构造函数 void __construct ([ mixed $args [, $… ]] ) PHP 5 允行开发者在一个类中定义一个方法作为构造函数。具有构造函数的类会在每次创建对象时先调用此方法，所以非常适合在使用对象之前做一些初始化工作。

Note: 如果子类中定义了构造函数则不会暗中调用其父类的构造函数。要执行父类的构造函数，需要在子类的构造函数中调用 parent::__construct()。

Example#1 使用新标准的构造函数

class BaseClass {

function __construct() { print “In BaseClass constructor\n”; } }

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

$obj = new BaseClass(); $obj = new SubClass(); ?> 为了实现向后兼容性，如果 PHP 5 在类中找不到 __construct() 函数，它就会尝试寻找旧式的构造函数，也就是和类同名的函数。因此唯一会产生兼容性问题的情况是：类中已有一个名为 __construct() 的方法，但它却又不是构造函数。

析构函数 void __destruct ( void ) PHP 5 引入了析构函数的概念，这类似于其它面向对象的语言，如 C++。析构函数会在到某个对象的所有引用都被删除或者当对象被显式销毁时执行。

Example#2 析构函数示例

class MyDestructableClass {

function __construct() { print “In constructor\n”; $this->name = “MyDestructableClass”; }

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

$obj = new MyDestructableClass(); ?> 和构造函数一样，父类的析构函数不会被引擎暗中调用。要执行父类的析构函数，必须在子类的析构函数体中显式调用 parent::__destruct()。

Note: 析构函数在脚本关闭时调用，此时所有的头信息已经发出。

Note: 试图在析构函数中抛出一个异常会导致致命错误。

访问控制 自动加载对象 ——————————————————————————– Last updated: Sun, 25 Nov 2007

add a note User Contributed Notes 构造函数和析构函数 Jeffrey 09-Oct-2008 02:04 Constructor Simplicity

If your class DOES CONTAIN instance members (variables) that need to be set, then your class needs to be initialized… and you should use __construct() to do that.

class MyClassA {

public $data1, $data2;

public function __construct($mcd1, $mcd2) { $this->data1 = $mcd1; // INITIALIZE $data1 $this->data2 = $mcd2; // INITIALIZE $data2 } }

$obj1 = new MyClassA(“Hello”, “World!”); // INSTANTIATE MyClassA $d1 = $obj1->data1; $d2 = $obj1->data2; ?>

If your class DOES NOT CONTAIN instance members or you DO NOT want to instantiate it, then there is no reason to initialize it or use __construct().

class MyClassB {

const DATA1 = “Hello”; public static $data2 = “World!”; }

$obj1 = new MyClassB(); // INSTANTIATE MyClassB – NO error. $d1 = $obj1::DATA1; // ERROR $d2 = $obj1::data2; // ERROR $d1 = MyClassB::DATA1; // ok $d2 = MyClassB::$data2; // ok ?>

The fact that $obj1 is useless and cannot be used as a reference, is further evidence that MyClassB objects should not be instantiated. NOTICE that MyClassB does not use private members or functions to make it behave that way. Rather, it is the collective nature of all the class members + what ISN’T there. Anonymous 09-Sep-2008 05:42 USE PARENT::CONSTRUCT() to exploit POLYMORPHISM POWERS

Since we are still in the __construct and __destruct section, alot of emphasis has been on __destruct – which I know nothing about. But I would like to show the power of parent::__construct for use with PHP’s OOP polymorphic behavior (you’ll see what this is very quickly).

In my example, I have created a fairly robust base class that does everything that all subclasses need to do. Here’s the base class def.

/*

• Animal.php
• This class holds all data, and defines all functions that all
• subclass extensions need to use.

*/ abstract class Animal { public $type; public $name; public $sound;

/*

• called by Dog, Cat, Bird, etc. */ public function __construct($aType, $aName, $aSound) { $this->type = $aType; $this->name = $aName; $this->sound = $aSound; }

/*

• define the sorting rules – we will sort all Animals by name. */ public static function compare($a, $b) { if($a->name < $b->name) return -1; else if($a->name == $b->name) return 0; else return 1; }

/*

• a String representation for all Animals. */ public function __toString() { return “$this->name the $this->type goes $this->sound”; } }

?>

Trying to instantiate an object of type Animal will not work…

$myPet = new Animal(“Parrot”, “Captain Jack”, “Kaaawww!”); // throws Fatal Error: cannot instantiate abstract class Animal.

