Editing Namespace

{{for multi|the use of namespaces in Wikipedia|Wikipedia:Namespace|a feature of the Linux kernel|Linux namespaces}}
{{short description|Container for a set of identifiers}}

In [[computing]], a '''namespace''' is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily [[Identifier|identified]].

Namespaces are commonly structured as hierarchies to allow reuse of names in different contexts. As an analogy, consider a system of [[Anthroponymy|naming of people]] where each person has a given name, as well as a family name shared with their relatives. If the first names of family members are unique only within each family, then each person can be uniquely identified by the combination of first name and family name; there is only one Jane Doe, though there may be many Janes. Within the namespace of the Doe family, just "Jane" suffices to unambiguously designate this person, while within the "global" namespace of all people, the full name must be used.

Prominent examples for namespaces include [[file system]]s, which assign names to files.<ref>{{cite conference
 |first1=Siddharth 
 |last1=Aniket
 |first2=Anushka 
 |last2=Aditya
 |first3=Chaya
 |last3=Deepa
 |first4=Batman 
 |last4=Cermak 
 |first5=Ronnie 
 |last5=Chaiken 
 |first6=John 
 |last6=Douceur 
 |first7=Jon 
 |last7=Howell 
 |first8=Jacob 
 |last8=Lorch 
 |first9=Marvin 
 |last9=Theimer 
 |first10=Roger 
 |last10=Wattenhofer 
 |title=FARSITE: Federated, Available, and Reliable Storage for an Incompletely Trusted Environment 
 |conference=Proc. [[USENIX]] Symp. on Operating Systems Design and Implementation 
 |year=2002 
 |url=http://paul.rutgers.edu/cs545/S06/papers/adya-farsite-osdi-2002.pdf 
 |quote=The primary construct established by a file system is a hierarchical directory namespace, which is the logical repository for files. 
 |archive-url=https://web.archive.org/web/20100728202945/http://paul.rutgers.edu/cs545/S06/papers/adya-farsite-osdi-2002.pdf 
 |archive-date=2010-07-28 
}}</ref>
Some [[programming language]]s organize their [[variable (computer science)|variables]] and [[subroutine]]s in namespaces.<ref>{{cite web|url=http://en.csharp-online.net/CSharp_FAQ:_What_is_a_namespace |title=C# FAQ: What is a namespace |publisher=C# Online Net |quote=A namespace is nothing but a group of assemblies, classes, or types. A namespace acts as a container—like a disk folder—for classes organized into groups usually based on functionality. C# namespace [[Syntax (programming languages)|syntax]] allows namespaces to be nested. |access-date=2010-02-23 |archive-url=https://web.archive.org/web/20131020150529/http://tutorials.csharp-online.net/index.php?title=CSharp_FAQ%3A_What_is_a_namespace |archive-date=2013-10-20 }}</ref><ref>{{Cite web|url=http://php.net/manual/en/language.namespaces.rationale.php|title=An overview of namespaces in PHP|website=PHP Manual|quote=What are namespaces? In the broadest definition, namespaces are a way of encapsulating items. This can be seen as an abstract concept in many places. For example, in any operating system directories serve to group related files, and act as a namespace for the files within them.}}</ref><ref>{{Cite web|url=https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html|title=Creating and Using Packages|website=Java Documentation|publisher=Oracle|quote=A package is a grouping of related types providing access protection and name space management. Note that types refers to [[Class (computer programming)|classes]], [[Interface (computing)|interfaces]], [[Enumerated type|enumerations]], and annotation types. Enumerations and annotation types are special kinds of classes and interfaces, respectively, so types are often referred to in this lesson simply as classes and interfaces.}}{{better source|date=July 2015}}</ref>
[[Computer network]]s and [[distributed system]]s assign names to resources, such as [[computer]]s, [[Printer (computing)|printer]]s, [[website]]s, and remote files. [[Operating system]]s can partition [[Kernel (operating system)|kernel]] resources by isolated namespaces to support [[OS-level virtualisation|virtualization]] containers.

