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Construtores e Destrutores

Construtores

__construct ([ mixed $args [, $... ]] ) : void

O PHP 5 permite aos desenvolvedores declararem métodos construtores para as classes. Classes que tem um método construtor, chamam o método a cada objeto recém criado, sendo apropriado para qualquer inicialização que o objeto necessite antes de ser utilizado.

Nota: Construtores pais não são chamados implicitamente se a classe filha define um construtor. Para executar o construtor da classe pai, uma chamada a parent::__construct() dentro do construtor da classe filha é necessária. Se a classe filha não definir um construtor, será herdado da classe pai como um método normal (se não foi declarado como privado).

Exemplo #1 Usando novos construtores unificados

<?php
class BaseClass {
   function 
__construct() {
       print 
"In BaseClass constructor\n";
   }
}

class 
SubClass extends BaseClass {
   function 
__construct() {
       
parent::__construct();
       print 
"In SubClass constructor\n";
   }
}

class 
OtherSubClass extends BaseClass {
    
// inherits BaseClass's constructor
}

// In BaseClass constructor
$obj = new BaseClass();

// In BaseClass constructor
// In SubClass constructor
$obj = new SubClass();

// In BaseClass constructor
$obj = new OtherSubClass();
?>

Para retrocompatibilidade com as versões do PHP 3 e 4, se o PHP não encontrar um __construct() para uma determinada classe, e a classe não herda um da classe, ele procurará pela função construtora à moda-antiga, que tenha o mesmo nome da classe. Efetivamente, isso significa que o único caso que pode gerar problemas de compatibilidade será quando a classe tiver um método chamado __construct() usado em semântica diferente.

Aviso

Construtores no estilo antigo se tornaram OBSOLETOS no PHP 7.0, e serão removidos em uma versão futura. Deve-se sempre utilizar __construct() em novos códigos.

Diferente de outros métodos, o PHP não irá gerar uma mensagem de erro de nível E_STRICT quando __construct() é sobrescrito com parâmetros diferentes do método __construct()da classe pai possuir.

A partir do PHP 5.3.3, métodos com o mesmo nome que o último elemento de uma classe com namespace não serão tratados como construtor. Esta modificação não afeta classe sem namespace.

Exemplo #2 Construtores em classes dentro de namespace

<?php
namespace Foo;
class 
Bar {
    public function 
Bar() {
        
// tratado como construtor no PHP 5.3.0-5.3.2
        // tratado como método comum a partir do PHP 5.3.3
    
}
}
?>

Destrutor

__destruct ( ) : void

O PHP 5 introduz um conceito de destrutor similar ao de outras linguagens orientadas a objeto, como C++. O método destrutor será chamado assim que todas as referências a um objeto particular forem removidas ou quando o objeto for explicitamente destruído ou qualquer ordem na sequência de encerramento.

Exemplo #3 Exemplo de Destrutor

<?php
class MyDestructableClass {
   function 
__construct() {
       print 
"In constructor\n";
       
$this->name "MyDestructableClass";
   }

   function 
__destruct() {
       print 
"Destroying " $this->name "\n";
   }
}

$obj = new MyDestructableClass();
?>

Como os construtores, destrutores da classe pai não serão chamados implicitamente pelo motor. Para executar o destrutor pai, deve-se fazer uma chamada explicita a parent::__destruct() no corpo do destrutor. Assim como construtores, uma classe filha pode herdar o destrutor pai caso não implemente um.

O destrutor será chamado mesmo se o script for terminado utilizando-se exit(). Chamar exit() em um destrutor irá impedir que as demais rotinas de encerramento executem.

Nota:

Destrutores chamados durante o encerramento da execução do script tem os cabeçalhos HTTP enviados. O diretório atual na fase de encerramento do script pode ser diferente em alguns SAPIs (e.g. Apache).

Nota:

Tentar disparar uma exceção em um destrutor (chamado no término do script), causará um erro fatal.

