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

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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.

language-design namespaces c++
5 Answers

Nested namespace definition is part of the C++17 working draft.
This topic is mentioned in the proposal as an example of this feature being a programmers' demand original n4026 updated version: n4230.

Current latest draft: n4567 (paragraph 7.3.1 item 10)

7.3.1 Namespace Definition
...
10 A nested-namespace-definition with an enclosing-namespace-specifier E, identifier I and namespace-body B is equivalent to namespace E { namespace I { B } }

Example:

namespace A::B::C {
  int i;
}    The above has the same effect as:

namespace A {
  namespace B {
    namespace C {
      int i;
    }
  }
}

Compiler Support

GCC since version 6 enable using -std=c++1z
Visual C++ since 2015 update 3 enable using /std:c++latest
Clang since version 3.6 enable using -std=c++1z

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