From: Cees Zeelenberg <lists@...>
Date: 2012-07-01T22:15:18+09:00
Subject: Re: Interactions between 'new' and 'initialize' ?

Many thanks for all the contributions.

Let me first try to explain a little bit further why I wanted tho change
the 'new' class method.
I have recently implemented a set of JRuby bindings for the QtJambi
Library. QtJambi is the Java version of the Qt GUI framework, a
cross-platform application development toolkit.
The library consists of several hundred Java classes and Interfaces. In
JRuby these can all be accessed through the Qt:: namespace.
Classes and Modules are 'lazily' loaded when they are accessed through
the 'const_missing' method.
In this method a constant is assigned to the underlying Java Class and a
class method 'new' is defined similar to the one mentioned in one of the
postings:
  def new(*a,&b)
    instance=self.allocate
    instance.send(:initialize,*args)
    if block_given?
      if block.arity == -1 || block.arity == 0
        instance.instance_eval(&block)
      elsif block.arity == 1
        block.call(instance)
      else
        raise ArgumentError, "Wrong number of arguments to
block(#{block.arity} ; should be 1 or 0)"
      end
    end
    return instance
  end

 The underlying object is an instance of a Java Class, sending an
initialize method is the equivalent of Ruby 'initialize'. JRuby takes
care of a great deal of 'type' conversions between Ruby and Java.
 The 'new' method also implements an optional Ruby Initializer Block.
Various other things take place upon loading the Java Class, but they
are not directly relevant to this discussion.
 Anyone actually interested in the code can download it from
http://github.com/CeesZ/qt_connect or get the Gem: gem install 
qt_connect
 This takes care of a basic API with access to all Classes and
Interfaces( Modules) in the Qt toolkit. The Java Classes can be extended
with Ruby methods or inherited by Ruby Classes.

 Bindings to the Qt toolkit for Ruby (not JRuby) have been available for
many years. Since these bindings and the QtJambi library are based on
the same underlying C++ library the resulting API's are very similar,
but not quite the same.
 To improve compatibility (and program portability) between the two
implementations, I have also added a compatibility layer, which
basically patches individual classes when they are loaded.
 These patches are mostly mutations to the input arguments which can be
done by simple Ruby wrappers round the relevant Java Methods. This works
well in practice and many QtRuby programs can now run unchanged on both
platforms.

 I ran into a problem however when the arguments of the constructor have
to be patched. I implemented that by wrapping the 'new' method (since
that is already a Ruby Method; the initializer is not).
 This works fine for the base class, but causes problems for inherited
classes as described in the initial posting.
 As far as I can see it, my best bet to solve this problem is to provide
Ruby wrappers for the 'initialize' methods and to apply any patches
there.
 Thanks to the posting of Robert Klemme I understand better what happens
when changing a 'new' class method, but I can't say I fully understand
it, so it seems safer not to interfere.
 For those interested, I found also a paragraph (13.1.4 Class methods in
-even more- depth) in The Well-Grounded Rubyist from David Black, on the
special behavior on singleton methods defined on objects of class Class.

 == Cees Zeelenberg

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