Clases y Objetos (PHP 5)
if you do this
<?php
$x = new b();
class b extends a {}
class a { }
?>
PHP will tell you "class b not found", because you've defined class b before a. However, the error tells you something different.... Got me a little confused :)
Las Bases
clase
Cada definición de clase empieza con la palabra "class", seguida por un nombre de clase, el cual puede ser cualquier nombre que no esté en la lista de palabras reservadas en PHP. Seguida por un par de llaves curvas, las cuales contienen la definición de los miembros de la clase y los métodos. Una seudo variable $this está disponible cuando un método es llamado dentro del contexto de un objeto. $this es una referencia al objeto que se está usando (usualmente el objeto al que el método pertenece, pero puede ser otro objeto, si un método es llamado estáticamente desde el contexto de un objeto secundario). Este es ilustrado en los siguientes ejemplos:
Example #1 $this variable in object-oriented language
<?php
class A
{
function foo()
{
if (isset($this)) {
echo '$this is defined (';
echo get_class($this);
echo ")\n";
} else {
echo "\$this is not defined.\n";
}
}
}
class B
{
function bar()
{
A::foo();
}
}
$a = new A();
$a->foo();
A::foo();
$b = new B();
$b->bar();
B::bar();
?>
El resultado del ejemplo seria:
$this is defined (a) $this is not defined. $this is defined (b) $this is not defined.
Example #2 Definición simple de una clase
<?php
class SimpleClass
{
// member declaration
public $var = 'a default value';
// method declaration
public function displayVar() {
echo $this->var;
}
}
?>
El valor por defecto debe ser una expresión constante, por ejemplo no una variable, ni un miembro de clase o llamada a función.
Example #3 Valor por defecto de un miembro de clase
<?php
class SimpleClass
{
// invalid member declarations:
public $var1 = 'hello '.'world';
public $var2 = <<<EOD
hello world
EOD;
public $var3 = 1+2;
public $var4 = self::myStaticMethod();
public $var5 = $myVar;
// valid member declarations:
public $var6 = myConstant;
public $var7 = self::classConstant;
public $var8 = array(true, false);
}
?>
Note: Hay algunas funciones útiles para el manejo de clases y objectos. Tal vez quieras checar las Funciones para Clases/Objectos.
Nuevo objeto
Para crear una instancia de un objeto, un nuevo objeto debe ser creado y asignado a una variable. Un objeto siempre será asignado cuando se crea un objeto nuevo a menos que el objeto tenga un constructor definido que arroje una excepción en caso de error. Las clases deben ser definidas antes de crear las instancias (y en algunos casos esto es un requerido).
Example #4 Creando una instancia
<?php
$instance = new SimpleClass()
?>
Cuando se asigna una instancia de un objeto previamente creado a una nueva variable, la nueva variable accesará la misma instancia que la del objeto a la que fue asignada. Este comportamiento es el mismo cuando se pasan instancias a una función. Una nueva instancia de un objeto previamente creado puede ser hecho clonandolo.
Example #5 Asignación del Objeto
<?php
$assigned = $instance;
$reference =& $instance;
$instance->var = '$assigned will have this value';
$instance = null; // $instance and $reference become null
var_dump($instance);
var_dump($reference);
var_dump($assigned);
?>
El resultado del ejemplo seria:
NULL NULL object(SimpleClass)#1 (1) { ["var"]=> string(30) "$assigned will have this value" }
Extendiendo objetos
Una clase puede heredar métodos y miembros de otra clase usando la palabra 'extends' en la declaración. No es posible extender de múltiples clases, una clase puede heredar solo de una clase base.
Los métodos heredados y sus miembros pueden ser evitados, redeclarandolos con el mismo nombre con el que los definió la clase padre, a menos que la clase padre haya definido un método como final. Es posible accesar a los métodos o miembros redeclarados haciendo referencia a ellos con parent::
Example #6 Herencia simple de una Clase
<?php
class ExtendClass extends SimpleClass
{
// Redefine the parent method
function displayVar()
{
echo "Extending class\n";
parent::displayVar();
}
}
$extended = new ExtendClass();
$extended->displayVar();
?>
El resultado del ejemplo seria:
Extending class a default value
add a note
User Contributed NotesLas Bases
info at youwanttoremovethisvakantiebaas dot nl
21-Apr-2008 12:40
21-Apr-2008 12:40
david dot schueler at tel-billig dot de
15-Feb-2008 03:16
15-Feb-2008 03:16
If you just want to create a new object that extends another object and you want to copy all variables from the father object, you may use this piece of code:
<?php
$father =& new father();
$father->a_var = "Hello World.";
$son = new son($event);
$son->say_hello();
class father {
public $a_var;
}
class son extends father {
public function __construct($father_class) {
foreach ($father_class as $variable=>$value) {
$this->$variable = $value;
}
}
public function say_hello() {
echo "Son says: ".$this->a_var;
}
}
?>
This outputs:
Son says: Hello World.
So you dont have to clone the entire object to get the contents of the variables from the father object.
aaron at thatone dot com
16-Dec-2007 03:46
16-Dec-2007 03:46
I was confused at first about object assignment, because it's not quite the same as normal assignment or assignment by reference. But I think I've figured out what's going on.