Declaring Animal as abstract is like killing two birds with one stone. 1. We stop it from being instantiated – which means we do not need a private __construct() or a static getInstance() method, and 2. We can use it for polymorphic behavior. In our case here, that means “__construct”, “__toString” and “compare” will be called for all subclasses of Animal that have not defined their own implementations.

The following subclasses use parent::__construct(), which sends all new data to Animal. Our Animal class stores this data and defines functions for polymorphism to work… and the best part is, it keeps our subclass defs super short and even sweeter.

class Dog extends Animal{

public function __construct($name){ parent::__construct(“Dog”, $name, “woof!”); } }

class Cat extends Animal{ public function __construct($name){ parent::__construct(“Cat”, $name, “meeoow!”); } }

class Bird extends Animal{ public function __construct($name){ parent::__construct(“Bird”, $name, “chirp chirp!!”); } }

# create a PHP Array and initialize it with Animal objects $animals = array( new Dog(“Fido”), new Bird(“Celeste”), new Cat(“Pussy”), new Dog(“Brad”), new Bird(“Kiki”), new Cat(“Abraham”), new Dog(“Jawbone”) );

# sort $animals with PHP’s usort – calls Animal::compare() many many times. usort($animals, array(“Animal”, “compare”));

# print out the sorted results – calls Animal->__toString(). foreach($animals as $animal) echo “$animal \n”;

?>

The results are “sorted by name” and “printed” by the Animal class:

Abraham the Cat goes meeoow! Brad the Dog goes woof! Celeste the Bird goes chirp chirp!! Fido the Dog goes woof! Jawbone the Dog goes woof! Kiki the Bird goes chirp chirp!! Pussy the Cat goes meeoow!

Using parent::__construct() in a subclass and a super smart base class, gives your child objects a headstart in life, by alleviating them from having to define or handle several error and exception routines that they have no control over.

Notice how subclass definitions are really short – no variables or functions at all, and there is no private __construct() method anywhere? Notice how objects of type Dog, Cat, and Bird are all sorted by our base class Animal? All the class definitions above address several issues (keeping objects from being instantiated) and enforces the desired, consistent, and reliable behavior everytime… with the least amount of code. In addition, new extenstions can easily be created. Each subclass is now super easy to redefine or even extend… now that you can see a way to do it. KK 16-May-2008 01:57 I ran into an interesting (and subtle) code error while porting some code to PHP 5.2.5 from PHP 4.4.8 that I think illustrates a noteworthy semantic.

I have a hierarchy of classes with both styles of constructors but where one in the middle was missing the __construct() function (it just had the old-style one that called the (nonexistent) __construct()). It worked fine in PHP4 but caused an endless loop (and stack overflow) in PHP5. I believe what happened is that in PHP4 the old-style constructor was not called, but in PHP5 it was (due to the “emulation” of PHP4), and since _construct() wasn’t defined for that class, the call to $this->__construct() caused a looping call to the original (lowest child) constructor. bolshun at mail dot ru 16-Apr-2008 06:13 Ensuring that instance of some class will be available in destructor of some other class is easy: just keep a reference to that instance in this other class. ashnazg at php dot net 12-Apr-2008 10:26 Since my last note, I’ve been instructed to _NEVER_ call “unset($this)”, never ever ever. Since my previous testing of behavior did not explicitly show me that it was indeed necessary, I’m inclined to trust those telling me not to do it. ashnazg at php dot net 28-Feb-2008 02:39 While experimenting with destructs and unsets in relation to memory usage, I found what seems to be one useful way to predict when __destruct() gets called… call it manually yourself.

I had previously assumed that explicitly calling unset($foo) would cause $foo->__destruct() to run implicitly, but that was not the behavior I saw (php 5.2.5). The destructors weren’t running until the script ended, even though I was calling unset($foo) in the middle of my script. So, having $foo->__destruct() unset all of $foo’s component objects was not helping my memory usage since my explicit unset($foo) was _not_ triggering $foo->__destruct()’s cleanup steps.

