Why is namespace composition so rarely used?

Question

In his book The C++ Programming Language (third edition) Stroustrup teaches to define individual components in their own namespace and import them in a general namespac

Answer 1

I guess because it reduces encapsulation. In your example, it's going to be a right pain when you write map_api to go to the api.h and import it into the api namespace. Having one big header api.h that imports every sub-namespace is not exactly include minimization.

• map.h:

  namespace map_api { /* fns */ }
  

• api.h:

  #include 
  namespace api { using map_api::map; }
  

Or, considering other header layouts: if the imports into the api namespace happen in another file:

• map_internal.h:

  namespace map_api { /* fns */ }
  

• map_api.h:

  #include 
  namespace api { using map_api::map; }
  

That's pretty painful too, having to split out the api into two files like that.

The final possibility is doing it all in one file:

• map.h:

  namespace map_api { /* fns */ }
  namespace api { using map_api::map; }
  

In that case, what's the point I wonder of putting things in two namespaces that are on the same level of abstraction, if they're both equally exposed to all clients/includers..?