decon.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
 <sect1 xml:id="language.oop5.decon" xmlns="http://docbook.org/ns/docbook">
  <title>Constructors and Destructors</title>

  <sect2 xml:id="language.oop5.decon.constructor">
   <title>Constructor</title>
   <methodsynopsis xml:id="object.construct">
    <type>void</type><methodname>__construct</methodname>
    <methodparam rep="repeat"><type>mixed</type><parameter>values</parameter><initializer>""</initializer></methodparam>
   </methodsynopsis>
   <para>
    PHP allows developers to declare constructor methods for classes.
    Classes which have a constructor method call this method on each
    newly-created object, so it is suitable for any initialization that the
    object may need before it is used.
   </para>
   <note>
    <simpara>
     Parent constructors are not called implicitly if the child class defines
     a constructor.  In order to run a parent constructor, a call to
     <function>parent::__construct</function> within the child constructor is
     required. If the child does not define a constructor then it may be inherited
     from the parent class just like a normal class method (if it was not declared
     as private).
    </simpara>
   </note>
   <example>
    <title>Constructors in inheritance</title>
    <programlisting role="php">
<![CDATA[
<?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();
?> 
]]>
    </programlisting>
   </example>
   <para>
    Unlike other methods, <link linkend="object.construct">__construct()</link>
    is exempt from the usual
    <link linkend="language.oop.lsp">signature compatibility rules</link>
    when being extended.
   </para>
   <para>
    Constructors are ordinary methods which are called during the instantiation of their
    corresponding object.  As such, they may define an arbitrary number of arguments, which
    may be required, may have a type, and may have a default value. Constructor arguments
    are called by placing the arguments in parentheses after the class name.
   </para>
   <example>
    <title>Using constructor arguments</title>
    <programlisting role="php">
<![CDATA[
<?php
class Point {
    protected int $x;
    protected int $y;

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

// Pass both parameters.
$p1 = new Point(4, 5);
// Pass only the required parameter. $y will take its default value of 0.
$p2 = new Point(4);
// With named parameters (as of PHP 8.0):
$p3 = new Point(y: 5, x: 4);
?>
]]>
    </programlisting>
   </example>
   <para>
    If a class has no constructor, or the constructor has no required arguments, the parentheses
    may be omitted.
   </para>
   <sect3>
    <title>Old-style constructors</title>
    <para>
     Prior to PHP 8.0.0, classes in the global namespace will interpret a method named
     the same as the class as an old-style constructor.  That syntax is deprecated,
     and will result in an <constant>E_DEPRECATED</constant> error but still call that function as a constructor.
     If both <link linkend="object.construct">__construct()</link> and a same-name method are
     defined, <link linkend="object.construct">__construct()</link> will be called.
    </para>
    <para>
     In namespaced classes, or any class as of PHP 8.0.0, a method named
     the same as the class never has any special meaning.
    </para>
    <para>Always use <link linkend="object.construct">__construct()</link> in new code.
    </para>
   </sect3>
   <sect3 xml:id="language.oop5.decon.constructor.promotion">
    <title>Constructor Promotion</title>
    <para>
     As of PHP 8.0.0, constructor parameters may also be promoted to correspond to an
     object property.  It is very common for constructor parameters to be assigned to
     a property in the constructor but otherwise not operated upon.  Constructor promotion
     provides a short-hand for that use case.  The example above could be rewritten as the following.
    </para>
    <example>
     <title>Using constructor property promotion</title>
     <programlisting role="php">
<![CDATA[
<?php
class Point {
    public function __construct(protected int $x, protected int $y = 0) {
    }
}
]]>
     </programlisting>
    </example>
    <para>
     When a constructor argument includes a modifier, PHP will interpret it as
     both an object property and a constructor argument, and assign the argument value to
     the property.  The constructor body may then be empty or may contain other statements.
     Any additional statements will be executed after the argument values have been assigned
     to the corresponding properties.
    </para>
    <para>
     Not all arguments need to be promoted. It is possible to mix and match promoted and not-promoted
     arguments, in any order.  Promoted arguments have no impact on code calling the constructor.
    </para>
    <note>
     <para>
      Using a <link linkend="language.oop5.visibility">visibility modifier</link> (<literal>public</literal>,
      <literal>protected</literal> or <literal>private</literal>) is the most likely way to apply property
      promotion, but any other single modifier (such as <literal>readonly</literal>) will have the same effect.
     </para>
    </note>
    <note>
     <para>
      Object properties may not be typed <type>callable</type> due to engine ambiguity that would
      introduce. Promoted arguments, therefore, may not be typed <type>callable</type> either. Any
      other <link linkend="language.types.declarations">type declaration</link> is permitted, however.
     </para>
    </note>
    <note>
     <para>
      As promoted properties are desugared to both a property as well as a function parameter, any
      and all naming restrictions for both properties as well as parameters apply.
     </para>
    </note>
    <note>
     <para>
      <link linkend="language.attributes">Attributes</link> placed on a
      promoted constructor argument will be replicated to both the property
      and argument.  Default values on a promoted constructor argument will be replicated only to the argument and not the property.
     </para>
    </note>
   </sect3>

