From: Stefano Crocco <stefano.crocco@...>
Date: 2008-03-08T01:48:19+09:00
Subject: Re: Dynamic class thingies? (Okay, not sure how to title this one)

Alle Friday 07 March 2008, coigner ha scritto:
> On Fri, 07 Mar 2008 10:11:43 -0500, Stefano Crocco wrote:
> > Alle Friday 07 March 2008, coigner ha scritto:
> >> Asking for ideas here but let me preface...
>
> <snip my long winded stuff>
>
> > You want Kernel.const_get. As the name suggests, it takes a string and
> > returns the value of the constant with that name. For instance:
> >
> > class C
> > end
> >
> > Kernel.const_get('C').new
> > => #<C:0xb7c55064>
> >
> > If your classes are defined top-level, that's all you need. If they're
> > defined inside a module, you need to call const_get for that module:
> >
> > module M
> >   class C
> >   end
> > end
> >
> > M.const_get('C').new
> >
> > I hope this helps
>
> Does and thanks. Though I'd already started getting it to work with eval.
> Which do you think is better? Or does it matter?

I'd use const_get because it's a tool made for exactly this task, while eval
is a much broader tool. Besides, const_get seems also faster:

require 'benchmark'
Benchmark.bm("const_get".size) do |b|
  b.report("const_get"){1_000_000.times{Kernel.const_get('C')}}
  b.report("eval"){1_000_000.times{eval('C')}}
end

               user     system      total        real
const_get  0.890000   0.040000   0.930000 (  0.932886)
eval       3.080000   0.100000   3.180000 (  3.177851)

Of course, if you have a deeply nested module hyerarchy, eval might be easier 
to use. Given this hyerarchy:

module A
 module B
  module C
   module D
    class E
    end
   end
  end
 end
end

to get the class E, you'd need the string "A::B::C::D::E". You can obtain E
using const_get with this code:

"A::B::C::D::E".split('::').inject(Kernel){|res, i| res.const_get i}

while using eval you can achieve the same with much less code:

eval "A::B::C::D::E"

In this case, eval is also faster:

require 'benchmark'
Benchmark.bm("const_get".size) do |b|
  b.report('const_get'){1_000_000.times{"A::B::C::D::E".split('::').inject(
Kernel){|res, i| res.const_get i}}}
  b.report('eval'){1_000_000.times{eval "A::B::C::D::E"}}
end

               user     system      total        real
const_get 15.360000   0.940000  16.300000 ( 16.299995)
eval       5.270000   0.130000   5.400000 (  5.394793)

Stefano