C++ Primer, Fifth Edition

18.2.3. Classes, Namespaces, and Scope

Name lookup for names used inside a namespace follows the normal lookup rules: The search looks outward through the enclosing scopes. An enclosing scope might be one or more nested namespaces, ending in the all-encompassing global namespace. Only names that have been declared before the point of use that are in blocks that are still open are considered:

namespace A {    int i;    namespace B {        int i;         // hides A::i within B        int j;        int f1()        {            int j;    // j is local to f1 and hides A::B::j            return i; // returns B::i        }    }  // namespace B is closed and names in it are no longer visible    int f2() {       return j;      // error: j is not defined ...