Similarly, [[Directory (computing)|hierarchical file systems]] organize files in directories. Each directory is a separate namespace, so that the directories "letters" and "invoices" may both contain a file "to_jane".

In [[computer programming]], namespaces are typically employed for the purpose of grouping symbols and identifiers around a particular functionality and to avoid [[name collision]]s between multiple identifiers that share the same name.

In [[Computer network|networking]], the [[Domain Name System]] organizes websites (and other resources) into [[hierarchical namespace]]s.

==Name conflicts==
Element names are defined by the developer. This often results in a conflict when trying to mix XML documents from different XML applications.

This XML carries [[HTML table]] information:
<syntaxhighlight lang="xml">
<table>
    <tr>
        <td>Apples</td>
        <td>Oranges</td>
    </tr>
</table>
</syntaxhighlight>
This XML carries information about a [[table (furniture)|table]] (i.e. a piece of furniture):
<syntaxhighlight lang="xml">
<table>
    <name>Mahogany Coffee Table</name>
    <width>80</width>
    <length>120</length>
</table>
</syntaxhighlight>

If these XML fragments were added together, there would be a name conflict. Both contain a {{tag|table}} element, but the elements have different content and meaning.

An XML parser will not know how to handle these differences.

===Solution via prefix===
Name conflicts in XML can easily be avoided using a name prefix.

The following XML distinguishes between information about the HTML table and furniture by prefixing "h" and "f" at the beginning of the elements.

<syntaxhighlight lang="xml">

<h:table>
    <h:tr>
        <h:td>Apples</h:td>
        <h:td>Oranges</h:td>
    </h:tr>
</h:table>

<f:table>
    <f:name>Mahogany Coffee Table</f:name>
    <f:width>80</f:width>
    <f:length>120</f:length>
</f:table>
</syntaxhighlight>

==Naming system==
A name in a namespace consists of a namespace name and a local name.<ref>{{cite web | url=http://www.w3.org/TR/REC-xml-names/ | title= Namespaces in XML 1.0 (Third Edition) | author=XML Core Working Group | work=W3C |date=8 December 2009 |access-date=2012-03-30}}</ref><ref>{{cite IETF  |title=URN Syntax |rfc=2141 |sectionname=Syntax |section=2 |page=1 |last= Moats |first= Ryan |date=May 1997 |publisher=[[Internet Engineering Task Force|IETF]] |access-date= 2012-03-30}}</ref> The namespace name is usually applied as a [[Prefix (computer science)|prefix]] to the local name.

In [[augmented Backus–Naur form]]:
<syntaxhighlight lang=abnf>
name = <namespace name> separator <local name>
</syntaxhighlight>
When local names are used by themselves, [[Name resolution (programming languages)|name resolution]] is used to decide which (if any) particular name is alluded to by some particular local name.

