From: Josh Cheek <josh.cheek@...>
Date: 2012-08-19T15:26:18+09:00
Subject: Re: A Ruby class is never closed

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On Thu, Aug 16, 2012 at 12:59 PM, Jan E. <lists@ruby-forum.com> wrote:

> What you do mean? A sealed class like in C# which doesn't allow
> subclasses? Or the singleton pattern where a class can only be
> instantiated once?
>
> You can actually do both. For a sealed/final class you can use the
> "inherited" hook which gets called every time a new class is derived
> from this class. If it raises an error, the class is "sealed" (the
> question is if this makes sense).
>
> class A
>   def self.inherited subclass
>     raise "cannot derive from sealed class #{self}"
>   end
> end
>
> # this doesn't work
> class B < A; end
>
> For the singleton pattern, there's the singleton module in the standard
> library.
>
> --
> Posted via http://www.ruby-forum.com/.
>
>
I can't think of any reason you would want to do this, but I don't think
you can ever *force* this sort of situation. e.g. if I decided that I
wanted to subclass it, I could just do this

class << A
  remove_method :inherited
end

I'd look very skeptically at any piece of code which did had such a hook,
though.

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On Thu, Aug 16, 2012 at 12:59 PM, Jan E. <span dir=3D"ltr">&lt;<a href=3D"m=
ailto:lists@ruby-forum.com" target=3D"_blank">lists@ruby-forum.com</a>&gt;<=
/span> wrote:<br><div class=3D"gmail_quote"><blockquote class=3D"gmail_quot=
e" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
What you do mean? A sealed class like in C# which doesn&#39;t allow<br>
subclasses? Or the singleton pattern where a class can only be<br>
instantiated once?<br>
<br>
You can actually do both. For a sealed/final class you can use the<br>
&quot;inherited&quot; hook which gets called every time a new class is deri=
ved<br>
from this class. If it raises an error, the class is &quot;sealed&quot; (th=
e<br>
question is if this makes sense).<br>
<br>
class A<br>
=A0 def self.inherited subclass<br>
=A0 =A0 raise &quot;cannot derive from sealed class #{self}&quot;<br>
=A0 end<br>
end<br>
<br>
# this doesn&#39;t work<br>
class B &lt; A; end<br>
<br>
For the singleton pattern, there&#39;s the singleton module in the standard=
<br>
library.<br>
<div class=3D"HOEnZb"><div class=3D"h5"><br>
--<br>
Posted via <a href=3D"http://www.ruby-forum.com/" target=3D"_blank">http://=
www.ruby-forum.com/</a>.<br>
<br>
</div></div></blockquote></div><br>I can&#39;t think of any reason you woul=
d want to do this, but I don&#39;t think you can ever *force* this sort of =
situation. e.g. if I decided that I wanted to subclass it, I could just do =
this<br>
<br>class &lt;&lt; A<br>=A0 remove_method :inherited<br>end<br><br>I&#39;d =
look very skeptically at any piece of code which did had such a hook, thoug=
h.<br>

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