[boost] unique_type

20 Feb 2004

      This code is extremely simple, but it begs several questions, and I hope
this forum is appropriate for discussion:

1. Is there something similar in boost already?
2. Are nested anonymous namespaces portable across most common
implementations?
3. Does anyone know of a better way of doing this, especially overcoming
the problem with having multiple declarations on the same line?
4. Does anyone know of a way to create a unique object id constant that
does not require something like using the address of a static variable
-- this would require memory for each instance of the object id?
5. Any other comments about this snippet, or related issues?

Thanks!!

#ifndef wjh__unique_type__hpp_
#define wjh__unique_type__hpp_

// Some of my meta-programming tasks require a completely
// unique type.  I am sure there is a better way, but this
// is what I currently have.
//
// Simply use the macro WJH_UNIQUE_TYPE in any source file,
// and a unique type will be defined.
//
// This depends on:
//     __LINE__ providing the line number of the source from which
//     the macro is called.
//
//     The nested anonymous namespace is specific to the compilation
//     unit.  So, each unit can have a type created with the same
//     line number, but the two types will still be unique.
//
//     Note that it "breaks" if you put more than one WJH_UNIQUE_TYPE
//     on a single line of code.
//
// I tried to create an OID at compile time, but was unsuccessful.
// This would certainly make the job of creating a unique type
// a lot easier.
//
// I almost always use the macro directly:
//      wjh::typemap<std::string, WJH_UNIQUE_TYPE>
// but there may be reasons for:
//      typedef WJH_UNIQUE_TYPE type_x;
namespace wjh
{

  namespace detail
  {

    namespace
    {
      template <int v>
      struct unique_type_helper
      {
      };
    } // end anonymous namespace

  } // end detail namespace

} // end wjh namespace

// Construct a "unique" type
#ifdef WJH_UNIQUE_TYPE
#warning WJH_UNIQUE_TYPE is already defined!
#else
#define WJH_UNIQUE_TYPE wjh::detail::unique_type_helper< __LINE__ >
#endif

#endif // wjh__unique_type__hpp_

-- 
Jody Hagins

Some men are born mediocre, some men achieve mediocrity, and some men
have mediocrity thrust upon them.
		-- Joseph Heller, "Catch-22"