Namespace declarations

A namespace-declaration consists of the keyword namespace, followed by a namespace name and body, optionally followed by a semicolon.

namespace-declaration:
namespace qualified-identifier namespace-body;_opt

qualified-identifier:
identifier
qualified-identifier. identifier

namespace-body:
{ extern-alias-directives_opt using-directives_opt namespace-member-declarations_opt }

A namespace-declaration may occur as a top-level declaration in a compilation-unit or as a member declaration within another namespace-declaration. When a namespace-declaration occurs as a top-level declaration in a compilation-unit, the namespace becomes a member of the global namespace. When a namespace-declaration occurs within another namespace-declaration, the inner namespace becomes a member of the outer namespace. In either case, the name of a namespace must be unique within the containing namespace.

Namespaces are implicitly public and the declaration of a namespace cannot include any access modifiers.

Within a namespace-body, the optional using-directives import the names of other namespaces and types, allowing them to be referenced directly instead of through qualified names. The optional namespace-member-declarations contribute members to the declaration space of the namespace. Note that all using-directives must appear before any member declarations.

The qualified-identifier of a namespace-declaration may be a single identifier or a sequence of identifiers separated by “.” tokens. The latter form permits a program to define a nested namespace without lexically nesting several namespace declarations. For example,

namespace N1.N2
{
class A {}

class B {}
}

is semantically equivalent to

namespace N1
{
namespace N2
{
class A {}

class B {}
}
}

Namespaces are open-ended, and two namespace declarations with the same fully qualified name contribute to the same declaration space (§3.3). In the example

namespace N1.N2
{
class A {}
}

namespace N1.N2
{
class B {}
}

the two namespace declarations above contribute to the same declaration space, in this case declaring two classes with the fully qualified names N1.N2.A and N1.N2.B. Because the two declarations contribute to the same declaration space, it would have been an error if each contained a declaration of a member with the same name.