Namespace definitions hold declarations. Since a namespace definition is a declaration itself, namespace definitions can be nested.

An alias can also be applied to a nested namespace.

namespace INTERNATIONAL_BUSINESS_MACHINES {
  int j;
    namespace NESTED_IBM_PRODUCT {
      void a() { j++; }
      int j;
      void b() { j++; }
  }
}
namespace NIBM = INTERNATIONAL_BUSINESS_MACHINES::NESTED_IBM_PRODUCT

In this example, the NIBM identifier is an alias for the namespace NESTED_IBM_PRODUCT. This namespace is nested within the INTERNATIONAL_BUSINESS_MACHINES namespace.