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User Contributed Notes 11 notes

up
105
david dot scourfield at llynfi dot co dot uk
9 years ago
Be aware of potential memory leaks caused by circular references within objects.  The PHP manual states "[t]he destructor method will be called as soon as all references to a particular object are removed" and this is precisely true: if two objects reference each other (or even if one object has a field that points to itself as in $this->foo = $this) then this reference will prevent the destructor being called even when there are no other references to the object at all.  The programmer can no longer access the objects, but they still stay in memory.

Consider the following example:

<?php

header
("Content-type: text/plain");

class
Foo {
   
   
/**
     * An indentifier
     * @var string
     */
   
private $name;
   
/**
     * A reference to another Foo object
     * @var Foo
     */
   
private $link;

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

    public function
setLink(Foo $link){
       
$this->link = $link;
    }

    public function
__destruct() {
        echo
'Destroying: ', $this->name, PHP_EOL;
    }
}

// create two Foo objects:
$foo = new Foo('Foo 1');
$bar = new Foo('Foo 2');

// make them point to each other
$foo->setLink($bar);
$bar->setLink($foo);

// destroy the global references to them
$foo = null;
$bar = null;

// we now have no way to access Foo 1 or Foo 2, so they OUGHT to be __destruct()ed
// but they are not, so we get a memory leak as they are still in memory.
//
// Uncomment the next line to see the difference when explicitly calling the GC:
// gc_collect_cycles();
//
// see also: http://www.php.net/manual/en/features.gc.php
//

// create two more Foo objects, but DO NOT set their internal Foo references
// so nothing except the vars $foo and $bar point to them:
$foo = new Foo('Foo 3');
$bar = new Foo('Foo 4');

// destroy the global references to them
$foo = null;
$bar = null;

// we now have no way to access Foo 3 or Foo 4 and as there are no more references
// to them anywhere, their __destruct() methods are automatically called here,
// BEFORE the next line is executed:

echo 'End of script', PHP_EOL;

?>

This will output:

Destroying: Foo 3
Destroying: Foo 4
End of script
Destroying: Foo 1
Destroying: Foo 2

But if we uncomment the gc_collect_cycles(); function call in the middle of the script, we get:

Destroying: Foo 2
Destroying: Foo 1
Destroying: Foo 3
Destroying: Foo 4
End of script

As may be desired.

NOTE: calling gc_collect_cycles() does have a speed overhead, so only use it if you feel you need to.
up
22
domger at freenet dot de
3 years ago
The __destruct magic method must be public.

public function __destruct()
{
    ;
}

The method will automatically be called externally to the instance.  Declaring __destruct as protected or private will result in a warning and the magic method will not be called.

Note: In PHP 5.3.10 i saw strange side effects while some Destructors were declared as protected.
up
15
spleen
12 years ago
It's always the easy things that get you -

Being new to OOP, it took me quite a while to figure out that there are TWO underscores in front of the word __construct.

It is __construct
Not _construct

Extremely obvious once you figure it out, but it can be sooo frustrating until you do.

I spent quite a bit of needless time debugging working code.

I even thought about it a few times, thinking it looked a little long in the examples, but at the time that just seemed silly(always thinking "oh somebody would have made that clear if it weren't just a regular underscore...")

All the manuals I looked at, all the tuturials I read, all the examples I browsed through  - not once did anybody mention this! 

(please don't tell me it's explained somewhere on this page and I just missed it,  you'll only add to my pain.)

I hope this helps somebody else!
up
6
prieler at abm dot at
13 years ago
i have written a quick example about the order of destructors and shutdown functions in php 5.2.1:

<?php
class destruction {
    var
$name;

    function
destruction($name) {
       
$this->name = $name;
       
register_shutdown_function(array(&$this, "shutdown"));
    }

    function
shutdown() {
        echo
'shutdown: '.$this->name."\n";
    }

    function
__destruct() {
        echo
'destruct: '.$this->name."\n";
    }
}

$a = new destruction('a: global 1');

function
test() {
   
$b = new destruction('b: func 1');
   
$c = new destruction('c: func 2');
}
test();

$d = new destruction('d: global 2');

