From: Brian Candler <b.candler@...>
Date: 2009-10-13T17:52:56+09:00
Subject: Re: Scope of constants in instance_eval

pharrington wrote:
> well in Ruby 1.8 something like this works (obviously not it all
> cases, but you get the gist and can fix it as needed):
> 
> Obj.new { puts "CONST = #{self.class.const_get("CONST")}" }

Or more simply, self.class::CONST

I find this particularly useful when I wish to override a constant in a 
subclass, e.g.

class Foo
  TIMEOUT = 10
  def run
    puts "Timeout is #{self.class::TIMEOUT}"
  end
end

class Bar < Foo
  TIMEOUT = 20
end

Bar.new.run

Note that you need this trick even in ruby 1.9.2. Without it, the 
constant is still resolved statically, depending on in which class the 
method was defined:

$ irb19 --simple-prompt
>> RUBY_VERSION
=> "1.9.2"
>> RUBY_DESCRIPTION
=> "ruby 1.9.2dev (2009-07-18 trunk 24186) [i686-linux]"
>> class Foo
>>   TIMEOUT = 10
>>   def run
>>     puts "Timeout is #{TIMEOUT}"
>>   end
>> end
=> nil
>> class Bar < Foo
>>   TIMEOUT = 20
>> end
=> 20
>> Bar.new.run
Timeout is 10
=> nil
>> class Bar
>>   def run2
>>     puts "Timeout is #{TIMEOUT}"
>>   end
>> end
=> nil
>> Bar.new.run2
Timeout is 20            ## ruby1.8 shows 10 here
=> nil
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