From: "Albert L. Wagner" <alwagner@...>
Date: 2002-01-19T05:47:21+09:00
Subject: Re: dynamic method creation

Phil Tomson wrote:
> 
> In article <3C4878FE.F0D83753@uark.edu>,
> Albert L. Wagner <alwagner@uark.edu> wrote:
> >I have a need to dynamically create methods with method names
> >that are not known until run time.  Below is my current best
> >shot.  Is there a better way? If methods allowed expressions as
> >names, and not just as qualifiers, I wouldn't need the eval.
> 
> A couple of questions about your example....
> 
> >
> >class Record
> >       def initialize(name)
> >               @name = name
> >       end
> >end
> >
> >fieldNames = ['abc', 'dof', 'ghi']
> >fieldTypes = ['String', 'Fixnum', 'Float']
> >rec = Record.new('aRecord')
> >class << rec
> >end
>  ^^^
> This 'end' is probably not supposed to be here, right?

I think it's needed to close the class << rec, just above it.  I
get a parse error without it.

> 
> >fieldNames.size.times do | i |
> >       f = fieldNames[i]
> >       defStr = "def rec.#{f}=(obj)\n"
> >       case fieldTypes[i]
> >               when 'String'           then defStr << "@#{f} = obj\n"
> >               when 'Fixnum'           then defStr << "@#{f} = obj\n"
> >               when 'Float'            then defStr << "@#{f} = obj\n"
> >       end
> What is the purpose of this 'case' statement?  All branches seem to do the
> same thing - am I missing something?

No, you aren't missing anything.  Those are fossils from earlier
code. Sorry.

> 
> >       defStr << "end\n"
> >       puts defStr
> >       eval defStr
> Maybe it's better to use instance_eval?

Like this?

	rec.instance_eval defStr

That works too.  What is the difference?
> 
> >end
>  ^^^
> I suspect that this 'end' is the one that corresponds to the
> class << rec above (?)
This one closes: fieldNames.size.times do | i |
> 
> >p rec.abc=(nil)
> >p rec.dof=(nil)
> >p rec.ghi=(nil)
> >p rec.methods
> 
> I can't come up with another way that doesn't use some kind of eval to do
> this.
> 
> Phil

Thanks, Phil. I think I'll try writing it to a file and then
require it. It shouldn't happen too often.

-- 
"It requires wisdom to understand wisdom:
 the music is nothing if the audience is deaf."
 - Walter Lippman