   <sect3 xml:id="language.oop5.decon.constructor.new">
    <title>New in initializers</title>
    <para>
     As of PHP 8.1.0, objects can be used as default parameter values,
     static variables, and global constants, as well as in attribute arguments.
     Objects can also be passed to <function>define</function> now.
    </para>
    <note>
     <para>
      The use of a dynamic or non-string class name or an anonymous class is not allowed.
      The use of argument unpacking is not allowed.
      The use of unsupported expressions as arguments is not allowed.
     </para>
    </note>
    <example>
     <title>Using new in initializers</title>
     <programlisting role="php">
<![CDATA[
<?php

// All allowed:
static $x = new Foo;

const C = new Foo;
 
function test($param = new Foo) {}
 
#[AnAttribute(new Foo)]
class Test {
    public function __construct(
        public $prop = new Foo,
    ) {}
}

// All not allowed (compile-time error):
function test(
    $a = new (CLASS_NAME_CONSTANT)(), // dynamic class name
    $b = new class {}, // anonymous class
    $c = new A(...[]), // argument unpacking
    $d = new B($abc), // unsupported constant expression
) {}
?>
]]>
     </programlisting>
    </example>
   </sect3>
   
   <sect3 xml:id="language.oop5.decon.constructor.static">
    <title>Static creation methods</title>
    <para>
     PHP only supports a single constructor per class.  In some cases, however, it may be
     desirable to allow an object to be constructed in different ways with different inputs.
     The recommended way to do so is by using static methods as constructor wrappers.
    </para>
    <example>
     <title>Using static creation methods</title>
     <programlisting role="php">
<![CDATA[
<?php
class Product {

    private ?int $id;
    private ?string $name;

    private function __construct(?int $id = null, ?string $name = null) {
        $this->id = $id;
        $this->name = $name;
    }

    public static function fromBasicData(int $id, string $name): static {
        $new = new static($id, $name);
        return $new;
    }

    public static function fromJson(string $json): static {
        $data = json_decode($json, true);
        return new static($data['id'], $data['name']);
    }

    public static function fromXml(string $xml): static {
        // Custom logic here.
        $data = convert_xml_to_array($xml);
        $new = new static();
        $new->id = $data['id'];
        $new->name = $data['name'];
        return $new;
    }
}

$p1 = Product::fromBasicData(5, 'Widget');
$p2 = Product::fromJson($some_json_string);
$p3 = Product::fromXml($some_xml_string);
]]>
     </programlisting>
    </example>
    <para>
     The constructor may be made private or protected to prevent it from being called externally.
     If so, only a static method will be able to instantiate the class. Because they are in the
     same class definition they have access to private methods, even if not of the same object
     instance. The private constructor is optional and may or may not make sense depending on
     the use case.
    </para>
    <para>
     The three public static methods then demonstrate different ways of instantiating the object.
    </para>
    <simplelist>
     <member><code>fromBasicData()</code> takes the exact parameters that are needed, then creates the
      object by calling the constructor and returning the result.</member>
     <member><code>fromJson()</code> accepts a JSON string and does some pre-processing on it itself
     to convert it into the format desired by the constructor. It then returns the new object.</member>
     <member><code>fromXml()</code> accepts an XML string, preprocesses it, and then creates a bare
     object.  The constructor is still called, but as all of the parameters are optional the method
     skips them.  It then assigns values to the object properties directly before returning the result.</member>
    </simplelist>
    <para>
     In all three cases, the <code>static</code> keyword is translated into the name of the class the code is in.
     In this case, <code>Product</code>.
    </para>
   </sect3>
  </sect2>
  <sect2 xml:id="language.oop5.decon.destructor">
   <title>Destructor</title>
   <methodsynopsis xml:id="object.destruct">
    <type>void</type><methodname>__destruct</methodname>
    <void />
   </methodsynopsis>
   <para>
    PHP possesses a destructor concept similar to that of other
    object-oriented languages, such as C++. The destructor method will be
    called as soon as there are no other references to a particular object,
    or in any order during the shutdown sequence.
   </para>
   <example>
    <title>Destructor Example</title>
    <programlisting role="php">
<![CDATA[
<?php

class MyDestructableClass 
{
    function __construct() {
        print "In constructor\n";
    }

    function __destruct() {
        print "Destroying " . __CLASS__ . "\n";
    }
}

$obj = new MyDestructableClass();
 
]]>
    </programlisting>
   </example>
   <para>
    Like constructors, parent destructors will not be called implicitly by
    the engine. In order to run a parent destructor, one would have to
    explicitly call <function>parent::__destruct</function> in the destructor
    body. Also like constructors, a child class may inherit the parent's
    destructor if it does not implement one itself.
   </para>
   <para>
    The destructor will be called even if script execution is stopped using
    <function>exit</function>. Calling <function>exit</function> in a destructor
    will prevent the remaining shutdown routines from executing.
   </para>
   <note>
    <para>
     Destructors called during the script shutdown have HTTP headers already
     sent. The working directory in the script shutdown phase can be different
     with some SAPIs (e.g. Apache).
    </para>
   </note>
   <note>
    <para>
     Attempting to throw an exception from a destructor (called in the time of
     script termination) causes a fatal error.
    </para>
   </note>
  </sect2>

 </sect1>
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