Interestingly, what _did_ appear to happen is that calling unset($bar) from inside a destructor like $foo->__destruct() _DID_ cause $bar->__destruct() to be implicitly executed. Perhaps this is because $bar has a “parent” reference of $foo whereas $foo does not, and the object destruction behaves differently… I don’t know.

Lastly, even after explicitly calling $foo->__destruct() (even when it had “unset($this);” inside it), the reference to the $foo object remained visible. I had to still explicitly call unset($foo) to get rid of it.

So, my advice based on the behavior I saw in my experiments: – always unset($bar) your class’s component objects from inside that class’s __destruct() method; no explicit component destructor calls seem to be required other than those unsets, though. – always explicitly call $foo->_destruct() in your code that _uses your class – always explicitly follow $foo->__destruct() with unset($foo).

This seems to be the best cleanup approach to take. Just for my own sanity, I also will always keep an unset($this) at the end of each __destruct() method. nerdystudmuffin at gmail dot com 09-Jan-2008 11:11 Correction to the previous poster about non public constructors. If I wanted to implement Singleton design pattern where I would only want one instance of the class I would want to prevent instantiation of the class from outside of the class by making the constructor private. An example follows:

class Foo {

private static $instance;

private __construct() { // Do stuff }

public static getInstance() {

if (!isset(self::$instance)) { $c = __CLASS__; $instance = new $c; }

return self::$instance; }

public function sayHello() { echo “Hello World!!”; }

}

$bar = Foo::getInstance();

// Prints ‘Hello World’ on the screen. $bar -> sayHello(); david at synatree dot com 29-Dec-2007 01:26 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. Typer85 at gmail dot com 21-Dec-2007 09:33 I mentioned in my post below this one that the solution is not perfect and has some flaws.

Because my note was too big and was not allowed to be submitted, I had to write a new note here containing the flaws. If an editor sees this, please combine both notes to allow for easier reading.

– This solution assumes that Class B and Class C extend or build upon Class AAbstract. If Class B or Class C extend from each other or from Class A, the original problem will be encountered again.

– There will be extra overhead in including an extra file … the dummy Class AAbstract. Typer85 at gmail dot com 21-Dec-2007 09:31 In regards to a Class Constructor visibility …

I too was having the same problem with Class Constructor visibility, in which I had one Class that was extended by several other Classes. The problem that I encountered was in one of the Child Classes, I wanted a weaker visibility. Consider the following example:

class A {

// Public Constructor … // No Problems Here.

public function __construct( ) {

}

// Class Function.

public function functionA( ) {

} }

class B extends A {

// Public Constructor … // Same As Parent Class … // Again No Problems Here.

public function __construct( ) {

}

// Class Function.

public function functionB( ) {

} }

class C extends A {

// Private Constructor … // Weaker Then Parent Class … // PHP Will Throw A Fatal Error.

private function __construct( ) {

}

// Class Function.

public function functionC( ) {

} }

?>

Nothing new in the above example that we have not seen before. My solution to solve this problem?

Create an Abstract Class with all the functionality of Class A. Make its Class Constructor have a visibility of Protected, then extend each of the three Classes above from that Abstract Class. In a way, the Abstract Class acts as a dummy Class to get rid of the visibility problem:

abstract class AAbstract {

// Protected Constructor … // Abstract Class Can Not Be Created Anyway … // No Problems Here.

protected function __construct( ) {

}

// Class Function … // Originally In Class A … // Which Was Used As A Super Class.

public function functionA( ) {

} }

class A extends AAbstract {

// Public Constructor … // Stronger Than Parent Class … // Again No Problems Here.

public function __construct( ) {

// By Moving All The Functionality Of // Class A To Class AAbstract Class A // Will Automatically Inherit All Of // Its Functionality. The Only Thing // Left To Do Is To Create A Constructor // Which Calls Class AABstract’s Constructor // To Mimic Similar Behavior.

parent::__construct( ); }

// No Need To Redeclare functionA( ) // Since It Was Moved To Class // AAbstract. }

class B extends AAbstract {

// Public Constructor … // Stronger Than Parent Class … // Again No Problems Here.