First, think of variables in PHP as data slots. Each one is a name that points to a data slot that can hold a value that is one of the basic data types: a number, a string, a boolean, etc. When you create a reference, you are making a second name that points at the same data slot. When you assign one variable to another, you are copying the contents of one data slot to another data slot.
Now, the trick is that object instances are not like the basic data types. They cannot be held in the data slots directly. Instead, an object's "handle" goes in the data slot. This is an identifier that points at one particular instance of an obect. So, the object handle, although not directly visible to the programmer, is one of the basic datatypes.
What makes this tricky is that when you take a variable which holds an object handle, and you assign it to another variable, that other variable gets a copy of the same object handle. This means that both variables can change the state of the same object instance. But they are not references, so if one of the variables is assigned a new value, it does not affect the other variable.
<?php
// Assignment of an object
Class Object{
public $foo="bar";
};
$objectVar = new Object();
$reference =& $objectVar;
$assignment = $objectVar
//
// $objectVar --->+---------+
// |(handle1)----+
// $reference --->+---------+ |
// |
// +---------+ |
// $assignment -->|(handle1)----+
// +---------+ |
// |
// v
// Object(1):foo="bar"
//
?>
$assignment has a different data slot from $objectVar, but its data slot holds a handle to the same object. This makes it behave in some ways like a reference. If you use the variable $objectVar to change the state of the Object instance, those changes also show up under $assignment, because it is pointing at that same Object instance.
<?php
$objectVar->foo = "qux";
print_r( $objectVar );
print_r( $reference );
print_r( $assignment );
//
// $objectVar --->+---------+
// |(handle1)----+
// $reference --->+---------+ |
// |
// +---------+ |
// $assignment -->|(handle1)----+
// +---------+ |
// |
// v
// Object(1):foo="qux"
//
?>
But it is not exactly the same as a reference. If you null out $objectVar, you replace the handle in its data slot with NULL. This means that $reference, which points at the same data slot, will also be NULL. But $assignment, which is a different data slot, will still hold its copy of the handle to the Object instance, so it will not be NULL.
<?php
$objectVar = null;
print_r($objectVar);
print_r($reference);
print_r($assignment);
//
// $objectVar --->+---------+
// | NULL |
// $reference --->+---------+
//
// +---------+
// $assignment -->|(handle1)----+
// +---------+ |
// |
// v
// Object(1):foo="qux"
?>
alexandre at dontspamme dot gaigalas dot net
07-Dec-2007 05:44
07-Dec-2007 05:44
In theory, it is possible to workaround and do multiple inheritance in PHP5 using magic methods. An example:
<?php
class foo
{
function fooMethod($a)
{
echo 'fooMethod called with argument: '.$a.'<br>';
}
}
class bar
{
function barMethod($b)
{
echo 'barMethod called with arguments: '.$b.'<br>';
}
}
class multiple
{
private $_parents = array();
function __construct()
{
$this->_parents[] = new foo;
$this->_parents[] = new bar;
}
function __call($m,$a)
{
foreach ($this->_parents as $extended)
{
if (method_exists($extended, $m)) {
return call_user_func_array(array($extended,$m),$a);
}
}
}
}
$test = new multiple;
$test->fooMethod('test');
$test->barMethod('me');
?>
This, ovbiously needs some modifications to work with static methods (using Reflection API) and obviously don't work with private methods.
alan at alan-ng dot net
10-Oct-2007 06:41
10-Oct-2007 06:41
The following odd behavior happens in php version 5.1.4 (and presumably some other versions) that does not happen in php version 5.2.1 (and possibly other versions > 5.1.4).
<?php
$_SESSION['instance']=...;
$instance=new SomeClass;
?>
The second line will not only create the $instance object successfully, it will also modify the value of $_SESSION['instance']!
The workaround I arrived at, after trial and error, was to avoid using object names which match a $_SESSION array key.
This is not intended to be a bug report, since it was apparently fixed by version 5.2.1, so it's just a workaround suggestion.
mep_eisen at web dot de
10-Aug-2007 03:06
10-Aug-2007 03:06
referring to steven's post:
****
Perhaps this is because =& statements join the 2 variable names in the symbol table, whereas = statements applied to objects simply create a new independent entry in the symbol table that simply points to the same location as other entries. I don't know for sure - I don't think this behavior is documented in the PHP manual, so perhaps somebody with more knowledge of PHP's internals can clarify what is going on.
****
lets talk about
a =& b;
b = c;
PHP internally marks a to be a reference to b. If You reassign b PHP does not update a. But if you access a once more PHP looks at the current value of b (now containing c).
Both statements (a=b and a=&b) seem to do the same but they don't. However this changed for objects from PHP4 to PHP5. Where PHP4 needed this operator to avoid object cloning, PHP5 does not need it.
It is explained in chapter 21 (References Explained). It's important to understand that a becomes a reference and the following code will not modify b:
a =& b;
a =& c;
Dan Dascalescu
27-Oct-2006 08:00
27-Oct-2006 08:00
If E_STRICT is enabled, the first example will generate the following error (and a few others akin to it):
Non-static method A::foo() should not be called statically on line 26
The example should have explicitly declared the methods foo() and bar() as static:
class A
{
static function foo()
{
...