{"id":215,"date":"2018-07-11T05:23:49","date_gmt":"2018-07-11T05:23:49","guid":{"rendered":"http:\/\/itp1.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=215"},"modified":"2018-07-30T10:28:49","modified_gmt":"2018-07-30T10:28:49","slug":"namespace-in-c","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/itp1\/chapter\/namespace-in-c\/","title":{"rendered":"Namespace in C++"},"content":{"raw":"Introduction<\/strong>\r\n\r\n \r\n

In C++, identifiers may have either local or global scope. C++ allows unique names in specific scope. When C++ application uses libraries from different vendors or when application is developed using multiple source files written by various programmers, there are chances of having identifiers with same name in global scope namespace. This introduces a problem of name conflict. Namespace helps to prevent such name conflict by allowing identifiers to be grouped in a user-defined namespace.<\/p>\r\n \r\n\r\nWhat is namespace?\r\n\r\n \r\n

Namespace is a declarative region providing a scope to the identifiers contained in it. The identifiers are named entities, such as variable, function, class, object, typename, struct, enum or other compound types. Identifiers should have unique names in a namespace.<\/p>\r\n \r\n\r\nWhy namespace?\r\n\r\n \r\n

Before understanding the need of namespace, let us recall the knowledge about global and local scope of variables. Namespace is sometimes referred to as scope also.<\/p>\r\n \r\n

Named entities, such as variables, functions, class need to be declared before being used in C++. Depending upon the location of their declaration in a program, entities are having their scope or visibility.<\/p>\r\n \r\n\r\n1.1. Local namespace<\/strong>\r\n\r\n \r\n

An entity declared within a block has block scope<\/em>. Block may be introduced using statements in a pair of curly braces. Some examples of blocks are functions, loops, conditions. Parameters of a function and entities declared within a block are local to the block. Such entities are visible only within the specific block in which they are declared, but not outside it. Their storage class is auto; i.e. they are created when the block is executed and their life is till the end of block. Variables with block scope are known as local variables<\/em>. They are not initialized with any default value.<\/p>\r\n \r\n\r\nLocal variables are in local namespace.\r\n\r\n \r\n\r\n1.2. Global namespace<\/strong>\r\n\r\n \r\n

An entity declared outside any block has global scope<\/em>, meaning that its name is valid anywhere in the code. Storage for global entities is static. Their life is for the entire duration of the program. Variables with global scope are known as global variables. By default, variables with static storage<\/em> are initialized to zeroes, so global variables have zero as their initial value.<\/p>\r\n \r\n\r\nGlobal variables are in global namespace.\r\n\r\n \r\n\r\n1.3. Unique name in namespace<\/strong>\r\n\r\n \r\n\r\nIn each scope, may be block (local) or global, a name can represent only one entity. Thus, there cannot be two variables with the same name in the same scope.\r\n\r\n \r\n\r\nint function1 ()\r\n\r\n{\r\n\r\nint x;\r\n\r\n \r\n\r\nx = 0;\r\n\r\n \r\n\r\ndouble x; \/\/ wrong: name already used in this scope x = 0.0;\r\n\r\n}\r\n\r\n \r\n

Blocks may be nested. The visibility of an entity with block scope<\/em> extends until the end of the block, including inner blocks. An inner block, being a different block, can re-utilize a name existing in an outer block scope to refer to a different entity. In such case, the name will refer to a different entity within the inner block, hiding the entity with the same name in outer block scope.<\/p>\r\n \r\n

In the following example, note that variable y of outer block has a scope in nested block also. There is no variable named y in inner block. But variable named x occurs in outer block as well as in inner block. Re-utilizing variable name x in inner block hides variable x of outer block. See global variable named x declared outside block. To refer it within block, it requires using scope resolution operator.<\/p>\r\n\"\"\r\n\r\n \r\n

cout << \"x: \" << x << '\\n';<\/p>\r\n

cout << \"y: \" << y << '\\n';<\/p>\r\n

}<\/p>\r\n

cout << \"outer block:\\n\";<\/p>\r\n

cout << \"x: \" << x << '\\n';<\/p>\r\n

cout << \"y: \" << y << '\\n';<\/p>\r\n

cout << \"global variable x: \" << ::x << '\\n';<\/p>\r\n

return 0;<\/p>\r\n

}<\/p>\r\n1.4. Need of namespace<\/strong>\r\n\r\n \r\n\r\nAs explained, only one entity can exist with a particular name in a particular scope or namespace in C++.\r\n\r\n \r\n

For local variables in block scope, there is hardly a chance for entities with same name; because blocks tend to be relatively short and names have particular purposes within them. But, for global scope, chances are very high for having multiple entities with same name as explained in following scenario.<\/p>\r\n \r\n

Real applications are usually very large and divided into multiple modules or tasks. These modules are mostly developed by more than one developer in an independent manner. It is possible that different developers used same variable name or class name for different purposes in their programs. To produce the final application, these separate source files are compiled and linked. Application will not be created successfully due to conflict in names. Error will occur considering re-declaration of a variable.<\/p>\r\n \r\n

For example, consider an event management application consisting of many program files. Assume one file having variable \u2018length\u2019 referring to number of days of an event. In another file, variable \u2018length\u2019 is used to store number of characters used in the string representing event name. While linking these two files, problem will arise due to name clash in variable \u2018length\u2019.<\/p>\r\n \r\n\r\nMany times, application may include libraries developed by different vendors. Hence, it is possible to have entities with same name in different libraries.\r\n\r\n \r\n

For example, assume library1 containing class Angle in degree and library2 containing class Angle in radian. When these two libraries are included in a program, occurrence of repeated entity name Angle will create a conflict.<\/p>\r\n \r\n

C++ do not allow two entities with the same name in the same scope. Entities declared in separate files or libraries share global namespace. Hence, multiple definitions of names or name clashes may be encountered while linking the separate modules.<\/p>\r\n \r\n\r\nTo solve this problem, some workarounds may be like\r\n