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

There’s no real advantage to adding such a feature to the language. Features generally don’t get added unless there’s demand.

What would namespaces inside classes buy you? Would you really rather say binary_tree::iterator::left_depth instead of simply binary_tree::left_depth? Perhaps if you had multiple namespaces inside, you use them to distinguish say binary_tree::depth_iterator::left and binary_tree::breadth_iterator::right.

In any event, you can achieve the desired result using internal classes as a poor-programmer’s namespace, which is even more reason why there isn’t demand for true namespaces inside classes.