A namespace is an abstract container or environment created to hold a logical grouping of unique identifier
Identifier
An identifier is a name that identifies either a unique object or a unique class of objects, where the "object" or class may be an idea, physical [countable] object , or physical [noncountable] substance...
A symbol is something which represents an idea, a physical entity or a process but is distinct from it. The purpose of a symbol is to communicate meaning. For example, a red octagon may be a symbol for "STOP". On a map, a picture of a tent might represent a campsite. Numerals are symbols for...
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, the meaning associated with 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.
For example, Bill works for company X and his employee ID is 123. Jane works for company Y and her employee ID is also 123. The reason Bill and Jane can be identified by the same ID number is because they work for different companies. The different companies in this case would symbolize different namespaces. There would be serious confusion if the two people worked for the same company, and still had the same employee ID. For instance, a paycheck issued to employee ID 123 would not identify which person should receive the check.
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
The term document has multiple meanings in ordinary language and in scholarship. WordNet 3.1. lists four meanings :* document, written document, papers...
s it is not uncommon 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
Modularity (programming)
Modular programming is a software design technique that increases the extent to which software is composed of separate, interchangeable components called modules by breaking down program functions into modules, each of which accomplishes one function and contains everything necessary to accomplish...
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
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 (eg. C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
), the identifiers naming namespaces are themselves associated with an enclosing namespace. Thus, in these languages namespaces can nest, forming a namespace tree
Tree (data structure)
In computer science, a tree is a widely-used data structure that emulates a hierarchical tree structure with a set of linked nodes.Mathematically, it is an ordered directed tree, more specifically an arborescence: an acyclic connected graph where each node has zero or more children nodes and at...
. At the root of this tree is the unnamed global namespace.
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
, a namespace is defined with a namespace block.
namespace abc {
int bar;
}
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 namespace abc, bar must be written abc::bar to be accessed. C++ includes another construct that makes this verbosity unnecessary. By adding the line
using namespace abc;
to a piece of code, the prefix abc:: is no longer needed.
Code that is not explicitly declared within a namespace is considered to be in the global namespace.
Namespace resolution in C++ is hierarchical. This means that within the hypothetical namespace food::soup, the identifier chicken refers to food::soup::chicken. If food::soup::chicken doesn't exist, it then refers to food::chicken. If neither food::soup::chicken nor food::chicken exist, chicken refers to ::chicken, an identifier in the global namespace.
A naming collision is a circumstance where two or more identifiers in a given namespace or a given scope cannot be unambiguously resolved, and such unambiguous resolution is a requirement of the underlying system.- Example: XML element names :...
s. Although namespaces are used extensively in recent C++ code, most older code does not use this facility. For example, the entire C++ standard library
C++ standard library
In C++, the C++ Standard Library is a collection of classes and functions, which are written in the core language and part of the C++ ISO Standard itself...
is defined within namespace std, but before standardization many components were originally in the global namespace.
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, the idea of a namespace is embodied in Java package
Java package
A Java package is a mechanism for organizing Java classes into namespaces similar to the modules of Modula. Java packages can be stored in compressed files called JAR files, allowing classes to download faster as a group rather than one at a time...
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 class String in package java.lang can be referred to as java.lang.String (this is known as the fully qualified class name
Fully qualified name
In computer programming, a fully qualified name is an unambiguous name that specifies which object, function, or variable a call refers to without regard to the context of the call...
). Like C++, Java offers a construct that makes it unnecessary to type the package name (import). However, certain features (such as reflection
Reflection (computer science)
In computer science, reflection is the process by which a computer program can observe and modify its own structure and behavior at runtime....
) 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.
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
, 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
.NET Framework
The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
). How specific these categories are and how deep the hierarchies go differ from language to language.
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
s can be viewed as implicit namespaces that are inextricably linked with visibility, accessibility, and object lifetime
Object lifetime
In computer science, the object lifetime of an object in object-oriented programming is the time between an object's creation till the object is no longer used, and is destructed or freed.In object-oriented programming , the meaning of creating objects is far more subtle than simple...
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
, namespaces are defined by the individual modules, and since modules can be contained in hierarchical packages, then name spaces are hierarchical too.
In general when a module is imported then the names defined in the module are defined via that module's name space, 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 : class1
import modulea
modulea.func1
modulea.func2
a = modulea.class1
The "from ... import ..." 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 : 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
Since this directly imports names (without qualification) it can overwrite existing names with no warnings.
