Namespace

This article is about namespaces in general. For their use in computing, see Namespace (computer science).

For their use in Wikipedia, see Wikipedia:Namespace.

In general, a namespace is a container for a set of identifiers (names), and allows the disambiguation of homonym identifiers residing in different namespaces.. Namespaces usually group names based on their functionality.

Naming System

A name in a namespace consists of a namespace identifier and a local name. The namespace name is usually applied as a prefix to the local name.

In Augmented_Backus–Naur_Form:

name = <namespace identifier> separator <local name>

Examples

Examples of names in a namespace

Context	Identifier	Namespace identifier	Local name
Path (computing)	/home/user/readme.txt	/home/user (path)	readme.txt (file name)
Domain name	www.example.com	example.com (domain)	www (host name)
C++	std::array	std	array
UN/LOCODE	US NYC	US (country)	NYC (locality)
XML	xmlns:xhtml="http://www.w3.org/1999/xhtml" <xhtml:body>	http://www.w3.org/1999/xhtml	body
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)

Delegation

Namespaces allow delegation of identifier assignment to multiple name issuing organisation whilst retaining global uniqueness. A central Registration authority registrars the assigned namespace identifiers allocated. Each namespace identifier 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 assign the names themselves, or another Registration authority which further delegates parts of their namespace to different organisations.

Hierarchy

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 identifier are the same for each subdelegation. An example of a recursive hierarchy is the Domain name system.

An example of a non-recursive hierarchy are Uniform resource name representing an ISBN-13 number.

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 identifier	Namespace
Uniform resource name (URN)	Internet Assigned Numbers Authority	urn:isbn:978-3-16-148410-0	urn	Formal URN namespace
Formal URN namespace	Internet Assigned Numbers Authority	urn:isbn:978-3-16-148410-0	isbn	International Standard Book Numbers as Uniform Resource Names
International Article Number (EAN)	GS1	978-3-16-148410-0	978	Bookland
International Standard Book Number (ISBN)	International ISBN Agency	3-16-148410-X	3	German-speaking countries
German publisher code	Agentur für Buchmarktstandards	3-16-148410-X	16	Mohr Siebeck

Namespace versus Scope

A namespace identifier may provide context (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.

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	Relative path in a File system
Global scope	Universally unique identifier	Domain Name System

Namespaces in Programming Languages

For many programming languages, namespace is a context for their identifiers. In an operating system, an example of namespace is a directory. Each name in a directory uniquely identifies one file or subdirectory, but one file may have the same name multiple times.

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, 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 concepts, be the namespace a 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, 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 in a process called name mangling.

Below is an example of a namespace in C++:

namespace Box1{
   int boxSide = 4;
}
 
namespace Box2{
   int boxSide = 12; 
}
 
int main () {
  int boxSide = 42;
  cout << Box1::boxSide << endl;  //output 4
  cout << Box2::boxSide << endl;  //output 12
  cout << boxSide << endl;  // output 42
  return 0;
}

See also

• 11-Digit Delivery point ZIP code
• Binomial nomenclature (genus-species in biology)
• Chemical nomenclature
• Dewey Decimal Classification
• Digital object identifier
• Domain name system (DNS) names (Internet addresses)
• IP address
• Library of Congress Classification
• Category:National identification numbers
• Star catalogues and astronomical naming conventions
• XML Namespaces