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https://github.com/sigmasternchen/php-doc-en
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git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@248045 c90b9560-bf6c-de11-be94-00142212c4b1
436 lines
14 KiB
XML
436 lines
14 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!-- $Revision: 1.3 $ -->
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<chapter xml:id="language.namespaces" xmlns="http://docbook.org/ns/docbook"
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version="1.1">
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<title>Namespaces</title>
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<sect1 xml:id="language.namespaces.rationale">
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<title>Namespaces overview</title>
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<simpara>
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Namespaces in PHP are designed to solve scoping problem in large PHP
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libraries. In PHP, all class definitions are global. Thus, when a library
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author creates various utility or public API classes for the library, he must
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be aware of the possibility that other libraries with similar functionality
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would exist and thus choose unique names so that these libraries could be
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used together. Usually it is solved by prefixing the class names with an
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unique string - e.g., database classes would have prefix
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<classname>My_Library_DB</classname>, etc. As the library grows, prefixes add up,
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leading to the very long names.
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</simpara>
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<simpara>
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The namespaces allow the developer to manage naming scopes without using the
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long names each time the class is referred to, and solve the problem of
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shared globals space without making code unreadable.
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</simpara>
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<simpara>
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Namespaces are available in PHP as of PHP 5.3.0. This section is experimental
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and subject to changes.
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</simpara>
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</sect1>
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<sect1 xml:id="language.namespaces.definition">
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<title>Namespace definition</title>
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<para>
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The namespace is declared using <literal>namespace</literal>
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keyword, which should be at the very beginning of the file. Example:
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<example>
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<title>Defining namespace</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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namespace MyProject::DB;
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const CONNECT_OK = 1;
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class Connection { /* ... */ }
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function connect() { /* ... */ }
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?>
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]]>
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</programlisting>
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</example>
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Same namespace name can be used in multiple files.
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</para>
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<para>
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Namespace can contain class, constant and function definitions, but no free code.
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</para>
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<para>
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Namespace definition does the following:
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<itemizedlist>
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<listitem>
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<simpara>
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Inside namespace, all class, function and constant names in definitions are
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automatically prefixed with namespace name. The class name is always the
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full name, i.e. in the example above the class is called
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<classname>MyProject::DB::Connection</classname>.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Constant definitions create constant which is composed of namespace name and constant name.
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Like class constants, namespace constant can only contains static values.
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</simpara>
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</listitem>
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<listitem>
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<para>
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Unqualified class name (i.e., name not containing <literal>::</literal>)
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is resolved at runtime following this procedure:
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<orderedlist>
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<listitem>
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<simpara>
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Class is looked up inside the current namespace (i.e. prefixing the
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name with the current namespace name) without attempting to
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<link linkend="language.oop5.autoload">autoload</link>.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Class is looked up inside the global namespace without attempting to
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autoload.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Autoloading for name in current namespace is attempted.
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</simpara>
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</listitem>
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<listitem>
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<simpara>If previous failed, lookup fails.</simpara>
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</listitem>
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</orderedlist>
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</para>
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</listitem>
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<listitem>
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<para>
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Unqualified function name (i.e., name not containing
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<literal>::</literal>) is looked up at runtime first in the current namespace
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and then in the global space.
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</para>
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</listitem>
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<listitem>
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<para>
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Unqualified constant names are looked up first at current namespace and then
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among globally defined constants.
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</para>
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</listitem>
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</itemizedlist>
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See also the full <link linkend="language.namespaces.rules">name resolution rules</link>.
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</para>
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</sect1>
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<sect1 xml:id="language.namespaces.using">
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<title>Using namespaces</title>
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<para>
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Every class and function in a namespace can be referred to by the full name -
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e.g. <classname>MyProject::DB::Connection</classname> or
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<classname>MyProject::DB::connect</classname> - at any time.
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<example>
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<title>Using namespaced name</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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require 'MyProject/Db/Connection.php';
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$x = new MyProject::DB::Connection;
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MyProject::DB::connect();
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<para>
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Namespaces can be imported into current context (global or namespace) using
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the <literal>use</literal> operator. The syntax for the operator is:
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<informalexample>
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<programlisting role="php">
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<![CDATA[
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<?php
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/* ... */
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use Some::Name as Othername;
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// The simplified form of use:
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use Foo::Bar;
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// which is the same as :
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use Foo::Bar as Bar;
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?>
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]]>
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</programlisting>
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</informalexample>
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The imported name works as follows: every time that the compiler encounters
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the local name <literal>Othername</literal> (as stand-alone name or as
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prefix to the longer name separated by <literal>::</literal>) the imported
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name <literal>Some::Name</literal> is substituted instead.
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</para>
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<para>
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<literal>use</literal> can be used only in global scope, not inside
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function or class. Imported names have effect from the point of import to
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the end of the current file. It is recommended to put imports at the
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beginning of the file to avoid confusion.
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</para>
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<para>
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<example>
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<title>Importing and accessing namespace</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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require 'MyProject/Db/Connection.php';
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use MyProject::DB;
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use MyProject::DB::Connection as DbConnection;
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$x = new MyProject::DB::Connection();
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$y = new DB::connection();
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$z = new DbConnection();
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DB::connect();
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<para>
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<note>
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<simpara>
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The import operation is compile-time only, all local names are converted to
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their full equivalents by the compiler. Note that it won't translate names
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in strings, so callbacks can't rely on import rules.
