Namespace (computer science)

A namespace is an abstract container or environment created to hold a logical grouping of unique identifiers (i.e., names). An identifier defined in a namespace is associated 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 which namespace an identifier (i.e., not its definition) belongs.

For example, Bill works for company X and his employee ID is 123. John works for company Y and his employee ID is also 123. The reason Bill and John 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 men 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 man should receive the check.

In large computer programs or documents 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.

Many modern computer languages provide support for namespaces. In some programming languages (eg. 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.

Use in common languages

In C++, a namespace is defined with a namespace block.

namespace foo {
  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 foo, bar must be written foo::bar. C++ includes another construct that makes this verbosity unnecessary. By adding the line

using namespace foo;

to a piece of code, the prefix foo:: 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 it exists. If it doesn't exist, it then refers to food::chicken if it exists. If neither exist, chicken refers to an identifier in the global namespace.

Namespaces in C++ are most often used to avoid naming collisions. Although namespaces are used extensively in recent C++ code, most older code does not use this facility. For example, the entire C++ standard library is defined within namespace std, but before standardization many components were originally in the global namespace.

In the Java programming language, 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 java.lang contains classes core to the language, and java.lang.reflect contains core classes specifically relating to reflection.

In Java (as well as 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.

XML Namespace

Main article: XML Namespace

Although it is not a programming language, XML makes extensive use of namespaces.

Emulating namespaces

In programming languages that do not provide language support for namespaces, namespaces can be emulated to some extent by using an identifier naming convention. For example, 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 gives reasonable assurance that the identifiers are unique and can therefore be used in larger programs without fear of identifier naming collisions.

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.
• All uses of the identifiers must, in effect, be fully namespace-qualified. 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.

See also

• Primary key
• Scope (programming)
• Name resolution
• Plan 9 - Taking everything is a file metaphor beyond files to names.
• Reiser4 - Naming systems should reflect rather than mold structure.
• Java package
• JavaScript Namespaces