Is there a specific reason nested namespace declarations are not allowed in C++?

Question

The standard does not allow code like this:

namespace Hello::World {

//Things that are in namespace Hello::World

}

and instead requires

namespace Hello { namespace World {

//Things that are in namespace Hello::World

}}

What is the rationale? Was this simply not thought of at the time, or is there a specific reason it is not included?

It seems that the first syntax more directly expresses in which namespace one is supposed to be, as the declaration mimics the actual use of the namespace in later code. It also results in less indentation if you are unfortunate enough to be using a "dumb" bracket counting indentation tool.

Answer 1

The reason is most likely "because that's how the language evolved."

There has been at least one proposal ("Nested Namespace Definition Proposal" in 2003) to allow nested namespace definitions, but it was not selected for inclusion in C++0x.

Answer 2

As I said in that "possible duplicate" thread, in C++ qualified names are reserved for referring to previously declared entities. This applies to both "sources" of qualified names: classes and namespaces.

Answer 3

I presume you'd rather want it to be so that, given namespace X::Y, it should be equivalent to "namespace X { namespace Y`. Which sounds fine on the surface of it, but consider a corner case:

namespace Hello {
   namespace {
      namespace World {}
   }
}

// Does this refer to an existing namespace? Or does it define a new one?
namespace Hello::World {}

Answer 4

I think it was a design choice.

The first syntax looks good. I would want to have it too. However, the second one is more structured. You don't create nested namespaces this way:

Hello::World::Everyone::Great {

}

Will you declare this beforehand?