Incomputing, anamespace 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 easilyidentified.
Namespaces are commonly structured as hierarchies to allow reuse of names in different contexts. As an analogy, consider a system ofnaming 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 includefile systems, which assign names to files.[1]Someprogramming languages organize theirvariables andsubroutines in namespaces.[2][3][4]Computer networks anddistributed systems assign names to resources, such ascomputers,printers,websites, and remote files.Operating systems can partitionkernel resources by isolated namespaces to supportvirtualization 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".
Incomputer programming, namespaces are typically employed for the purpose of grouping symbols and identifiers around a particular functionality and to avoidname collisions between multiple identifiers that share the same name.
Innetworking, theDomain Name System organizes websites (and other resources) intohierarchical namespaces.
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 carriesHTML table information:
<table><tr><td>Apples</td><td>Oranges</td></tr></table>
This XML carries information about atable (i.e. a piece of furniture):
<table><name>MahoganyCoffeeTable</name><width>80</width><length>120</length></table>
If these XML fragments were added together, there would be a name conflict. Both contain a<table>...</table> element, but the elements have different content and meaning.
An XML parser will not know how to handle these differences.
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.
<h:table><h:tr><h:td>Apples</h:td><h:td>Oranges</h:td></h:tr></h:table><f:table><f:name>MahoganyCoffeeTable</f:name><f:width>80</f:width><f:length>120</f:length></f:table>
A name in a namespace consists of a namespace name and a local name.[5][6] The namespace name is usually applied as aprefix to the local name.
name=<namespacename>separator<localname>
When local names are used by themselves,name resolution is used to decide which (if any) particular name is alluded to by some particular local name.
| 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[7] | 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.[8] A centralRegistration authority registers theassigned 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 thatassign the names themselves, or anotherRegistration authority which further delegates parts of their namespace to different organisations.
A naming scheme that allows subdelegation of namespaces to third parties is ahierarchical 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 theDomain name system.
An example of a non-recursive hierarchy areUniform Resource Name representing anInternet Assigned Numbers Authority (IANA) number.
| 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 | 3-16-148410-X | 16 | Mohr Siebeck |
A namespace name 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.
| Without a namespace | With a namespace | |
|---|---|---|
| Local scope | Vehicle registration plate | Filesystem Hierarchy Standard |
| Global scope | Universally unique identifier | Domain Name System |
For many programming languages, namespace is a context for theiridentifiers. In an operating system, an example of namespace is a directory. Each name in a directory uniquely identifies one file or subdirectory.[9]
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 acontext, because the same name in different namespaces can have different meanings, each one appropriate for its namespace.
Following are other characteristics of namespaces:
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++:
#include<iostream>// This is how one brings a name into the current scope. In this case, it's// bringing them into global scope.usingstd::cout;usingstd::endl;namespacebox1{intbox_side=4;}namespacebox2{intbox_side=12;}intmain(){intbox_side=42;cout<<box1::box_side<<endl;// Outputs 4.cout<<box2::box_side<<endl;// Outputs 12.cout<<box_side<<endl;// Outputs 42.}
A namespace in computer science (sometimes also called aname scope) is an abstract container or environment created to hold a logical grouping of uniqueidentifiers orsymbols (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.[10]
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 largecomputer programs or documents it is common to have hundreds or thousands of identifiers. Namespaces (or a similar technique, seeEmulating namespaces) provide a mechanism for hiding local identifiers. They provide a means of grouping logically related identifiers into corresponding namespaces, thereby making the system moremodular.
Data storage devices and many modernprogramming 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 namespacetree. At the root of this tree is the unnamedglobal namespace.
It is possible to use anonymous structs as namespaces inC sinceC99.
// helper.cstaticint_add(inta,intb){returna+b;}conststruct{doublepi;int(*add)(int,int);}helper={3.14,_add};// helper.hconststruct{doublepi;int(*add)(int,int);}helper;// main.c#include<stdio.h>#include"helper.h"intmain(){printf("3 + 2 = %d\n",helper.add(3,2));printf("pi is %f\n",helper.pi);}
InC++, a namespace is defined with a namespace block.[11]
namespaceabc{intbar;}
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 ofnamespace abc,bar must be writtenabc::bar to be accessed. C++ includes another construct that makes this verbosity unnecessary. By adding the line
usingnamespaceabc;
to a piece of code, the prefixabc:: is no longer needed.
