}

class C extends multipleInheritance
{
   function insideC()
   {
       $a = parent::callClass('A');
       $a->insideA();
       $b = parent::callClass('B');
       $b->insideB();
   }
}

$c = new C();
$c->insideC();
---
This will succesfully echo:
I'm inside A!
I'm inside B!
schultz at rancon dot de
16-Aug-2002 01:37
class a {
  function samename(){
   echo 'a';
} }

class b extends a{
  function samename(){
   echo 'b';
  }
  function b(){
   a::samename();
   b::samename();
} }

$test_obj = new b();

Quiet confusing though, as this prints out 'ab'.
No need to create a new instance of a,
therefor both methods still exists with same name.
griffon9 at hotmail dot com
18-Jul-2002 07:42
Just to clarify something about inheritance. The following code :

class a
{  function call()
     {  $this->toto();
     }
    
     function toto()
     { echo('Toto of A');
     }
}
 
class b extends a
{  function toto()
   { echo('Toto of B');
   }
 }

$b=new b;
$b->call();

...will correctly display "toto of B" (that is, the function declared in the parent is correctly calling the redefined function in the child)
php_AT_undeen_DOT_com
12-Dec-2001 03:31
if the class B that extends class A does not have a constuctor function (i.e. a function named B), then the constructor function of A will be used instead, you don't need to make a constructor in B just to call the constructor of A.

For example:
class A
{
  function A()
   {
     echo "HEY! I'm A!\n";

   }
}
class B extends A
{
}
$b = new B();

produces the output:
HEY! I'm A!
bpotier at edreamers dot org
08-Nov-2001 04:08
Just one thing that may seem obvious but not mentionned in this page is that you need to include/require the file containing the parent class or else you'll get an error:
<?php
require(dirname(__FILE__).'/'.'myParent.php');
...
myChild extends myParent {
...
}
...
?>
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后退起点前进
类与对象（PHP 4）上一级构造函数