===Examples===
{| class=wikitable
|+ Examples of names in a namespace
|-
! Context !! Name !! Namespace name !! Local name
|-
| [[Path (computing)|Path]] || /home/user/readme.txt || /home/user (directory)|| readme.txt (file name)
|-
| [[Domain name]] || www.example.com || example.com (domain name)|| www (leaf domain name)
|-
| [[#Use in common languages|C++]] || std::array || std (C++ namespace)|| array (struct)
|-
| [[UN/LOCODE]] || US NYC  || US (country or territory)|| NYC (locality)
|-
| [[XML namespace|XML]] || xmlns:xhtml="http://www.w3.org/1999/xhtml" <br /> &lt;xhtml:body&gt; || xhtml (previously declared XML namespace xhtml="http://www.w3.org/1999/xhtml")|| body (element)
|- 
| [[Perl module#Perl packages and namespaces|Perl]] || $DBI::errstr || $DBI (Perl module)|| errstr (variable)
|-
| [[Java package|Java]] || java.util.Date || java.util (Java namespace)|| Date (class)
|-
| [[Uniform Resource Name]] (URN) || urn:nbn:fi-fe19991055 || urn:nbn (National Bibliography Numbers) || fi-fe19991055
|-
| [[Handle System]] || 10.1000/182  || 10 (handle naming authority) || 1000/182 (handle local name)
|-
| [[Digital object identifier]] || 10.1000/182 || 10.1000 (publisher) || 182 (publication)
|-
| [[MAC address]] || 01-23-45-67-89-ab  || 01-23-45 ([[organizationally unique identifier]]) || 67-89-ab (NIC specific)
|-
| [[PCI ID]] || 1234 abcd || 1234 (vendor ID) || abcd (device ID)
|-
| [[USB Implementers Forum#Obtaining a vendor ID|USB VID/PID]] || 2341 003f<ref>
Stephen J. Gowdy.
[http://www.linux-usb.org/usb.ids "List of USB ID's"].
2013.
</ref> || 2341 (vendor ID) || 003f (product ID)
|-
| [[SPARQL]] || dbr:Sydney || dbr (previously declared ontology, e.g. by specifying @prefix dbr: <http://dbpedia.org/resource/>) || Sydney 
|}
===Delegation===
Delegation of responsibilities between parties is important in real-world applications, such as the structure of the World Wide Web. Namespaces allow delegation of identifier assignment to multiple name issuing organisations whilst retaining global uniqueness.<ref>{{cite IETF  |title= Functional Requirements for Uniform Resource Names |rfc=1731 |sectionname=Requirements for functional capabilities |section=2 |page=3 |last= Sollins & Masinter |date=December 1994 |publisher=[[Internet Engineering Task Force|IETF]] |access-date= 2012-03-30}}</ref> A central [[Registration authority]] registers the [[National Internet registry|assigned namespace names allocated]]. Each namespace name is allocated to an organisation which is subsequently responsible for the assignment of names in their allocated namespace. This organisation may be a name issuing organisation that [[Metadata registry|assign the names]] themselves, or another [[Registration authority]] which further delegates parts of their namespace to different organisations.

===Hierarchy{{anchor|hierarchical namespace}}===
A naming scheme that allows subdelegation of namespaces to third parties is a '''hierarchical namespace'''.

A hierarchy is recursive if the syntax for the namespace names is the same for each subdelegation. An example of a recursive hierarchy is the [[Domain name|Domain name system]].

An example of a non-recursive hierarchy are [[Uniform Resource Name]] representing an [[Internet Assigned Numbers Authority]] (IANA) number.

{| class=wikitable
|+ Hierarchical namespace breakdown for urn:isbn:978-3-16-148410-0, an identifier for the book The Logic of Scientific Discovery by Karl Popper, 10th edition.
|-
! Registry !! Registrar !! Example Identifier !! Namespace name !! Namespace
|-
| [[Uniform Resource Name]] (URN) || [[Internet Assigned Numbers Authority]] || urn:isbn:978-3-16-148410-0 || urn || [https://www.iana.org/assignments/urn-namespaces/urn-namespaces.xml Formal URN namespace]
|-
| [[Uniform resource name|Formal URN namespace]] || [[Internet Assigned Numbers Authority]] || urn:isbn:978-3-16-148410-0 || ISBN || [http://tools.ietf.org/html/rfc3187 International Standard Book Numbers as Uniform Resource Names]
|-
| [[International Article Number (EAN)]] || [[GS1]] || 978-3-16-148410-0 || 978 || [[List of GS1 country codes|Bookland]]
|-
| [[International Standard Book Number]] (ISBN) || [[International ISBN Agency]] || 3-16-148410-X || 3 || [[German-speaking countries]]
|-
| [[International Standard Book Number#Publisher code|German publisher code]] || [http://www.german-isbn.de/ Agentur für Buchmarktstandards] || {{ISBNT|3-16-148410-X}} || 16 || [[Mohr Siebeck]]
|}

===Namespace versus scope===
A namespace name may provide context ([[Scope (computer science)|scope]] in computer science) to a name, and the terms are sometimes used interchangeably. However, the context of a name may also be provided by other factors, such as the location where it occurs or the syntax of the name.

