From: Tim Hunter <cyclists@...>
Date: 2003-08-13T20:46:32+09:00
Subject: Re: Exposing C "enums" through extensions

On Wed, 13 Aug 2003 17:01:03 +0900, Brian Candler wrote:

> 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.

This is the practice that RMagick follows. Each attribute accessor that
accepts a constant has code that tests the input argument against the list
of valid values.

That, however, does not prevent somebody from assigning a constant with a
correct value but incorrect name. For example, if APPLES = 3 then
miter_line_join would happily accept APPLES as an argument.

I don't regard this as a major problem.