From: Robert Klemme <bob.news@...>
Date: 2005-02-09T18:20:07+09:00
Subject: Re: Creating objects from a template

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"David McCabe" <davemccabe@gmail.com> schrieb im Newsbeitrag
news:1107929791.839839.293300@c13g2000cwb.googlegroups.com...
> Hi folks.
>
> I'm pretty comfortable with Python, and I just decided to give Ruby a
> shot. I'm trying to duplicate a nifty method I use in Python to
> instantiate objects with properties that are loaded from a file.
>
> My program is a war strategy game, which involves several different
> types of artillery units. Each type has properties such as range,
> speed, power, etc. I need to load these values from a file.
>
> In python, my parser returns a hash of unit_type objects, where the
> names of units are the keys. Each time a unit is instantiated, it
> receives a unit_type object as an argument. Now here's the interesting
> part: it copies the instance variables from the unit_type into itself.
>
> class Unit(object):
>       def __init__(self, type):
>   self.type = type
>   self.__dict__.update(self.type.__dict__)
>
> There's more to it than that, but that's the interesting part. So now
> I can say:
>
>       my_new_unit = Unit(types["Panzer"])
>
> And I'll get a Unit object with all the values for Panzers
> initialized.
>
> drbrain on #ruby-lang came up with this:
>
> http://rafb.net/paste/results/lgb4xv42.html
>
> It looks like it'll do what I want. There's only one deficiency: I
> would have to list out every possible proporty of a unit in two
> seperate places in the source. In the python version, I don't have to
> list it at all. Whatever properties are in the config file are loaded
> into the object.
>
> So, now I'm looking for any pointers on ways to do this.

You need two simple modifications: make the initialization more general
and add accessors automatically.  (see file attached).

If those properties never need to change for instances, then of course
another approach applies (see access-2.rb).

Kind regards

    robert



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class ArtilleryUnit=0A=
=0A=
  attr_reader :range=0A=
=0A=
  ##=0A=
  # Metaprograming to get the right constant (our subclass' DEFAULTS)=0A=
=0A=
  def self.defaults=0A=
    const_get 'DEFAULTS'=0A=
  end=0A=
=0A=
  ##=0A=
  # So #initialize doesn't look so lame=0A=
=0A=
  def defaults=0A=
    self.class.defaults=0A=
  end=0A=
=0A=
end=0A=
=0A=
# load config, pretend it comes back as a hash=0A=
=0A=
#            class name     # default for class=0A=
defaults =3D { 'LongRange' =3D> { :range =3D> 5, :size =3D> 10 } }=0A=
#                             ^^^^^^ a Symbol=0A=
=0A=
defaults.each do |klass_name, defaults|=0A=
=0A=
  # Create the new class=0A=
=0A=
  klass =3D Class.new(ArtilleryUnit) do=0A=
    # setup DEFAULTS constant=0A=
    const_set 'DEFAULTS', defaults=0A=
=0A=
    # create accessors=0A=
    defaults.each do |k,v|=0A=
      define_method(k){ v }=0A=
    end=0A=
  end=0A=
=0A=
  # add klass_name klass to the toplevel namespace (rooted at Object in =
Ruby)=0A=
  Object.const_set klass_name, klass=0A=
end=0A=
=0A=
p LongRange.ancestors=0A=
p LongRange::DEFAULTS=0A=
p LongRange.new=0A=
p LongRange.new.range=0A=
p LongRange.new.public_methods.grep /range|size/=0A=

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class ArtilleryUnit=0A=
=0A=
  attr_reader :range=0A=
=0A=
  ##=0A=
  # Metaprograming to get the right constant (our subclass' DEFAULTS)=0A=
=0A=
  def self.defaults=0A=
    const_get 'DEFAULTS'=0A=
  end=0A=
=0A=
  ##=0A=
  # So #initialize doesn't look so lame=0A=
=0A=
  def defaults=0A=
    self.class.defaults=0A=
  end=0A=
=0A=
  def initialize=0A=
    # generalized initialization=0A=
    defaults.each do |k, v|=0A=
      send "#{k}=3D", v=0A=
    end=0A=
  end=0A=
=0A=
end=0A=
=0A=
# load config, pretend it comes back as a hash=0A=
=0A=
#            class name     # default for class=0A=
defaults =3D { 'LongRange' =3D> { :range =3D> 5, :size =3D> 10 } }=0A=
#                             ^^^^^^ a Symbol=0A=
=0A=
defaults.each do |klass_name, defaults|=0A=
=0A=
  # Create the new class=0A=
=0A=
  klass =3D Class.new(ArtilleryUnit) do=0A=
    # setup DEFAULTS constant=0A=
    const_set 'DEFAULTS', defaults=0A=
=0A=
    # create accessors=0A=
    defaults.each do |k,|=0A=
      attr_accessor k=0A=
    end=0A=
  end=0A=
=0A=
  # add klass_name klass to the toplevel namespace (rooted at Object in =
Ruby)=0A=
  Object.const_set klass_name, klass=0A=
end=0A=
=0A=
p LongRange.ancestors=0A=
p LongRange::DEFAULTS=0A=
p LongRange.new=0A=
p LongRange.new.range=0A=
p LongRange.new.public_methods.grep /range/=0A=

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