From: Paul Brannan <pbrannan@...>
Date: 2003-08-14T23:25:57+09:00
Subject: Re: Exposing C "enums" through extensions

On Wed, Aug 13, 2003 at 08:53:53PM +0900, Brian Candler wrote:
> Nor would I; but if it were a concern you could use symbols instead, and a
> hash to map to the correct constant value.
> 
>   class MyClass
>     LINES = {
> 	:thin => 1,
> 	:thick => 2,
> 	:monster => 3,
>     }
>     def self.set_line_size(x)
>       @@line_size = LINES[x] || (raise "Bad value")
>     end
>   end
>   MyClass.set_line_size(:thick)

An interesting solution.

I prefer to use instances of a particlar class instead, e.g.:

  MyEnum = Struct.new(:name, :value)
  class MyEnum
    def to_s
      return name()
    end
  end

  THIN_LINE    = MyEnum.new("THIN"    , 0)
  THICK_LINE   = MyEnum.new("THICK"   , 1)
  MONSTER_LINE = MyEnum.new("MONSTER" , 2)

Then I can write something like:

  case line_style
  when THIN_LINE    then ...
  when THICK_LINE   then ...
  when MONSTER_LINE then ...
  end

Or something like:

  puts THIN_LINE #=> "THIN"

It's also possible to add singleton methods to each of these objects to
avoid case/when.

I've written a few classes to help me generate these type of enums in
Ruby code and in extensions:

  http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/rubycollections/rubycollections/rbc/enum.rb?rev=1.7&content-type=text/vnd.viewcvs-markup
  http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/excruby/excruby/excruby/enum_wrapper.hpp?rev=1.2&content-type=text/vnd.viewcvs-markup

The latter has no sample code, but in my extensions I write something
like:

  Exc_Ruby::Enum_Wrapper<Line_Style> line_style;

  extern "C"
  void Init_extension()
  {
    line_style
      .initialize(rb_cObject, "Line_Style")
      .define_value("THIN_LINE"    , THIN_LINE)
      .define_value("THICK_LINE"   , THICK_LINE)
      .define_value("MONSTER_LINE" , MONSTER_LINE)
  }

And on days when I'm not in the mood to deal with something this
heavyweight, I can always cheat a little and just use instances of Class
instead of instances of a particular enum type:

  class THIN_LINE    ; end
  class THICK_LINE   ; end
  class MONSTER_LINE ; end

Paul