public function __construct( ) {

parent::__construct( ); }

// Class Function … // Specific To This Class.

public function functionB( ) {

} }

class C extends AAbstract {

// Protected Constructor … // Same As Parent Class … // Again No Problems Here.

protected function __construct( ) {

}

// Class Function … // Specific To This Class.

public function functionC( ) {

} }

?>

As you can see the problem is more or less fixed. Class AAbstract acts a dummy class, containing all the original functionality of Class A. But because it has a protected Constructor, in order to make its functionality available, Class A is redeclared as Child Class with the only difference of it having a public Constructor that automatically calls the Parent Constructor.

Notice that Class B does not extend from Class A but also from Class AAbstract! If I wanted to change Class B’s Constructor to protected, I can easily do it! Notice that an extra Method was added to Class B … this is because Class B has extra functionality specific to itself. Same applies to Class C.

Why don’t Class B and Class C extend from Class A? Because Class A has a public Constructor, which pretty much defies the point of this solution.

This solution is not perfect however and has some flaws.

Good Luck, randallgirard at hotmail dot com 26-Oct-2007 10:17 I’ve been working on a project for a while now, and for example in my DB handler I wanted to load var’s to the objects late; However, without doing it manually on the object itself but through a single static call. For other reasons, in my Sessions I then wanted to end them early without registering a shutdown routine for every possible session. Hence, I needed a way to track EVERY instance or object of the class.

(Of course in a normal website design, this would be overly coded. It’s more of a framework for a CMS, Daemon service, anything I wanna create etc…)

class example {

public static $OBJS = array(); public $id;

public function __construct() { example::$OBJS[] = $this; # I actually use a ‘lastKey’ routine here, but in this example the following should work: $this->id = key( array_slice(example::$OBJS, count(example::$OBJS) -1, 1, true) ); }

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

# unset the tracked object private function destruct() { if ( isset(example::$OBJS[$this->id]) ) unset(example::$OBJS[$this->id]); }

# public shutdown routine public function close() { if ( isset($this->destructed) ) return; # … $this->destruct(); $this->destructed = true; }

}

?>

You could then also use register_shutdown_function(…) to call a static class method, which loops through ALL the objects and calls the $obj->close routine.

# register a shutdown function which triggers before destructors (hence when such results are desired for a Session class, etc)

register_shutdown_function(“example::destructors”);

class example {

# … see above code example

# static class method to destruct all objects public static function destructors() { foreach ( example::$OBJS as $obj ) $obj->destruct(); }

}

This will probably give some people new ideas, and other will probably be confused. Happy coding…

NOTE: I didn’t test the code(s) above. They were a rewritten example from my frameworks.

?> James Laver 25-Oct-2007 02:45 I recently found, while implementing a database backed session class, that PHP has an apparently less than desirably structured destruction order. Basically, my session class which would have saved when destructed, was being destructed after one of the classes it depends on. Apparently we cannot, therefore depend on PHP to use reverse initialisation order.

My solution was to use register_shutdown_function, which is called before any objects are killed on script end.

Quick example (if you’re going to use it, I recommend tidying up the code somewhat) : class Session

function __construct() { //Initialise the session here…

//Register the shutdown function register_shutdown_function(array($this,”save”)); }

function save() { //Save it here }

function __destruct() { //Persisting the session here will not work. } ?> magus do t xion a t g mail d ot c o m 11-Oct-2007 12:20 Looking through the notes I noticed a few people expressing concern that PHP5 does not support multiple constructors…

Here is an example of a method that I use which seems to work fine:

class Example { function __construct() { echo “do some basic stuff here”; }

function Example($arg) { echo $arg; } }

You then can call with or without arguments without having notices and/or warnings thrown at you… Of course this is limited but if you don’t need something complex this can help to get the job done in some situations. I believe you could also add arguments to the __construct() function and as long as it is different than Example() ‘s args you would be fine. Although I have yet to test this. prieler at abm dot at 27-Jul-2007 12:42 i have written a quick example about the order of destructors and shutdown functions in php 5.2.1:

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 fredrik at rambris dot com 16-Jul-2007 06:59 The fact that class names are case-insensitive in PHP5 also applies to constructors. Make sure you don’t have any functions named like the class *at all*.

