From: Paul Brannan <pbrannan@...>
Date: 2002-09-06T22:39:37+09:00
Subject: Re: Ruby aesthetics

On Fri, Sep 06, 2002 at 09:45:32AM +0900, Yukihiro Matsumoto wrote:
> It's in the ToDo list.  Private instance variables will be something
> like @_foo.  But it's pretty hard to implement.  If someone come up
> with a good idea, I'm glad to hear.

The way I implemented it in Ruby was:

1) Each class has a Hash which maps object id to a table of private
   instance variables.

2) When I access a private instance variable, I use a function get_piv,
   which uses some set_trace_func magic to know the class from which
   it was called.  It then looks up the variable in the above Hash.

3) I also hook the initialize method of every class that has private
   instance variables to define a finalizer that removes the entry in
   the Hash for the given object when it is destroyed.

Problems:

1) Since the private instance variable is not part of the object itself,
   it will not get marshalled/demarshalled.

2) Determining which class the variable belongs to can be tricky in
   Ruby.  I would imagine that this would be much easier in C.

3) set_trace_func is slow.

4) I'm not sure how well this maps to C.  It should work, but it may not
   be the best solution.

BTW, when you say that private instance variables are "hard to
implement," what exactly do you mean?  What barriers have you run into
while trying to implement them?

Paul