From: Tim Pease <tim.pease@...>
Date: 2009-04-07T08:08:49+09:00
Subject: Re: class C < B < A


On Apr 6, 2009, at 4:12 PM, Iņaki Baz Castillo wrote:

> Hi, I have this case:
>
>  class A     ; end
>  class B < A ; end
>  class C < B ; end
>
>  a = A.new
>  b = B.new
>  c = C.new
>
>
> I look for a way (***) to get the following results:
>
> a)
>  a *** A => true
>  a *** B => false
>  a *** C => false
>
> b)
>  b *** A => false  (*1)
>  b *** B => true
>  b *** C => false
>
> c)
>  c *** A => false  (*2)
>  c *** B => true
>  c *** C => true
>
>
> Note that I could use ===, so:
>  A === a => true
>  A === b => false
>
> but in cases *1 and *2 the result is not what I want:
>
>  A === b => true (I want false)
>  A === c => true (I want false)
>

Well, it's kinda hackish but it does give the correct output. Tailor  
to suit your needs:


class A
   alias :like? :instance_of?
end

class B < A; end

class C < B
   def like?( clazz )
     return true if clazz == B
     super
   end
end

a = A.new
b = B.new
c = C.new

[A, B, C].each do |clazz|
   puts "a.like? #{clazz.name}  => #{a.like? clazz}"
end
puts

[A, B, C].each do |clazz|
   puts "b.like? #{clazz.name}  => #{b.like? clazz}"
end
puts

[A, B, C].each do |clazz|
   puts "c.like? #{clazz.name}  => #{c.like? clazz}"
end


==== OUTPUT ====
a.like? A  => true
a.like? B  => false
a.like? C  => false

b.like? A  => false
b.like? B  => true
b.like? C  => false

c.like? A  => false
c.like? B  => true
c.like? C  => true


Again, I'm not proud of this code -- it works, though, without being  
too tricky.

Blessings,
TwP