c++ - Why can't we declare a namespace within a class?

Question

Declaring a class within a class is valid. (Nested classes)

Declaring a namespace within a class is invalid.

The question is: is there any good reason (other than c++ grammar/syntax problems) to forbid the declaration of a namespace within a class ?

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As for why would i want to do that, here is an exemple :

Let's have a basic delcaration of a binary tree container

template<typename Data>
class binary_tree
{
 public:
  ... stuff ....     

 private:
  ... iterators class declaration ...

 public:
  typedef left_depth_iterator_impl     left_depth_iterator;
  typedef right_depth_iterator_impl    right_depth_iterator;
  typedef left_breadth_iterator_impl   left_breadth_iterator;
  typedef right_breadth_iterator_impl  right_breadth_iterator;

  ... stuff ....     

 private:
  Data         data;
  binary_tree* left;
  binary_tree* right;
};

Now i notice that there are a lot of iterators in my class, so i would like to regroup them within the same namespace like this :

template<typename Data>
class binary_tree
{
 public:
  ... stuff ....     

 private:
  ... iterators class declaration ...

 public:
  namespace iterator
  {
    typedef left_depth_iterator_impl     left_depth;
    typedef right_depth_iterator_impl    right_depth;
    typedef left_breadth_iterator_impl   left_breadth;
    typedef right_breadth_iterator_impl  right_breadth;
  }

  ... stuff ....     

 private:
  Data         data;
  binary_tree* left;
  binary_tree* right;
};

This would allow a simple usage :

void  function()
{
  binary_tree::iterator::left_depth   it;

  ...stuff...
}

This works if i use a class instead of a namespace, but i am then forced to declare a class that will never be instantiated which is quite a namespace.

Why allow nested classes and forbid nested namespaces within classes ? is it a legacy burden ?

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Answers with semantic reasons that do not only quote part of the standard(especially syntax parts) will be apreciated :)

Answer 1

Since you asked which parts of the standard mandate namespace location, we hit that up first:

C++11 7.3-p4: Every namespace-definition shall appear in the global scope or in a namespace scope (3.3.6).

Regarding class definitions and the proposition of declaring a namespace within, I bring you to...

C++11 9.2-p2: A class is considered a completely-defined object type (3.9) (or complete type) at the closing } of the class-specifier. Within the class member-specification, the class is regarded as complete within function bodies, default arguments, exception-specifications, and brace-or-equal-initializers for non-static data members (including such things in nested classes). Otherwise it is regarded as incomplete within its own class member-specification.

Ergo, a class definition is finite once the closing curly is reached. It cannot be opened back up and extended (derivation is something different, but it is NOT extending the class just defined).

But lurking at the very beginning of the standard definition of a namespace is the ability to extend it; to expand it for lack of a better term:

C++ 7.3-p1: A namespace is an optionally-named declarative region. The name of a namespace can be used to access entities declared in that namespace; that is, the members of the namespace. Unlike other declarative regions, the definition of a namespace can be split over several parts of one or more translation units. (emphasis added).

Therefore, a namespace within a class would violate the definition in 7.3-p4. Assuming that was not present, it would be possible to declare a namespace anywhere, including in a class, but since the definition of a class is formalized once it is closed, you would be left with only the ability to do the following if you maintained compliance with 7.3-p1:

class Foo
{
   namespace bar
   {
       ..stuff..
   }

   .. more stuff ..

   namespace bar
   {
       ..still more stuff..
   }
};

The usefulness of this feature was likely debated for about 3-full-seconds before 7.3-p4 was established to settle it.