From: Brian Candler <B.Candler@...>
Date: 2003-08-13T17:01:03+09:00
Subject: Re: Exposing C "enums" through extensions

On Wed, Aug 13, 2003 at 01:03:14PM +0900, Scott Thompson wrote:
> I'm allowing Ruby to access a C based library through an extension.  
> I'd like to expose some of the enums that are in the C interface, but 
> wasn't sure about the best way to do this.
> 
> My first thought was to define them as class variables within the 
> relevant class.  But then, as I discovered, there wasn't a good way I 
> could keep external code from modifying the values of those variables.  
> It would be really bad if someone set the value of the "miter line 
> join" constant to 7 when the only valid range of values is 0 - 3.

The closest thing to enums would be class constants:

     class MyClass
       THIN_LINE = 1
       THICK_LINE = 2
       MONSTER_LINE = 3
     end

     p MyClass::MONSTER_LINE

(and being constants, you will at least get a warning if someone tries to
reassign them)

Also, you don't have to expose variables in your classes, just provide
accessor methods (either instance methods or class methods) for settings
values; then you can validate the values yourself.

     class MyClass
       def self.miter_line_join=(x)
         raise "Naughty!" unless (0..3) === x
         @@miter_line_join = x
       end
     end

     MyClass.miter_line_join = 3

Regards,

Brian.