{| class=wikitable
|+Examples of naming systems with local and global scope, and with and without namespaces
!   !! Without a namespace !! With a namespace
|-
| ''Local scope'' || [[Vehicle registration plate]] || [[Filesystem Hierarchy Standard]]
|-
| ''Global scope'' || [[Universally unique identifier]] || [[Domain Name System]]
|}

==In programming languages== <!--Anchor for Redirect: [Namespace (computer science). -->

For many programming languages, namespace is a context for their [[Identifier (computer languages)|identifiers]]. In an operating system, an example of namespace is a directory. Each name in a directory uniquely identifies one file or subdirectory.<ref>{{cite web|url=http://tutorials.csharp-online.net/CSharp_FAQ:_What_is_a_namespace|title=C# FAQ: What is a namespace|website=C# Online Net|archive-url=https://web.archive.org/web/20131020153832/http://tutorials.csharp-online.net/CSharp_FAQ:_What_is_a_namespace|archive-date=October 20, 2013|access-date=2010-02-23|quote=For instance, [under [[Microsoft Windows|Windows]]], to access the built-in input-output (I/O) classes and members, use the System.IO namespace. Or, to access Web-related classes and members, use the System{{Not a typo|.}}Web namespace.}}</ref>

As a rule, names in a namespace cannot have more than one meaning;  that is, different meanings cannot share the same name in the same namespace. A namespace is also called a [[context (computing)|context]], because the same name in different namespaces can have different meanings, each one appropriate for its namespace.

Following are other characteristics of namespaces:
* Names in the namespace can represent objects as well as [[concept]]s, be the namespace a [[natural language|natural or ethnic language]], a [[constructed language]], the [[technical terminology]] of a profession, a [[dialect]], a [[sociolect]], or an artificial language (e.g., a [[programming language]]).
* In the [[Java (programming language)|Java programming language]], identifiers that appear in namespaces have a short (local) name and a unique long "qualified" name for use outside the namespace.
* Some compilers (for languages such as [[C++]]) combine namespaces and names for internal use in the compiler in a process called ''[[name mangling]]''.

As well as its abstract language technical usage as described above, some languages have a specific keyword used for explicit namespace control, amongst other uses. Below is an example of a namespace in C++:

<syntaxhighlight lang="cpp">
#include <iostream>

// This is how one brings a name into the current scope.  In this case, it's
// bringing them into global scope.
using std::cout;
using std::endl;

namespace box1 {
    int box_side = 4;
}

namespace box2 {
    int box_side = 12;
}

int main() {
    int box_side = 42;
    cout << box1::box_side << endl;  // Outputs 4.
    cout << box2::box_side << endl;  // Outputs 12.
    cout << box_side << endl;  // Outputs 42.
}
</syntaxhighlight>

===Computer-science considerations===
A namespace in computer science (sometimes also called a '''name scope''') is an abstract container or environment created to hold a logical grouping of unique [[identifier]]s or [[symbol]]s (i.e. names). An identifier defined in a namespace is associated only with that namespace. The same identifier can be independently defined in multiple namespaces. That is, an identifier defined in one namespace may or may not have the same meaning as the same identifier defined in another namespace. Languages that support namespaces specify the rules that determine to which namespace an identifier (not its definition) belongs.<ref>{{cite web |url=http://www.webopedia.com/TERM/N/namespace.html |title=A namespace is "a logical grouping of the names used within a program."|date=10 April 2002 | publisher=Webopedia.com| access-date=2011-07-26}}</ref>

This concept can be illustrated with an analogy. Imagine that two companies, X and Y, each assign ID numbers to their employees. X should not have two employees with the same ID number, and likewise for Y; but it is not a problem for the same ID number to be used at both companies. For example, if Bill works for company X and Jane works for company Y, then it is not a problem for each of them to be employee #123. In this analogy, the ID number is the identifier, and the company serves as the namespace. It does not cause problems for the same identifier to identify a different person in each namespace.

In large [[computer program]]s or documents it is common to have hundreds or thousands of identifiers. Namespaces (or a similar technique, see [[#Emulating namespaces|Emulating namespaces]]) provide a mechanism for hiding local identifiers. They provide a means of grouping logically related identifiers into corresponding namespaces, thereby making the system more [[modularity (programming)|modular]].

