archived-doc-en/language/oop5/late-static-bindings.xml

<?xml version="1.0" encoding="utf-8"?>
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 <sect1 xml:id="language.oop5.late-static-bindings" xmlns="http://docbook.org/ns/docbook">
  <title>Late Static Bindings</title>
  <para>
   PHP implements a feature called late static bindings which
   can be used to reference the called class in a context of static inheritance.
  </para>

  <para>
   More precisely, late static bindings work by storing the class named in the
   last "non-forwarding call". In case of static method calls, this is the
   class explicitly named (usually the one on the left of the
   <link linkend="language.oop5.paamayim-nekudotayim"><literal>::</literal></link>
   operator); in case of non static method calls, it is the class of the object. A
   "forwarding call" is a static one that is introduced by <literal>self::</literal>,
   <literal>parent::</literal>, <literal>static::</literal>, or, if going
   up in the class hierarchy, <function>forward_static_call</function>.
   <!-- technically, static:: may be non forwarding, but it's irrelevant -->
   
   The function <function>get_called_class</function> can be used to retrieve
   a string with the name of the called class and <literal>static::</literal>
   introduces its scope.
  </para>
  
  <para>
   This feature was named "late static bindings" with an internal perspective in
   mind. "Late binding" comes from the fact that <literal>static::</literal>
   will not be resolved using the class where the method is defined but it will
   rather be computed using runtime information.

   It was also called a "static binding" as it can be used for (but is not
   limited to) static method calls.
  </para>

  <sect2 xml:id="language.oop5.late-static-bindings.self">
   <title>Limitations of <literal>self::</literal></title>
   <para>
    Static references to the current class like <literal>self::</literal> or
    <literal>__CLASS__</literal> are resolved using the class in which the
    function belongs, as in where it was defined:
   </para>
   <example>
    <title><literal>self::</literal> usage</title>
    <programlisting role="php">
<![CDATA[
<?php

class A
{
    public static function who()
    {
        echo __CLASS__;
    }

    public static function test()
    {
        self::who();
    }
}

class B extends A
{
    public static function who()
    {
        echo __CLASS__;
    }
}

B::test();

?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
A
]]>
    </screen>
   </example>

  </sect2>

  <sect2 xml:id="language.oop5.late-static-bindings.usage">
   <title>Late Static Bindings' usage</title>

   <para>
    Late static bindings tries to solve that limitation by introducing a
    keyword that references the class that was initially called at runtime.
    Basically, a keyword that would allow referencing
    <literal>B</literal> from <literal>test()</literal> in the previous
    example. It was decided not to introduce a new keyword but rather use
    <literal>static</literal> that was already reserved.
   </para>

   <example>
    <title><literal>static::</literal> simple usage</title>
    <programlisting role="php">
<![CDATA[
<?php

class A
{
    public static function who()
    {
        echo __CLASS__;
    }

    public static function test()
    {
        static::who(); // Here comes Late Static Bindings
    }
}

class B extends A
{
    public static function who()
    {
        echo __CLASS__;
    }
}

B::test();

?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
B
]]>
    </screen>
   </example>
   <note>
    <para>
     In non-static contexts, the called class will be the class of the object
     instance. Since <literal>$this-></literal> will try to call private
     methods from the same scope, using <literal>static::</literal> may give
     different results. Another difference is that <literal>static::</literal>
     can only refer to static properties.
    </para>
   </note>
   <example>
    <title><literal>static::</literal> usage in a non-static context</title>
    <programlisting role="php">
<![CDATA[
<?php

class A
{
    private function foo()
    {
        echo "Success!\n";
    }

    public function test()
    {
        $this->foo();
        static::foo();
    }
}

class B extends A
{
    /* foo() will be copied to B, hence its scope will still be A and
    * the call be successful */
}

class C extends A
{
    private function foo()
    {
        /* Original method is replaced; the scope of the new one is C */
    }
}

$b = new B();
$b->test();

$c = new C();
try {
    $c->test();
} catch (Error $e) {
    echo $e->getMessage();
}

?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
Success!
Success!
Success!
Call to private method C::foo() from scope A
]]>
    </screen>
   </example>
   <note>
    <para>
     Late static bindings' resolution will stop at a fully resolved static call
     with no fallback. On the other hand, static calls using keywords like
     <literal>parent::</literal> or <literal>self::</literal> will forward the
     calling information.
    </para>
    <example>
     <title>Forwarding and non-forwarding calls</title>
     <programlisting role="php">
<![CDATA[
<?php

class A
{
    public static function foo()
    {
        static::who();
    }

    public static function who()
    {
        echo __CLASS__ . "\n";
    }
}

class B extends A
{
    public static function test()
    {
        A::foo();
        parent::foo();
        self::foo();
    }

    public static function who()
    {
        echo __CLASS__ . "\n";
    }
}

class C extends B
{
    public static function who()
    {
        echo __CLASS__ . "\n";
    }
}

C::test();

?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
A
C
C
]]>
     </screen>
    </example>
   </note>
  </sect2>
 </sect1>
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