From: gwtmp01@...
Date: 2007-01-11T23:03:22+09:00
Subject: Re: Sharing constants between C and Ruby code


On Jan 11, 2007, at 8:23 AM, Nathaniel Trellice wrote:
> What I was thinking of doing was writing a Ruby class (in C) which has
> the constants as class constants. This way, compiling the class will
> pull in the #defines from the C header file, and they'll be available
> within Ruby from my class. But, being new to ruby, I was hoping to  
> avoid
> having to figure out how to write a Ruby class in C at this stage  
> of my
> ruby development!

I don't think that approach would buy you much.  You would still have
to maintain (by hand) a C file that mapped the C constants (from the .h
file) into Ruby constants.  I'm not a Ruby's internal programmer but
I imagine you would have to have something like:

	rb_constant_set( klass, 'MY_CONSTANT', MY_CONSTANT)

for every single constant. Note: I made up the function name and API,  
but
you would have to explicitly bind the string 'MY_CONSTANT' to the value
represented by the MY_CONSTANT macro via some sort of function call.

I can't imagine that maintaining that C file by hand is any easier than
the simple parsing suggestion that has already been described to  
generate
a pure Ruby class.  Alternatively, you could describe your constants
via something like YAML and write (in Ruby!) code to generate a C header
file for the constants and Ruby class for your use in the Ruby side  
of things.

Even easier, something like:

class Constants
   A = 100
   B = 200
   C = A + B

   def self.to_h
     constants.sort.inject("") { |t,c|
       t << "#define #{c} #{const_get(c)}\n"
     }
   end
end

Now you can maintain this class directly and have it generate your  
C .h file
via:

	open("constants.h", "w") { |f|
	  f.write Constants.to_h
         }

Anyway, I think there are lots of ways to handle your situation  
without having
to dive into writing Ruby extensions and/or classes in C.



Gary Wright