Identifiers that are not explicitly declared within a namespace are considered to be in the global namespace.
intfoo;
These identifiers can be used exactly as they are declared, or, since the global namespace is unnamed, the namespace specifier:: can be prefixed. For example,foo can also be written::foo.
Namespace resolution in C++ is hierarchical. This means that within the hypothetical namespacefood::soup, the identifierchicken refers tofood::soup::chicken. Iffood::soup::chicken doesn't exist, it then refers tofood::chicken. If neitherfood::soup::chicken norfood::chicken exist,chicken refers to::chicken, an identifier in the global namespace.
Namespaces in C++ are most often used to avoidnaming collisions. 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 entireC++ Standard Library is defined withinnamespace std, but before standardization many components were originally in the global namespace. A programmer can insert theusing 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 theusing directive for reasons other than backwards compatibility (e.g., convenience) is considered to be against good code practices.
InJava, the idea of a namespace is embodied inJava 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 exampleclass String inpackagejava.lang can be referred to asjava.lang.String (this is known as thefully qualified class name). Like C++, Java offers a construct that makes it unnecessary to type the package name (import). However, certain features (such asreflection) 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 withjava are a part of theJava platform—the packagejava.lang contains classes core to the language, andjava.lang.reflect contains core classes specifically relating to reflection.
In Java (andAda,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 andclassscopes can be viewed as implicit namespaces that are inextricably linked with visibility, accessibility, andobject lifetime.
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 avoidnaming collisions.When referencing a class, one should specify either its fully qualified name, which means namespace followed by the class name:
System.Console.WriteLine("Hello World!");inti=System.Convert.ToInt32("123");
or add ausing statement. This eliminates the need to mention the complete name of all classes in that namespace.
usingSystem;Console.WriteLine("Hello World!");inti=Convert.ToInt32("123");
In the above examples,System is a namespace, andConsole andConvert are classes defined withinSystem.
InPython, namespaces are defined by the individual modules, and since modules can be contained in hierarchical packages, then namespaces are hierarchical too.[12][13]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.
# assume modulea defines two functions : func1() and func2() and one class : Class1importModuleaModulea.func1()Modulea.func2()a=Modulea.Class1()
Thefrom ... import ... 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:
# assume Modulea defines two functions : func1() and func2() and one class : Class1fromModuleaimportfunc1func1()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()
Since this directly imports names (without qualification) it can overwrite existing names with no warnings.
A special form of the statement isfrom ... import * 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.[14]
Python also supportsimport x as y as a way of providing an alias or alternative name for use by the calling module:
importnumpyasnpa=np.arange(1000)
InXML, 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.
Namespaces were introduced intoPHP from version 5.3 onwards. Naming collision of classes, functions and variables can be avoided.InPHP, a namespace is defined with a namespace block.
# File phpstar/foobar.phpnamespacephpstar;classFooBar{publicfunctionfoo():void{echo'Hello world, from function foo';}publicfunctionbar():void{echo'Hello world, from function bar';}}
We can reference a PHP namespace with the following different ways:
# File index.php# Include the fileinclude"phpstar/foobar.php";# Option 1: directly prefix the class name with the namespace$obj_foobar=new\phpstar\FooBar();# Option 2: import the namespaceusephpstar\FooBar;$obj_foobar=newFooBar();# Option 2a: import & alias the namespaceusephpstar\FooBarasFB;$obj_foobar=newFB();# Access the properties and methods with regular way$obj_foobar->foo();$obj_foobar->bar();
In programming languages lacking language support for namespaces, namespaces can be emulated to some extent by using anidentifier naming convention. For example,C libraries such aslibpng 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_structpng_get_signaturepng_read_rowpng_set_invalid
Thisnaming convention provides reasonable assurance that theidentifiers are unique and can therefore be used in larger programs withoutnaming collisions.[15] Likewise, many packages originally written inFortran (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:
DGEMM indicates that it operates on double-precision floating-point numbers (D) and general matrices (GE), with only the last two characters (MM) showing what it actually does: matrix–matrix multiplication.It also has a few advantages:
The primary construct established by a file system is a hierarchical directory namespace, which is the logical repository for files.
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# namespacesyntax allows namespaces to be nested.
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.
A package is a grouping of related types providing access protection and name space management. Note that types refers toclasses,interfaces,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 needed]
For instance, [underWindows], 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.Web namespace.
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