?>

this will output:
shutdown: a: global 1
shutdown: b: func 1
shutdown: c: func 2
shutdown: d: global 2
destruct: b: func 1
destruct: c: func 2
destruct: d: global 2
destruct: a: global 1

conclusions:
destructors are always called on script end.
destructors are called in order of their "context": first functions, then global objects
objects in function context are deleted in order as they are set (older objects first).
objects in global context are deleted in reverse order (older objects last)

shutdown functions are called before the destructors.
shutdown functions are called in there "register" order. ;)

regards, J
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4
bolshun at mail dot ru
12 years ago
Ensuring that instance of some class will be available in destructor of some other class is easy: just keep a reference to that instance in this other class.
up
9
Per Persson
8 years ago
As of PHP 5.3.10 destructors are not run on shutdown caused by fatal errors.

For example:
<?php
class Logger
{
    protected
$rows = array();

    public function
__destruct()
    {
       
$this->save();
    }

    public function
log($row)
    {
       
$this->rows[] = $row;
    }

    public function
save()
    {
        echo
'<ul>';
        foreach (
$this->rows as $row)
        {
            echo
'<li>', $row, '</li>';
        }
        echo
'</ul>';
    }
}

$logger = new Logger;
$logger->log('Before');

$nonset->foo();

$logger->log('After');
?>

Without the $nonset->foo(); line, Before and After will both be printed, but with the line neither will be printed.

One can however register the destructor or another method as a shutdown function:
<?php
class Logger
{
    protected
$rows = array();

    public function
__construct()
    {
       
register_shutdown_function(array($this, '__destruct'));
    }
   
    public function
__destruct()
    {
       
$this->save();
    }
   
    public function
log($row)
    {
       
$this->rows[] = $row;
    }
   
    public function
save()
    {
        echo
'<ul>';
        foreach (
$this->rows as $row)
        {
            echo
'<li>', $row, '</li>';
        }
        echo
'</ul>';
    }
}

$logger = new Logger;
$logger->log('Before');

$nonset->foo();

$logger->log('After');
?>
Now Before will be printed, but not After, so you can see that a shutdown occurred after Before.
up
3
Yousef Ismaeil cliprz[At]gmail[Dot]com
7 years ago
<?php

/**
* a funny example Mobile class
*
* @author Yousef Ismaeil Cliprz[At]gmail[Dot]com
*/

class Mobile {

   
/**
     * Some device properties
     *
     * @var string
     * @access public
     */
   
public $deviceName,$deviceVersion,$deviceColor;
   
   
/**
     * Set some values for Mobile::properties
     *
     * @param string device name
     * @param string device version
     * @param string device color
     */
   
public function __construct ($name,$version,$color) {
       
$this->deviceName = $name;
       
$this->deviceVersion = $version;
       
$this->deviceColor = $color;
        echo
"The ".__CLASS__." class is stratup.<br /><br />";
    }
   
   
/**
     * Some Output
     *
     * @access public
     */
   
public function printOut () {
        echo
'I have a '.$this->deviceName
           
.' version '.$this->deviceVersion
           
.' my device color is : '.$this->deviceColor;
    }
   
   
/**
     * Umm only for example we will remove Mobile::$deviceName Hum not unset only to check how __destruct working
     *
     * @access public
     */
   
public function __destruct () {
       
$this->deviceName = 'Removed';
        echo
'<br /><br />Dumpping Mobile::deviceName to make sure its removed, Olay :';
       
var_dump($this->deviceName);
        echo
"<br />The ".__CLASS__." class is shutdown.";
    }

}

// Oh ya instance
$mob = new Mobile('iPhone','5','Black');

// print output
$mob->printOut();

?>

The Mobile class is stratup.

I have a iPhone version 5 my device color is : Black

Dumpping Mobile::deviceName to make sure its removed, Olay :
string 'Removed' (length=7)

The Mobile class is shutdown.
up
0
iwwp at outlook dot com
10 months ago
To better understand the __destrust method:

class A {
    protected $id;

    public function __construct($id)
    {
        $this->id = $id;
        echo "construct {$this->id}\n";
    }

    public function __destruct()
    {
        echo "destruct {$this->id}\n";
    }
}

$a = new A(1);
echo "-------------\n";
$aa = new A(2);
echo "=============\n";

The output content:

construct 1
-------------
construct 2
=============
destruct 2
destruct 1
up
1
Jonathon Hibbard
10 years ago
Please be aware of when using __destruct() in which you are unsetting variables...