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</simpara>
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</note>
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</para>
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</sect1>
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<sect1 xml:id="language.namespaces.global">
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<title>Global space</title>
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<para>
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Without any namespace definition, all class and function definitions are
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placed into the global space - as it was in PHP before namespaces were
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supported. Prefixing a name with <literal>::</literal> will specify that
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the name is required from the global space even in the context of the
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namespace.
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<example>
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<title>Using global space specification</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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namespace A::B::C;
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/* This function is A::B::C::fopen */
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function fopen() {
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/* ... */
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$f = ::fopen(...); // call global fopen
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return $f;
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}
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?>
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]]>
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</programlisting>
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</example>
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</para>
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</sect1>
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<sect1 xml:id="language.namespaces.constant">
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<title>__NAMESPACE__</title>
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<para>
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The compile-time constant <constant>__NAMESPACE__</constant> is defined to
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the name of the current namespace. Outside namespace this constant has the
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value of empty string. This constant is useful when one needs to compose
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full name for local namespaced names.
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<example>
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<title>Using __NAMESPACE__</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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namespace A::B::C;
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function foo() {
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// do stuff
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}
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set_error_handler(__NAMESPACE__ . "::foo");
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?>
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]]>
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</programlisting>
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</example>
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</para>
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</sect1>
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<sect1 xml:id="language.namespaces.rules">
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<title>Name resolution rules</title>
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<para>
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Names are resolved following these resolution rules:
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<orderedlist>
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<listitem>
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<simpara>
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All qualified names are translated during compilation according to current
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import rules. In example, if the namespace A::B::C is imported, a call to
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<code>C::D::e()</code> is translated to <code>A::B::C::D::e()</code>.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Unqualified class names are translated during compilation according to current
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import rules (full name substituted for short imported name). In example, if
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the namespace <literal>A::B::C</literal> is imported, <code>new C()</code> is
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translated to <code>new A::B::C()</code>.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Inside namespace, calls to unqualified functions that are defined in the
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current namespace (and are known at the time the call is parsed) are
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interpreted as calls to these namespace functions, at compile time.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Inside namespace (say A::B), calls to unqualified functions that are not
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defined in current namespace are resolved at run-time. Here is how a
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call to function <literal>foo()</literal> is resolved:
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</simpara>
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<orderedlist>
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<listitem>
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<simpara>
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It looks for a function from the current namespace:
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<literal>A::B::foo()</literal>.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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It tries to find and call the <emphasis>internal</emphasis> function
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<literal>foo()</literal>.
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</simpara>
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</listitem>
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</orderedlist>
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<simpara>
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To call a user defined function in the global namespace,
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<literal>::foo()</literal> has to be used.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Inside namespace (say <literal>A::B</literal>), calls to unqualified class names are
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resolved at run-time. Here is how a call to
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<code>new C()</code> is resolved:
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</simpara>
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<orderedlist>
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<listitem>
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<simpara>
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It looks for a class from the current namespace:
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<literal>A::B::C</literal>.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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It tries to find and call the <emphasis>internal</emphasis> class
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<literal>C</literal>.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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It attemts to autoload <literal>A::B::C</literal>.
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</simpara>
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</listitem>
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</orderedlist>
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<simpara>
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To reference a user defined class in the global namespace,
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<code>new ::C()</code> has to be used.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Calls to qualified functions are resolved at run-time. Here is how a call
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to <literal>A::B::foo()</literal> is resolved:
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</simpara>
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<orderedlist>
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<listitem>
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<simpara>
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It looks for a function <literal>foo()</literal> in the namespace
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<literal>A::B</literal>.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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It looks for a class <literal>A::B</literal> and call its static
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method <literal>foo()</literal>. It will autoload the class if
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necessary.
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</simpara>
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</listitem>
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</orderedlist>
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</listitem>
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<listitem>
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<simpara>
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Qualified class names are resolved in compile-time as class from corresponding
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namespace. For example, <code>new A::B::C()</code> refers to class
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<classname>C</classname> from namespace <literal>>A::B</literal>.
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</simpara>
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</listitem>
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</orderedlist>
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</para>
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<example>
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<title>Name resolutions illustrated</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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namespace A;
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// function calls
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foo(); // first tries to call "foo" defined in namespace "A"
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// then calls internal function "foo"
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::foo(); // calls function "foo" defined in global scope
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// class references
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new B(); // first tries to create object of class "B" defined in namespace "A"
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// then creates object of internal class "B"
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new ::B(); // creates object of class "B" defined in global scope
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// static methods/namespace functions from another namespace
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B::foo(); // first tries to call function "foo" from namespace "A::B"
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// then calls method "foo" of internal class "B"
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::B::foo(); // first tries to call function "foo" from namespace "B"
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// then calls method "foo" of class "B" from global scope
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// static methods/namespace functions of current namespace
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A::foo(); // first tries to call function "foo" from namespace "A::A"
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// then tries to call method "foo" of class "A" from namespace "A"
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// then tries to call function "foo" from namespace "A"
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// then calls method "foo" of internal class "A"
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::A::foo(); // first tries to call function "foo" from namespace "A"
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// then calls method "foo" of class "A" from global scope
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?>
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]]>
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</programlisting>
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</example>
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</sect1>
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</chapter>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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indent-tabs-mode:nil
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sgml-parent-document:nil
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sgml-default-dtd-file:"../../manual.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:nil
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sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
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vim: et tw=78 syn=sgml
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vi: ts=1 sw=1
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-->
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