This has bitten me a few times.

class Example extends Base

{ function example() { echo “This gets called”; } }

class Base { function __construct() { echo “Not this”; } }

?> theubaz at gmail dot com 11-Jul-2007 04:35 What you could do is write the constructor without any declared arguments, then iterate through the arguments given and check their types/values to determine what other function to use as the constructor. soapthgr8 at gmail dot com 06-Jul-2007 07:46 This is just to clarify that the Singleton pattern is a bit more complex than just making the constructor private. It also involves caching an instance of the object and always returning the cached value. So, in the previous example, the getNewInstance() function would undermine the intent of the Singleton pattern. Instead you would just need a getInstance() function, like so.

class A {

// cached instance private static oInst = null;

/**

• Prevent an object from being constructed. / private function __construct( ) {} /*
• Function to return the instance of this class. */ public static function getInstance( ) { if (is_null(self::$oInst)) { self::$oInst = new A( ); } return self::$oInst; } } ?> Typer85 at gmail dot com 03-Jun-2007 07:16 I am not sure if the following is known or not, but here goes.

I am sure most are aware of the concept of making a Class Constructor private to prevent an Object of that Class from being created as follows:

class A

{ /**

• Prevent An Object From Being Constructed. */ private function __construct( ) {

} }

?>

In some code, if I try to do the following:

$Obj = new A( );

?>

PHP will fail with a fatal error. This is useful when creating Classes that are composed of only static functions or in a more advanced example, when applying the Singleton Pattern to a Class design.

However what is not properly documented in this manual but is pretty simple to note, especially if you read the notes regarding the Singleton Pattern, is that you can create an Object of a Class that has a private Constructor from within the Class itself.

Confused … so am I, so allow me to provide a visual example as follows:

class A

{ /**

• Prevent An Object From Being Constructed. */ private function __construct( ) {

} /**

• Function To Return An Instance Of This Class. */ public static function getNewInstance( ) {

return new A( ); } }

?>

In some code, if I try to do the following:

$Obj = A::getNewInstance( );

?>

PHP in this case will not fail with a fatal error and instead the variable ‘$Obj’ becomes an Object with an instance of Class ‘A’.

This is a simple example and pretty advanced things can be done using this method. I am sure advanced developers are aware of this so this is just a little note going out to new PHP developers. Peter Molnar 17-May-2007 08:32 The manual says: “Destructor is called during the script shutdown so headers are always already sent.”

This is obviously not true. If you instantiate a class in a function or class method, but it is not returned by the method, nor is it saved in a global or object member variable, the object is cleaned up, and it’s destructor is called. This can of course occur before anything is printed or sent to the client. frederic dot barboteu at laposte dot net 14-Apr-2007 07:11 The manual says: “Like constructors, parent destructors will not be called implicitly by the engine.”

This is true ONLY when a __destruct() function has been defined by the child class. If no __destruct() function exists in the child class, the parent’s one will be implicitly executed.

So be carefull if you have some ancestor executing a particular task in its __destruct() function, an you plan its childs to execute it or not, wether you include “parent::__destruct()” or not. If you want the child not to execute its parent __destruct() function, you must ensure that it has its own __destruct() function, even if empty. Then the parent’s one will not be executed.

This can be verified with the following code:

class AncestorClass { function __destruct() { echo ‘ AncestorClass: destructing ‘.get_class($this); } }

class ParentDestructClass extends AncestorClass { function __destruct() { echo ‘ParentDestructClass: destructing itself’; parent::__destruct(); } }

class EmptyDestructClass extends AncestorClass { function __destruct() { echo ‘EmptyDestructClass: destructing itself’; } }

class NoDestructClass extends AncestorClass { } #— echo ‘

‘; $p=new ParentDestructClass(); unset($p); echo ‘

‘; $e=new EmptyDestructClass(); unset($e); echo ‘

‘; $n=new NoDestructClass(); unset($n); echo ‘

‘; ?> which displays: — ParentDestructClass: destructing itself AncestorClass: destructing ParentDestructClass — EmptyDestructClass: destructing itself —

