Triedy a Objekty (PHP 5)
@info -- 20-April
This is because you requested class "b" before defining it, not because you defined class "b" before "a". It doesn't make a difference which class you define first.
Základy
class
Ka¾dá definícia triedy zaèína kµúèovým slovom class nasledované názvom triedy, ktorý je µubovoµný, s výnimkou rezervovaných slov v PHP. Nasleduje pár zlo¾ených zátvoriek, ktoré obsahujú definíciu vlastností a metód triedy. V ka¾dej metóde, okrem statických, je dostupná pseudo premenná $this. $this je referencia na tú istú in¹tanciu, z ktorej bola metóda volaná.
Example#1 Jednoduchá definícia triedy
<?php
class SimpleClass
{
// member declaration
public $var = 'a default value';
// method declaration
public function displayVar() {
echo $this->var;
}
}
?>
new
Na vytvorenie in¹tancie objektu, musí by» vytvorený nový objekt a priradený premennej. Ak sa vytvára nový objekt, v¾dy je priradený premennej, s jedinou výnimkou, a to ak objekt obsahuje kon¹truktor, v ktorom je definovaná výnimka, vracajúca chybu.
Example#2 Vytváranie in¹tancie
<?php
$instance = new SimpleClass()
?>
Ak priraïujeme u¾ existujúcu in¹tanciu objektu novej premennej, tak nová premenná bude pristupova» k tej istej in¹tancii ako premenná, ktorej bol pôvodný objekt priradený. Toto isté správanie je aj pri predávaní in¹tancie ako parametru funkcii. Nová in¹tancia u¾ existujúceho objektu mo¾e by» vytvorená klonovaním.
Example#3 Priraïovanie objektu
<?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);
?>
Výstup horeuvedeného príkladu bude:
NULL NULL object(SimpleClass)#1 (1) { ["var"]=> string(30) "$assigned will have this value" }
extends
Trieda mô¾e zdedi» metódy a vlastnosti inej triedy pou¾itím kµúèového slova extends v deklarácii. Nie je mo¾né dedenie z viacerých tried, trieda mô¾e dedi» iba z jednej základnej triedy.
Zdedené metódy a vlastnosti mô¾u by» nahradené, s jednou výnimkou, a to, ak je v rodièovskej triede definovaná metóda ako final, opakovanou deklaráciou s tým istým menom metódy, aké bolo pou¾ité v rodièovskej triede. Je taktie¾ mo¾né pristupova» k nahradeným metódam alebo vlastnostiam rodièovskej triedy, odkazovaním sa pomocou parent::
Example#4 Jednoduché dedenie triedy
<?php
class ExtendClass extends SimpleClass
{
// Redefine the parent method
function displayVar()
{
echo "Extending class\n";
parent::displayVar();
}
}
$extended = new ExtendClass();
$extended->displayVar();
?>
Výstup horeuvedeného príkladu bude:
Extending class a default value
add a note
User Contributed NotesZáklady
ashraf dot samhouri at hotmail dot com
24-May-2008 06:35
24-May-2008 06:35
info at youwanttoremovethisvakantiebaas dot nl
20-Apr-2008 03:40
20-Apr-2008 03:40
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 :)
david dot schueler at tel-billig dot de
15-Feb-2008 06:16
15-Feb-2008 06:16
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
15-Dec-2007 06:46
15-Dec-2007 06:46
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"
?>
alan at alan-ng dot net
09-Oct-2007 09:41
09-Oct-2007 09:41
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 06:06
10-Aug-2007 06:06
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
26-Oct-2006 11:00
26-Oct-2006 11:00
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()
{
...