Forward reference to class in external namespace

[Castalia]

When declaring a forward reference to a class in external namespaces I've found that the following syntax works as desired:

namespace A {
namespace B {
class C;
}}

This is a bit awkward, but the syntax -

class A::B::C;

- is rejected.

Is there an acceptable, and succinct, alternative syntax for this case?

[JLBorges]

> Is there an acceptable, and succinct, alternative syntax for this case?

AFAIK, no.

[vlad from moscow]

A translation unit has to know how the names A and B were declared. That is it has to know what entities are denoted by A and B.