AncestorClass: destructing NoDestructClass — robert at lax-berlin dot de 12-Nov-2006 12:48 In response to prauscher at gmx dot net:

As tcknetwork wrote earlier, if you try to access a file from a destructor, you have to be aware that you are probably in the webservers root directory, because the destructor already “forgot” what you working directory was. If you try to write a file there, you will probably have no permission to do so. Read tcknetwork’s post for a solution. maniac_warlord at web dot de 04-Nov-2006 04:50 as reported in bug 34206 the working dir is changed to the server root the best workaround is class Foo

{ public function bar() { $this->_cwd = getcwd(); }

public function __destruct() { chdir($this->_cwd); } } ?> Be aware the booty trap! http://bugs.php.net/bug.php?id=34206 phaxius 01-Nov-2006 09:23 Hello, I’ve been messing with php for about a week but am learning a lot. It occurred to me that it might be necessary in some cases to write a class that takes a variable number of arguments. After some experimentation, this example was formed:

class variableArgs{ public $a = array(); protected $numOfArgs; public function __construct() { $numOfArgs=func_num_args(); if(func_num_args()==0) { $numOfArgs=1; $a[0]=’No arguments passed’; $this->Arg[0]=$a[0]; } else for($i=0; $i { $a[$i]=func_get_arg($i); $this->Arg[$i]=$a[$i]; } } public function showArgs() { echo ‘showArgs() called ‘; for ($i=0; $i<$numOfArgs; $i++) { echo ‘$i: ‘ . $i . ‘ ‘; echo $this->Arg[$i]; echo ‘ ‘; } } public function __destruct(){}

}

$test1 = new variableArgs; $test2 = new variableArgs(“arg1”); $test3 = new variableArgs(“arg1”, “arg2”); $test4 = new variableArgs(“arg1”, “arg2”, “arg3”);

$test1->showArgs(); $test2->showArgs(); $test3->showArgs(); $test4->showArgs();

This outputs the following:

showArgs() called $i: 0 No arguments passed showArgs() called $i: 0 arg1 showArgs() called $i: 0 arg1 $i: 1 arg2 showArgs() called $i: 0 arg1 $i: 1 arg2 $i: 2 arg3

I have no idea how efficient this is, but it works at any rate. Hopefully this helps someone. 01-Oct-2006 11:03 This is a simple thing to bear in mind but it’s also easy to forget it. When chaining object constructors and destructors, always remember to call the superclass __construct() method in the subclass __construct() so that all superclass members are properly initialized before you start initializing the ones belonging to your subclass.

Also, you will usually want to do your own cleanup first in your subclass __destruct() method so you will probably want to call the superclass __destruct() as the last thing in your subclass so that you can use resources defined in the superclass during the cleanup phase.

For example, if your superclass includes a database connection and your subclass __destruct method commits things to the database then if you call the superclass destruct before doing so then the database connection will no longer be valid and you will be unable to commit your changes. chanibal at deltasoft dot int dot pl dot SPAMPROTECT 20-Aug-2006 09:56 Note that if a class contains another class, the contained class’s destructor will be triggered after the destructor of the containing class.

class contained {

protected $parent;

public function __construct(&$p) { # $this->parent=&$p; }

public function __destruct() { /* unset $this->parent */ print ‘contained ‘; } }

class containing {

protected $contained;

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

public function __destruct() { // unset($this->contained); print ‘containing ‘; } }

new containing(); ?>

Will output containing contained

After uncommenting the // comment, the output will change to contained containing

Adding a reference from the contained class to the containing one (the # comment) will not change that, but beware, because it can cause random errors in other destructors in the parts of the script which seem unrelated! (PHP Version 5.1.2) prauscher at gmx dot net 14-Aug-2006 04:05 I saw no note in the manual about my function. If you want to write a file in a __destruct – function, it will fail with a “Permission denied” Error. Reza Mahjourian 10-Jul-2006 02:18 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.

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. Peter Molnar 18-May-2006 06:24 There were many notes about the inability of defining multiple constructors for the class.