Consider the following code:
<?php
class my_class {
  public
$error_reporting = false;

  function
__construct($error_reporting = false) {
   
$this->error_reporting = $error_reporting;
  }

  function
__destruct() {
    if(
$this->error_reporting === true) $this->show_report();
    unset(
$this->error_reporting);
  }
?>

The above will result in an error:
Notice: Undefined property: my_class::$error_reporting in my_class.php on line 10

It appears as though the variable will be unset BEFORE it actually can execute the if statement.  Removing the unset will fix this.  It's not needed anyways as PHP will release everything anyways, but just in case you run across this, you know why ;)
up
1
Reza Mahjourian
14 years ago
Peter has suggested using static methods to compensate for unavailability of multiple constructors in PHP.  This works fine for most purposes, but if you have a class hierarchy and want to delegate parts of initialization to the parent class, you can no longer use this scheme.  It is because unlike constructors, in a static method you need to do the instantiation yourself.  So if you call the parent static method, you will get an object of parent type which you can't continue to initialize with derived class fields.

Imagine you have an Employee class and a derived HourlyEmployee class and you want to be able to construct these objects out of some XML input too.

<?php
class Employee {
   public function
__construct($inName) {
      
$this->name = $inName;
   }

   public static function
constructFromDom($inDom)
   {
      
$name = $inDom->name;
       return new
Employee($name);
   }

   private
$name;
}

class
HourlyEmployee extends Employee {
   public function
__construct($inName, $inHourlyRate) {
      
parent::__construct($inName);
      
$this->hourlyRate = $inHourlyRate;
   }

   public static function
constructFromDom($inDom)
   {
      
// can't call parent::constructFromDom($inDom)
       // need to do all the work here again
      
$name = $inDom->name// increased coupling
      
$hourlyRate = $inDom->hourlyrate;
       return new
EmployeeHourly($name, $hourlyRate);
   }

   private
$hourlyRate;
}
?>

The only solution is to merge the two constructors in one by adding an optional $inDom parameter to every constructor.
up
0
david at synatree dot com
12 years ago
When a script is in the process of die()ing, you can't count on the order in which __destruct() will be called.

For a script I have been working on, I wanted to do transparent low-level encryption of any outgoing data.  To accomplish this, I used a global singleton class configured like this:

class EncryptedComms
{
    private $C;
    private $objs = array();
    private static $_me;
   
    public static function destroyAfter(&$obj)
    {
        self::getInstance()->objs[] =& $obj;
        /*
            Hopefully by forcing a reference to another object to exist
            inside this class, the referenced object will need to be destroyed
            before garbage collection can occur on this object.  This will force
            this object's destruct method to be fired AFTER the destructors of
            all the objects referenced here.
        */
    }
    public function __construct($key)
    {
            $this->C = new SimpleCrypt($key);
            ob_start(array($this,'getBuffer'));
    }
    public static function &getInstance($key=NULL)
    {
        if(!self::$_me && $key)
            self::$_me = new EncryptedComms($key);
        else
            return self::$_me;
    }
   
    public function __destruct()
    {
        ob_end_flush();
    }
   
    public function getBuffer($str)
    {
        return $this->C->encrypt($str);
    }

}

In this example, I tried to register other objects to always be destroyed just before this object.  Like this:

class A
{

public function __construct()
{
     EncryptedComms::destroyAfter($this);
}
}

One would think that the references to the objects contained in the singleton would be destroyed first, but this is not the case.  In fact, this won't work even if you reverse the paradigm and store a reference to EncryptedComms in every object you'd like to be destroyed before it.

In short, when a script die()s, there doesn't seem to be any way to predict the order in which the destructors will fire.
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