[[Data storage device]]s and many modern [[programming language]]s support namespaces. Storage devices use directories (or folders) as namespaces. This allows two files with the same name to be stored on the device so long as they are stored in different directories. In some programming languages (e.g. [[C++]], [[Python (programming language)|Python]]), the identifiers naming namespaces are themselves associated with an enclosing namespace. Thus, in these languages namespaces can nest, forming a namespace [[tree (data structure)|tree]]. At the root of this tree is the unnamed '''global namespace'''.

====Use in common languages====
=====C=====
It is possible to use anonymous structs as namespaces in [[C (programming language)|C]] since [[C99]].
<syntaxhighlight lang="c">
// helper.c
static int _add(int a, int b) {
    return a + b;
}

const struct {
    double pi;
    int (*add) (int, int);
} helper = { 3.14, _add };

// helper.h
const struct {
    double pi;
    int (*add) (int, int);
} helper;

// main.c
#include <stdio.h>
#include "helper.h"

int main(){
    printf("3 + 2 = %d\n", helper.add(3, 2));
    printf("pi is %f\n", helper.pi);
}

</syntaxhighlight>
=====C++=====
In [[C++]], a namespace is defined with a namespace block.<ref>{{cite web|url=http://www.cplusplus.com/doc/tutorial/namespaces/ |title=Namespaces allow to group entities like classes, objects and functions under a name. |publisher=Cplusplus.com |access-date=2011-07-26}}</ref>
<syntaxhighlight lang="cpp">
namespace abc {
    int bar;
}
</syntaxhighlight>
Within this block, identifiers can be used exactly as they are declared. Outside of this block, the namespace specifier must be prefixed. For example, outside of <code>namespace abc</code>, <code>bar</code> must be written <code>abc::bar</code> to be accessed. C++ includes another construct that makes this verbosity unnecessary. By adding the line
<syntaxhighlight lang="cpp">
using namespace abc;
</syntaxhighlight>
to a piece of code, the prefix <code>abc::</code> is no longer needed.

Identifiers that are not explicitly declared within a namespace are considered to be in the global namespace.
<syntaxhighlight lang="cpp">
int foo;
</syntaxhighlight>
These identifiers can be used exactly as they are declared, or, since the global namespace is unnamed, the namespace specifier <code>::</code> can be prefixed. For example, <code>foo</code> can also be written <code>::foo</code>.

Namespace resolution in C++ is hierarchical. This means that within the hypothetical namespace <code>food::soup</code>, the identifier <code>chicken</code> refers to <code>food::soup::chicken</code>. If <code>food::soup::chicken</code> doesn't exist, it then refers to <code>food::chicken</code>. If neither <code>food::soup::chicken</code> nor <code>food::chicken</code> exist, <code>chicken</code> refers to <code>::chicken</code>, an identifier in the global namespace.

Namespaces in C++ are most often used to avoid [[naming collision]]s. Although namespaces are used extensively in recent C++ code, most older code does not use this facility because it did not exist in early versions of the language. For example, the entire [[C++ Standard Library]] is defined within <code>namespace std</code>, but before standardization many components were originally in the global namespace. A programmer can insert the <code>using</code> directive to bypass namespace resolution requirements and obtain backwards compatibility with older code that expects all identifiers to be in the global namespace. However the use of the <code>using</code> directive for reasons other than backwards compatibility (e.g., convenience) is considered to be against good code practices.

=====Java=====
In [[Java (programming language)|Java]], the idea of a namespace is embodied in [[Java package]]s. All code belongs to a package, although that package need not be explicitly named. Code from other packages is accessed by prefixing the package name before the appropriate identifier, for example <code>class String</code> in <code>package [[java.lang]]</code> can be referred to as <code>java.lang.String</code> (this is known as the [[fully qualified name|fully qualified class name]]). Like C++, Java offers a construct that makes it unnecessary to type the package name (<code>import</code>). However, certain features (such as [[reflection (computer science)|reflection]]) require the programmer to use the fully qualified name.

