From: Eric Mahurin <eric_mahurin@...>
Date: 2005-05-11T03:20:43+09:00
Subject: Re: a class that looks like (kind_of?) several classes

> > > > Is there a way to make a class that looks like several
> > > classes?
> > > >  For example:
> > > >
> > > > class IOArray < IO
> > > >     include Array # doesn't work because Array is not a
> > > Module
> > > >     # use IO methods calls to accomplish the Array
> methods
> > > > end
> > > >
> > > > I don't need multiple inheritance.  All I want is these
> > > > additional classes to appear in the ancestors list.  In
> the
> > > > above example, IOArray could act like an IO or an
> Array.
> > > It
> > > > looks like evil.rb might have a solution, but is there
> a
> > > > non-evil way?
> > >
> > > class IOArray < IO
> > >   def kind_of?(cl)
> > >     Array == cl || super
> > >   end
> > > end
> > 
> > Yep, that is pretty obvious.  I think that is all I need,
> but I
> > guess overriding ancestors would complete this.
> > 
> > > >> x=IOArray.allocate
> > > => #<IOArray:0x10184258>
> > > >> x.kind_of? Array
> > > => true
> > >
> > > But seriously, for what do you need that?  I can't think
> of
> > > an application
> > > where I would want to make something look like an Array
> > > without being one.
> > > Maybe it's rather Enumerable that you want?
> > 
> > Enumerable doesn't have random access and can't modify like
> > Array (or String).  I think it would have been nice to have
> a
> > module like "Indexable" that would cover common methods in
> > Array and String.
> > 
> > The example I have above should probably be combining an IO
> and
> > a String since an IO accesses characters like a String:
> > 
> > class IOString < IO
> >     def kind_of?(c); String==c || super; end
> >     # use IO methods (seek/tell/read/write) to accomplish
> >     # the String methods
> > end
> 
> This is generally unneeded in Ruby; If duck typing is used
> properly,
> the ancestors and kind_of? shouldn't matter. And if it *does*
> matter,
> then masquerading like this could cause problems:
> 
>   if obj.kind_of? String
>     var.replace obj
>   end
> 
> This code would break if obj is an IOString as defined above,
> even
> though this code has no logic errors.
> 
> If you check out the StringIO class (require 'stringio'), you
> will
> find that it inherits neither from IO nor String. But it
> still works
> very nicely :)


Thanks.  I assumed that the StringIO class inherited from the
IO class since it has all of the same methods.  So the proper
Ruby style is to use respond_to? instead of kind_of? to see
what "type" your arguments are?

You still should be able to make the above "replace" code work
above.  For the IOString class I described, you'd just replace
the contents of the file with the string passed.  seek(0);
truncate(write(value)).  That would seem to accomplish the
replace function.



	
		
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