My solution is to define separate static methods for each type of constructor. class Vector {

private $x; private $y;

public function __construct() { $this->x = 0; $this->y = 0; }

public static function createXY($x, $y) { $v = new Vector(); $v->x = $x; $v->y = $y; return $v; } } ?> ckoschmied at web dot de 07-Apr-2006 07:58 Be aware of the fact that if you create a new instance of a class like this:

$instance = new Class();

$instance will not contain a valid reference to the newly created object until the constructor is finished. So don’t use $instance while the constructor is still running.

Well, on the other side, why would you want to do it? I wanted to, and it took me some hours to figure out. Even though it’s quite obvious if you think about it 🙂 jcaplan at bogus dot amazon dot com 24-Mar-2006 09:52 __construct and __destruct must be declared public in any class that you intend to instantiate with new. However, in an abstract (or never-instantiated base) class you can declare them private or protected, and subclasses can still refer to them via parent::__construct (!) (tested in PHP 5.1.2). 09-Feb-2006 10:55 (Refering to: caliban at darklock dot com)

To force a constructor always to be called, and still be able to define a constructor on a derived class use the model below. Ideal for module architectures, because you only have to know the file and classname to construct an object.

class Parameter {}

abstract class BaseClass { protected $param;

public final function __construct( Parameter $param ) { $this->param = $param; $this->pseudoConstruct(); }

protected abstract function pseudoConstruct(); }

class ConcreteClass extends BaseClass { protected function pseudoConstruct() { echo __CLASS__.’ constructor’; } }

$refl = new ReflectionClass( ‘ConcreteClass’ ); if( !$refl->isSubclassOf( ‘BaseClass’ ) ) throw new Exception( ‘Invalid base class!’ ); $refl->newInstance( new Parameter() ); ?> jochem AT mondrian-it d_o_t nl 30-Jan-2006 04:07 at: derk AT oneindig DOT com

You can achieve identical functionality by doing this: class Parent {

function __construct() { echo “Parent constructor called\n”; } }

class Child extends Parent { function __construct() { parent::__construct(); echo ” Child ‘contructor’ called”; } }

$c = new Child(); ?> Added advantage is that Parent doesn’t need to have the method myConstruct(), and that you’re using constructors like they were intended. developit at mail dot ru 25-Jan-2006 02:32 as [kida at keymail dot it] said you can’t weaken a visibility of constructor when extending some class. but suggested trick that uses both old and new constructor namimg syntaxes to weaken visibility from ‘protected’ to ‘public’ seems a little bit odd. allthough it works allright. declaring extended class as ‘abstract’ with ‘public’ constructor will do quite the same thing in a more elegant manner and without any syntax mess.

class A

{ public function __construct() { //do smth } }

abstract class B extends A { public function __construct() { parent::__construct(); } } ?>

thus, you avoid instanciating class B as if it had a protected contructor aya at eh dot org 01-Dec-2005 02:20 For those who aren’t already aware, PHP5 currently suffers from the classic reference counting leak. See http://en.wikipedia.org/wiki/Reference_counting for more info.

Example code:

class Noisy

{ private $name;

public function __construct($name) { $this->name = $name; echo “Noisy::__construct($this->name)\n”; }

public function __destruct() { echo “Noisy::__destruct($this->name)\n”; } }

function foo($num) { $noisy = new Noisy($num); //$noisy->me = $noisy; // Uncomment this line to create a cyclic reference }

for ($i = 0; $i < 10; ++$i) foo($i);

?>

As it stands, the destructor of class ‘Noisy’ will be called on ‘$noisy’ when it goes out of scope in function ‘foo’, but uncommenting the second line in function ‘foo’ will prevent this, and cause a memory leak.

See http://bugs.php.net/bug.php?id=33595 for a bug report, which reads as if this is not likely to get fixed in the near future, so watch out! derk AT oneindig DOT com 03-Nov-2005 02:02 If a constructor is not present in a child class, php5 will try to call a constructor from the parent class. This behaviour can be used to somewhat simulate constructor chaining.

abstract class Parent {

function __construct() { echo “Parent constructor called\n”; $this->myConstruct(); } }

class Child extends Parent { function myConstruct() { echo ” Child ‘contructor’ called”; } }

$c = new Child(); ?>

will output: Parent constructor called Child ‘constructor’ called contact at tcknetwork dot com 21-Sep-2005 12:54 be careful while trying to access files with __destruct() because the base directory (getcwd()) will be the root of your server and not the path of your script, so add before all your path called in __destruct() : EITHER dirname($_SERVER[“SCRIPT_FILENAME”]).”my/path/” OR dirname(__FILE__).”my/path/” (be careful with includes, it will give the path of the file processed and not the main file) php dot net at lk2 dot de 13-Aug-2005 09:50 It looks like `echo()`ed output from the _destructor() function is displayed onto screen _before other output that the class may have have already sent before.