Unlike C++, namespaces in Java are not hierarchical as far as the syntax of the language is concerned. However, packages are named in a hierarchical manner. For example, all packages beginning with <code>java</code> are a part of the [[Java platform]]—the package {{Javadoc:SE|package=java.lang|java/lang}} contains classes core to the language, and {{Javadoc:SE|package=java.lang.reflect|java/lang/reflect}} contains core classes specifically relating to reflection.

In Java (and [[Ada (programming language)|Ada]], [[C Sharp (programming language)|C#]], and others), namespaces/packages express semantic categories of code. For example, in C#, <code>namespace System</code> contains code provided by the system (the [[.NET Framework]]). How specific these categories are and how deep the hierarchies go differ from language to language.

[[function (programming)|Function]] and [[class (computer science)|class]] [[scope (programming)|scope]]s can be viewed as implicit namespaces that are inextricably linked with visibility, accessibility, and [[object lifetime]].

=====C#=====
Namespaces are heavily used in C# language. All .NET Framework classes are organized in namespaces, to be used more clearly and to avoid chaos. Furthermore, custom namespaces are extensively used by programmers, both to organize their work and to avoid [[naming collision]]s.
When referencing a class, one should specify either its fully qualified name, which means namespace followed by the class name:
<syntaxhighlight lang="c#">
System.Console.WriteLine("Hello World!");
int i = System.Convert.ToInt32("123");
</syntaxhighlight>

or add a '''using''' statement. This eliminates the need to mention the complete name of all classes in that namespace.

<syntaxhighlight lang="c#">
using System;

Console.WriteLine("Hello World!");
int i = Convert.ToInt32("123");
</syntaxhighlight>

In the above examples, '''System''' is a namespace, and '''Console''' and '''Convert''' are classes defined within '''System'''.

[[File:Dotnet-system-namespace-uml.svg|UML diagram with a Console and a Convert class.]]

=====Python=====
In [[Python (programming language)|Python]], namespaces are defined by the individual modules, and since modules can be contained in hierarchical packages, then namespaces are hierarchical too.<ref>{{cite web|title=6. Modules|url=https://docs.python.org/tutorial/modules.html|work=The Python Tutorial|publisher=Python Software Foundation|access-date=25 October 2010}}</ref><ref>{{cite web|url=https://docs.python.org/tutorial/classes.html#python-scopes-and-namespaces |title=Python Scopes and Namespaces |publisher=Docs.python.org |access-date=2011-07-26}}</ref>
In general when a module is imported then the names defined in the module are defined via that module's namespace, and are accessed in from the calling modules by using the fully qualified name.
<syntaxhighlight lang="python">
# assume modulea defines two functions : func1() and func2() and one class : Class1
import Modulea

Modulea.func1()
Modulea.func2()
a = Modulea.Class1()
</syntaxhighlight>

The <code>from ... import ...</code> statement can be used to insert the relevant names directly into the calling module's namespace, and those names can be accessed from the calling module without the qualified name:
<syntaxhighlight lang="python">
# assume Modulea defines two functions : func1() and func2() and one class : Class1
from Modulea import func1

func1()
func2() # this will fail as an undefined name, as will the full name Modulea.func2()
a = Class1() # this will fail as an undefined name, as will the full name Modulea.Class1()
</syntaxhighlight>
Since this directly imports names (without qualification) it can overwrite existing names with no warnings.

A special form of the statement is <code>from ... import *</code> which imports all names defined in the named package directly in the calling module's namespace. Use of this form of import, although supported within the language, is generally discouraged as it pollutes the namespace of the calling module and will cause already defined names to be overwritten in the case of name clashes.<ref>https://docs.python.org/3/tutorial/modules.html "in general the practice of importing * from a module or package is frowned upon"</ref>

Python also supports <code>import x as y</code> as a way of providing an alias or alternative name for use by the calling module:

<syntaxhighlight lang="numpy">
import numpy as np

a = np.arange(1000)
</syntaxhighlight>

===== XML namespace <!-- This section is linked from [[XML]] --> =====
{{Main|XML namespace}}

In [[XML]], the XML namespace specification enables the names of elements and attributes in an XML document to be unique, similar to the role of namespaces in programming languages. Using XML namespaces, XML documents may contain element or attribute names from more than one XML vocabulary.

