From: mirian@... (Mirian Crzig Lennox)
Date: 2001-06-10T23:26:52+09:00
Subject: [ruby-talk:16388] Re: other ways to create class instance?

After thinking about the question Samantha raised (creating class
instances without new), I realized that perl has a rather useful
mechanism for this, which is "bless".  In perl, any object (usually a
hash) can be turned into an object of class X by calling bless on it.
This allows the object to be hand-crafted beforehand.

Of course, in perl the structure of objects are exposed to such a
low-level that working with them is much clumsier than in a pure OO
language like Ruby.  But you can still do something remarkably
similar:

module HashAttrs
  def HashAttrs.construct clazz, hash
    clazz.new.set_attrs_from_hash hash
  end

  def set_attrs_from_hash hash
    hash.each {
      |key, value|
      setter = key.to_s + "="
      send setter, value
    }
    self
  end
end

Then, any object that you wish to be constructible in this way need
only include the HashAttrs module and declare its attr accessors,
e.g.

class MyClass
  include HashAttrs

  attr_accessor :foo, :bar, :baz
  public :foo, :bar, :baz
  private :foo=, :bar=, :baz=
end

And then you can easily instantiate an object of that class by
constructing a hash mapping attrs to values, and passing that to
HashAttrs.construct along with the class name:

my_object_attrs = { "foo", 32, "bar", 54, "baz", 102 }
my_object = HashAttrs.construct MyClass, my_object_attrs

You can also combine this with the marshalling technique I described
previously, so that objects can be stored/loaded easily as mere
key,value pairs, and hashes acts as intermediaries between that format
and the fully-instantiated object form -- with all of the code to do
this in one place only, and abstracted away from the implementation of
the specific classes themselves.

--Mirian