From: Robert Klemme <shortcutter@...>
Date: 2006-09-03T03:50:37+09:00
Subject: Re: Class behaviour

Geoff Barnes wrote:
> Just curious, can I write a class that can check values as they are 
> assigned to objects?
> 
> foo = Klass.new
>  foo.legal_values = (4..10)
> 
> foo.value = 5  # OK
> foo.value = 11 # ERROR - out of bounds, raise exception or something

You can define a special class method as a replacement for attr_accessor:

class Module
   def checked_attr(name,&test)
     define_method("#{name}=") do |val|
       test[val] or raise ArgumentError
       instance_variable_set "@#{name}", val
     end

     attr_reader name
   end
end

class T
   checked_attr :foo do |x|
     x >= 0
   end
end

irb(main):018:0* t=T.new
=> #<T:0x392088>
irb(main):019:0> t.foo = 10
=> 10
irb(main):020:0> t.foo = -10
ArgumentError: ArgumentError
         from (irb):4:in `foo='
         from (irb):20
         from :0
irb(main):021:0>

> Also, do I have to have an explicit method for the 'value', or can it 
> operate like a "builtin" class, like this :
> 
> foo = Klass.new
>  foo.legal_values = (4..10)
> 
> foo = 5  # OK
> foo = 11 # ERROR - out of bounds, raise exception or something

You cannot interfere here so there is no way to check these assignments.

> In Perl, I would probably use tiescalar for something like this, where I 
> can intercept an assignment...
> 
> Also, it may not be numbers, but enums as well.

I'm not sure I understand you here.

Kind regards

	robert