===== PHP <!-- This section is linked from [[PHP]] --> =====
Namespaces were introduced into [[Php|PHP]] from version 5.3 onwards. Naming collision of classes, functions and variables can be avoided.
In [[PHP]], a namespace is defined with a namespace block.
<syntaxhighlight lang="php">
# File phpstar/foobar.php

namespace phpstar;

class FooBar
{
    public function foo(): void
    {
        echo 'Hello world, from function foo';
    }

    public function bar(): void
    {
        echo 'Hello world, from function bar';
    }
}
</syntaxhighlight>
We can reference a PHP namespace with the following different ways:
<syntaxhighlight lang="php">
# File index.php

# Include the file
include "phpstar/foobar.php";

# Option 1: directly prefix the class name with the namespace
$obj_foobar = new \phpstar\FooBar();

# Option 2: import the namespace
use phpstar\FooBar;
$obj_foobar = new FooBar();

# Option 2a: import & alias the namespace
use phpstar\FooBar as FB;
$obj_foobar = new FB();

# Access the properties and methods with regular way
$obj_foobar->foo();
$obj_foobar->bar();
</syntaxhighlight>

[[File:UML phpstar package diagram.svg|UML diagram of the phpstar package with the class FooBar.]]

===Emulating namespaces===
In programming languages lacking language support for namespaces, namespaces can be emulated to some extent by using an [[identifier naming convention]]. For example, [[C (programming language)|C]] libraries such as [[libpng]] often use a fixed prefix for all functions and variables that are part of their exposed interface. Libpng exposes identifiers such as:

 png_create_write_struct
 png_get_signature
 png_read_row
 png_set_invalid

This [[naming conventions (programming)|naming convention]] provides reasonable assurance that the [[identifier]]s are unique and can therefore be used in larger programs without [[naming collision]]s.<ref>{{Cite web|url=http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=286 |title=Why I Hate Namespaces |author=Danny Kalev |url-status=bot: unknown |archive-url=https://web.archive.org/web/20160709115609/http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=286 |archive-date=2016-07-09 }}</ref> Likewise, many packages originally written in [[Fortran]] (e.g., [[BLAS]], [[LAPACK]]) reserve the first few letters of a function's name to indicate the group to which the function belongs.

This technique has several drawbacks:
* It doesn't scale well to nested namespaces; identifiers become excessively long since all uses of the identifiers must be [[fully qualified name|fully namespace-qualified]];
* Individuals or organizations may use inconsistent naming conventions, potentially introducing unwanted obfuscation;
* Compound or "query-based" operations on groups of identifiers, based on the namespaces in which they are declared, are rendered unwieldy or unfeasible;
* In languages with restricted identifier length, the use of prefixes limits the number of characters that can be used to identify what the function does; this is a particular problem for packages originally written in [[Fortran|FORTRAN 77]], which offered only 6 characters per identifier; for example, the name of the [[BLAS]] function <code>DGEMM</code> indicates that it operates on double-precision floating-point numbers (<code>D</code>) and general matrices (<code>GE</code>), with only the last two characters (<code>MM</code>) showing what it actually does: matrix–matrix multiplication.

It also has a few advantages:
* No special software tools are required to locate names in source-code files; a simple program like [[grep]] suffices;
* There are no namespace-related name conflicts;
* There is no need for name mangling, and thus no potential incompatibility problems.

==See also==
{{Wiktionary|namespace}}
* 11-digit [[delivery point]] [[ZIP code]]
* [[Binomial nomenclature]] (genus-species in [[biology]])
* [[Chemical nomenclature]]
* [[Dewey Decimal Classification]]
* [[Digital object identifier]]
* [[Domain Name System]]
* [[Fourth wall]]
* [[Identity (object-oriented programming)]]
* [[Library of Congress Classification]]
* [[Star catalogue]]s and [[astronomical naming conventions]]
* [[Violation of abstraction level]]
* [[XML namespace]]
* [[Argument-dependent name lookup]]

==References==
{{Reflist|30em}}

{{DEFAULTSORT:Namespace}}
[[Category:Naming conventions]]