6 Basics [basic]

6.4 Scope [basic.scope]

6.4.6 Namespace scope [basic.scope.namespace]

1
#
The declarative region of a namespace-definition is its namespace-body.
Entities declared in a namespace-body are said to be members of the namespace, and names introduced by these declarations into the declarative region of the namespace are said to be member names of the namespace.
A namespace member name has namespace scope.
Its potential scope includes its namespace from the name's point of declaration onwards; and for each using-directive that nominates the member's namespace, the member's potential scope includes that portion of the potential scope of the using-directive that follows the member's point of declaration.
[ Example
: 
namespace N {
  int i;
  int g(int a) { return a; }
  int j();
  void q();
}
namespace { int l=1; }
// the potential scope of l is from its point of declaration to the end of the translation unit

namespace N {
  int g(char a) {   // overloads N​::​g(int)
    return l+a;     // l is from unnamed namespace
  }

  int i;            // error: duplicate definition
  int j();          // OK: duplicate function declaration

  int j() {         // OK: definition of N​::​j()
    return g(i);    // calls N​::​g(int)
  }
  int q();          // error: different return type
}
— end example
 ]
2
#
If a translation unit Q is imported into a translation unit R ([module.import]), the potential scope of a name X declared with namespace scope in Q is extended to include the portion of the corresponding namespace scope in R following the first module-import-declaration or module-declaration in R that imports Q (directly or indirectly) if
  • (2.1)
    X does not have internal linkage, and
  • (2.2)
    X is declared after the module-declaration in Q (if any), and
  • (2.3)
    either X is exported or Q and R are part of the same module.
[ Note
:
A module-import-declaration imports both the named translation unit(s) and any modules named by exported module-import-declarations within them, recursively.
[ Example
:

Translation unit #1:

export module Q;
export int sq(int i) { return i*i; }

Translation unit #2:

export module R;
export import Q;

Translation unit #3:

import R;
int main() { return sq(9); }    // OK: sq from module Q
— end example
 ]
— end note
 ]
3
#
A namespace member can also be referred to after the ​::​ scope resolution operator ([expr.prim.id.qual]) applied to the name of its namespace or the name of a namespace which nominates the member's namespace in a using-directive; see [namespace.qual].
4
#
The outermost declarative region of a translation unit is also a namespace, called the global namespace.
A name declared in the global namespace has global namespace scope (also called global scope).
The potential scope of such a name begins at its point of declaration ([basic.scope.pdecl]) and ends at the end of the translation unit that is its declarative region.
A name with global namespace scope is said to be a global name.