From: Paul Brannan <pbrannan@...>
Date: 2004-09-08T23:21:21+09:00
Subject: Re: Marshal and allocating objects

On Sat, Sep 04, 2004 at 07:45:22PM +0900, Stephen  Sykes wrote:
> I would find it useful if Marshal were to use "allocate" to generate
> new objects.

It does use allocate, for objects of type T_DATA (objects created from C
extensions).

> This is so that I can load unknown objects by doing something like
> this:
> 
> class Object
> def self.const_missing(name)
> const_set(name, kl = Class.new)
> kl
> end
> end
> 
> And then be able to load objects with marshal without getting
> ArgumentError: undefined class/module.

Even if allocate is used, this won't work.  The lookup for the class is
failing before allocate is called.

> For example (after running the above code)...
> 
> irb(main):026:0> M.allocate
> => #<M:0x2bb2168>
> 
> That works ok, but...
> 
> irb(main):029:0> Marshal.load("\004\010o:\006N\000")
> ArgumentError: undefined class/module N
> from (irb):29:in `load'
> from (irb):29
> from (null):0
> 
> If Mashal used allocate, and hence called const_missing, then the load
> would not fail.  Sound reasonable?

I don't like your solution, since now if I write:

  class Foo
    SOME_CONSTANT = 1
    def foo
      return SOME_CONSTATN
    end
  end

my code will no longer break, but will silently do the wrong thing.

Arguably better would be a callback for when Marshal.load cannot find
the class it needs.  I say arguably because there really is no good way
to create a class with the correct behavior unless you already know
something about the class.

Paul