A special form is "from ... import *", which imports all names defined in the named package directly in the calling modules 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.
Python also supports "from ... import ... as ...." as a way of providing an alias or alternative name for use by the calling module.
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
, 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 is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
from version 5.3 onwards. Naming collision of classes, functions and variables can be avoided.
In PHP
PHP
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
, a namespace is defined with a namespace block.
assume this is a class file defines two functions : foo and bar
location of the file phpstar/foobar.php
namespace phpstar;
class fooBar
{
public function foo
{
echo 'hello world, from function foo';
}
public function bar
{
echo 'hello world, from function bar';
}
}
We can reference a PHP namespace with following different ways:
location of the 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 & alise 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;
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
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
libpng is the official Portable Network Graphics reference library . It is a platform-independent library that contains C functions for handling PNG images...
often use a fixed prefix for all functions and variables that are part of their exposed interface. Libpng exposes identifiers such as:
This provides reasonable assurance that the identifier
Identifier
An identifier is a name that identifies either a unique object or a unique class of objects, where the "object" or class may be an idea, physical [countable] object , or physical [noncountable] substance...
s are unique and can therefore be used in larger programs without fear of identifier naming collision
Naming collision
A naming collision is a circumstance where two or more identifiers in a given namespace or a given scope cannot be unambiguously resolved, and such unambiguous resolution is a requirement of the underlying system.- Example: XML element names :...
s. Likewise, many packages originally written in Fortran
Fortran
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...
Blas is mainly a Spanish given name and surname, related to Blaise. It may refer to-Places:*Piz Blas, mountain in Switzerland*San Blas , many places - see separate article, also**Cape San Blas Light, lighthouse...
-External links:* : a modern replacement for PLAPACK and ScaLAPACK* on Netlib.org* * * : a modern replacement for LAPACK that is MultiGPU ready* on Sourceforge.net* * optimized LAPACK for Solaris OS on SPARC/x86/x64 and Linux* * *...
) reserve the first few letters of a function's name to indicate which group it belongs to.
Unfortunately, this technique has several drawbacks:
It doesn't scale well to nested namespaces; identifiers become excessively long.
Individuals or organizations may use dramatically 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, is rendered unwieldy or unfeasible.
In computer programming, a fully qualified name is an unambiguous name that specifies which object, function, or variable a call refers to without regard to the context of the call...
. Languages with direct support for namespaces usually provide ways for the programmer to declare up front that they wish to use some or all identifiers from a specific namespace, which they can then use without qualification for the remainder of the block.
In languages in which identifier length is restricted, 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 77
Fortran
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...
that offered only 6 characters per identifier. For example, the BLAS
Blas
Blas is mainly a Spanish given name and surname, related to Blaise. It may refer to-Places:*Piz Blas, mountain in Switzerland*San Blas , many places - see separate article, also**Cape San Blas Light, lighthouse...
function DGEMM function indicates that it operates on double precision numbers (the 'D'), general matrices (the 'GE') and only the last two characters show what it actually does: a matrix-matrix multiplication (the 'MM').
An Identity column is a column in a database table that is made up of values generated by the database. This is much like an AutoNumber field in Microsoft Access or a sequence in Oracle...
In relational databases and flat file databases, a table is a set of data elements that is organized using a model of vertical columns and horizontal rows. A table has a specified number of columns, but can have any number of rows...
An identity in object-oriented programming, object-oriented design and object-oriented analysis describes the property of objects that distinguishes them from other objects. This is closely related to the philosophical concept of identity....
A Java package is a mechanism for organizing Java classes into namespaces similar to the modules of Modula. Java packages can be stored in compressed files called JAR files, allowing classes to download faster as a group rather than one at a time...
-In computer languages:Expressions in computer languages can contain identifiers. The semantics of such expressions depend on the entities that the identifiers refer to. The algorithm that determines what an identifier in a given context refers to is part of the language definition.The complexity...
In general, a namespace is a container that provides context for the identifiers it holds, and allows the disambiguation of homonym identifiers residing in different namespaces....
(general concept)
Plan 9, extends everything is a file metaphor beyond files to names
In relational databases and flat file databases, a table is a set of data elements that is organized using a model of vertical columns and horizontal rows. A table has a specified number of columns, but can have any number of rows...
Reiser4 is a computer file system, successor to the ReiserFS file system, developed from scratch by Namesys and sponsored by DARPA as well as Linspire...
, naming systems should reflect rather than mold structure
In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
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