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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 identified. Namespaces are commonly structured as hierarchies to allow reuse of names in different contexts. As an analogy, consider a system of 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 systems, which assign names to files. Some programming languages organize their variables and subroutines in namespaces. Computer networks and distributed systems assign names to resources, such as computers, printers, websites, and remote files. Operating systems can partition kernel resources by isolated namespaces to support virtualization containers. Similarly, 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 collisions between multiple identifiers that share the same name. In networking, the Domain Name System organizes websites (and other resources) into hierarchical namespaces.

+ Examples of names in a namespace Context Name Namespace name Local name 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) C++ std::array std (C++ namespace) array (struct) UN/LOCODE US NYC US (country or territory) NYC (locality) XML xmlns:xhtml="http://www. w3. org/1999/xhtml" <xhtml:body> xhtml (previously declared XML namespace xhtml="http://www. w3. org/1999/xhtml") body (element) Perl $DBI::errstr $DBI (Perl module) errstr (variable) 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 VID/PID 2341 003f 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 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. A central Registration authority registers the 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 assign the names themselves, or another Registration authority which further delegates parts of their namespace to different organisations.

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 system. An example of a non recursive hierarchy are Uniform Resource Name representing an Internet Assigned Numbers Authority (IANA) 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 name 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 16 Mohr Siebeck

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 identifiers or symbols (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. 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 programs or documents it is common to have hundreds or thousands of identifiers. Namespaces (or a similar technique, see 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 modular. Data storage devices and many modern programming languages 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), the identifiers naming namespaces are themselves associated with an enclosing namespace. Thus, in these languages namespaces can nest, forming a namespace tree. At the root of this tree is the unnamed global namespace.

In Java, the idea of a namespace is embodied in Java packages. 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 class String in package java. lang can be referred to as java. lang. String (this is known as the fully qualified class name). Like C++, Java offers a construct that makes it unnecessary to type the package name (import). However, certain features (such as 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 java are a part of the Java platform—the package contains classes core to the language, and contains core classes specifically relating to reflection. In Java (and Ada, C#, and others), namespaces/packages express semantic categories of code. For example, in C#, namespace System 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 and class scopes can be viewed as implicit namespaces that are inextricably linked with visibility, accessibility, and object lifetime.

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.