Get certified by completing a course today! If you want to report an error, or if you want to make a suggestion, do not hesitate to send us an e-mail:. Syntax Declare a namespace called Html: namespace Html;. Report Error. Your message has been sent to W3Schools. W3Schools is optimized for learning and training. In the program above, we can see how nested namespaces are declared and accessed from outside of the namespace.
We also demonstrated the way we access a variable in the namespace without the scope resolution operator :: and use the using directive. We have also seen how we can create the namespace myalias that allows us to use it explicitly using the name of the alias. We have seen the benefits of using namespace in our programs, with our examples above.
By using namespace, we can prevent conflicts in files with the same variable names and function names within the same program. Peer Review Contributions by: Saiharsha Balasubramaniam. Dawe is passionate about innovation, new technology and software development. Dawe is particularly motivated in learning all about android and wants a future full of it.
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For Infrastructure Providers. Simple, centralized, intelligent management of distributed compute locations on massive scale. What is a namespace? An inline namespace is a namespace that uses the optional keyword inline in its original-namespace-definition. Members of an inline namespace are treated as if they are members of the enclosing namespace in many situations listed below.
This property is transitive: if a namespace N contains an inline namespace M, which in turn contains an inline namespace O, then the members of O can be used as though they were members of M or N. Note: the rule about specializations allows library versioning: different implementations of a library template may be defined in different inline namespaces, while still allowing the user to extend the parent namespace with an explicit specialization of the primary template.
This definition is treated as a definition of a namespace with unique name and a using-directive in the current scope that nominates this unnamed namespace Note: implicitly added using directive makes namespace available for the qualified name lookup and unqualified name lookup , but not for the argument-dependent lookup.
Even though names in an unnamed namespace may be declared with external linkage, they are never accessible from other translation units because their namespace name is unique.
Unnamed namespaces as well as all namespaces declared directly or indirectly within an unnamed namespace have internal linkage , which means that any name that is declared within an unnamed namespace has internal linkage. Introduces a name that is defined elsewhere into the declarative region where this using-declaration appears. A using-declaration with more than one using-declarator is equivalent to a corresponding sequence of using-declarations with one using-declarator.
For the use in derived class definitions, see using declaration. Names introduced into a namespace scope by a using-declaration can be used just like any other names, including qualified lookup from other scopes:.
If, after the using-declaration was used to take a member from a namespace, the namespace is extended and additional declarations for the same name are introduced, those additional declarations do not become visible through the using-declaration in contrast with using-directive.
One exception is when a using-declaration names a class template: partial specializations introduced later are effectively visible, because their lookup proceeds through the primary template.
Each declarator in a using-declaration introduces one and only one name, for example using-declaration for an enumeration does not introduce any of its enumerators. All restrictions on regular declarations of the same names, hiding, and overloading rules apply to using-declarations:. If a function was introduced by a using-declaration, declaring a function with the same name and parameter list is ill-formed unless the declaration is for the same function.
If a function template was introduced by a using-declaration, declaring a function template with the same name, parameter type list, return type, and template parameter list is ill-formed. Two using-declarations can introduce functions with the same name and parameter list, but if a call to that function is attempted, the program is ill-formed.
If an entity is declared, but not defined in some inner namespace, and then declared through using-declaration in the outer namespace, and then a definition appears in the outer namespace with the same unqualified name, that definition is a member of the outer namespace and conflicts with the using-declration:.
More generally, a declaration that appears in any namespace scope and introduces a name using an unqualified identifier always introduces a member into the namespace it's in and not to any other namespace. The exceptions are explicit instantiations and explicit specializations of a primary template that is defined in an inline namespace: because they do not introduce a new name, they may use unqualified-id in an enclosing namespace. A using-directive is a block-declaration with the following syntax:.
Using-directives are allowed only in namespace scope and in block scope. From the point of view of unqualified name lookup of any name after a using-directive and until the end of the scope in which it appears, every name from namespace-name is visible as if it were declared in the nearest enclosing namespace which contains both the using-directive and namespace-name. Using-directive does not add any names to the declarative region in which it appears unlike the using-declaration , and thus does not prevent identical names from being declared.
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