This can be misleading if you have debug info printed in the destructor but not a problem if you know it. stanley dot turnteen at gmail dot com 05-Aug-2005 12:43 IMHO using func_get_args() is superior to constructor polymorphism, because you don’t have to define constructors for every possible way a class can be initialized.

The pattern I use looks like this; all you have to do is pass the parameters in the correct order.

class Movie

{ public $title; public $director; public $stars; public $year_released;

public function __construct() { $args = func_get_args();

foreach(array(“title”, “director”, “stars”, “year_released”) as $i) { if(empty($args)) { break; }

$this->$i = array_shift($args); } }

} ?> dominics at gmail dot com 10-Jul-2005 07:12 If you’re using E_STRICT error reporting, PHP will tell you if you define both __construct() and an old-style constructor (a function with the same name as the class) together in a class. Note that this occurs even if the old constructor function is abstract or final (for instance, if you were intending to only use it in a sub-class). Be wary of this if you’re trying to implement the ‘command’ design pattern.

The solution? Either turn E_STRICT off (and possibly forgo some other important notices), rename your function (and possibly make things a little more complicated), or look at using an interface. rocco at bluora dot com dot au 16-Apr-2005 08:29 Before PHP reaches the point where it calls the __destruct functions, it has already done a session_write_close() so you can no longer write anything to the session.

I wanted it to copy some variables from my class into the session once the script had finished but now having to get the last function to call a SaveToSession() function.

In php versions 5.0.2 and 5.0.4 contact at tcknetwork dot com 15-Apr-2005 08:45 Note that php5 use in priority __construct() instead of [classname](). So you could build a constructed/destructed class for php4/5 very easily using this. class test {

function test() { $this->__construct(); register_shutdown_function(array($this,”__destruct”)); } function __construct() { echo “construct\n”; } function __destruct() { echo “destruct\n”; } }; $t=new test(); ?> In case you use unset($t) in php4, the destructor is not called. so be careful. apfelsaft 30-Mar-2005 01:59 at the end of a script all remaining objects aren’t in fact destructed. it is only their __destruct() method, which will be called. the objects still exist after that.

so, if your database connection object has no __destruct() or at least it doesn’t disconnects the database, it will still work.

in general, there is no need to disconnect the database (especially for persistent connections). 04-Mar-2005 03:48

To caliban at darklock dot com: Why not just define a dummy constructor

Because you don’t always get to modify your base classes. Once you get beyond the “build to suit” range of software development, you end up having to work with other people’s code, and sometimes you just plain can’t change it. When Bob is in charge of making changes to that object, you can’t add a dummy constructor. You have to tell Bob to do it, and until Bob does it, you don’t get it. So if you want to hit your deadlines, you don’t count on Bob caring enough about your job to make the changes you want… you work around it. It might be convenient for *you* to have a constructor on that object, but when you’re only one of several thousand people that are using it, your convenience isn’t generally among the design criteria.

Smaller projects where you can add whatever you want wherever you want will not have this problem, in which case the dummy constructor is indeed a better solution. 24-Feb-2005 01:08 To caliban at darklock dot com: Why not just define a dummy constructor in the base class? This adds little overhead, and allows you to both extend the class worry-free and later add construct functionality to the base class.

And now, about destructors: I haven’t seen this clarified anywhere in the manual, but object destructors are called implicitly at script shutdown for all objects that still exist at that Tpoint. his happens *after* any shutdown functions set with have been called.

Objects appear to be destructed in the order they were defined, which means you have to be careful with destruct methods that rely on the functionality of other objects (e.g. on a database-handler) as they will have shut down already.

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