Re: Idle curiosity re. using directive/declaration scoped to a given class - is this technique sensible?

From:

Paul Bibbings <paul.bibbings@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 19 Apr 2010 00:17:48 +0100

Message-ID:

<87hbn8ic4j.fsf@gmail.com>

Stuart Golodetz <blah@blah.com> writes:

Hi guys,

Just curious what you think of the technique described below. It seems
to solve an issue I've occasionally encountered, but it involves
putting a using directive/declaration in a header file (but within a
class-specific namespace). Just wondering whether this would be
"frowned upon" or whether you think it's a valid technique please.

The problem:

I occasionally find I want to bring things from a namespace into scope
within a class definition in a header file. AFAIK I can't do that in a
trivial way, however. (Obviously I don't want to put the using
directive/declaration somewhere where it will pollute the global
namespace, or the actual namespace in which the class should reside.)

A suggested solution:

Put a class-specific namespace around the class and shove the using
directive/declaration in that, e.g.

#include <boost/shared_ptr.hpp>

namespace N
{

namespace N__X
{
    using namespace boost;
    struct X
    {
        shared_ptr<int> p;
    };
}

using N__X::X; // bring N__X::X into N (as originally intended)

}

(a) Does this seem reasonable?
(b) Is there a better way at all?

Looks good, and would certainly appear to meet the requirements of your
use case. What is more, it extends to functions without breaking ADL.

   // appended to original code
   namespace N
   {

   namespace N__X
   {
      void default_init_X(X& x) { // illustrative: not necessarily useful
         x.p = shared_ptr<int>(new int());
      }
   }

   using N__X::default_init_X;

   }

   int main()
   {
      X::X x;
      default_init_X(x);
   }

   00:11:45 Paul Bibbings@JIJOU
   /cygdrive/d/CPPProjects/nano $gcc -Id:/boost_1_39_0 -o
      class_specific_namespace.o -c class_specific_namespace.cpp

   00:11:55 Paul Bibbings@JIJOU
   /cygdrive/d/CPPProjects/nano $

Regards

Paul Bibbings