Re: Object spec

[Dan Sugalski]

At 5:35 PM -0500 2/13/04, Simon Glover wrote:
 A few questions:

  1) How is the search order for the parents of a particular class
     specified? In particular, is this determined at the Parrot level
     or at the language level? Can it change at runtime?
It's determined by the method invocation code.

  2) Re. the classoffset op: how does this work when multiple parent
     classes specify the same attribute?
     For instance, suppose 'foo' is the first attribute specified in
     classes 'Bar' and 'Baz', which are theselves unrelated (i.e. 'Bar'
     doesn't inherit from 'Baz', nor vice versa), but which are both
     parent classes of a third class 'Qux'. Now, if P0 holds an object
     of class 'Qux', does:
       classoffset I0, P0, 'Bar'

     return the same value as:

       classoffset I0, P0, 'Baz'

     or not?
Nope. When the Qux class is constructed the offsets for each of its 
parent classes (and their parent classes,a nd so on) are calculated, 
and are effectively custom on a per-class basis. So if we had the 
following classes with attributes:

  Bar:
    a
    b
    c
  Baz:
    x
    y
    z
and we had Qux and Xyzzy like so:

  Qux: isa(Bar, Baz)
  Xyzzy: isa(Baz, Bar)
Then you had code that looked like:

  new P0, .Qux
  new P1, .Xyzzy
  classoffset I0, P0, 'Bar'
  classoffset I1, P1, 'Bar'
  print I0
  print "\n"
  print I1
  print "\n"
you'd get:

  2
  5
(Because the first few attributes are reserved) *However*.... 
Attribute "b" is always at classoffset("Bar") + 1, regardless of 
where Bar is in the attribute list.

Basically any class code that directly accesses attributes should 
first query where that class's base offset is for the object in 
question and work from there.

I think I need to add in an example section.

   3) If more than one parent class specifies a particular attribute,
      do all of these parents have to be removed before the attribute
      is removed?
Currently yes. That also means a class can have its attributes in the 
hierarchy only once, regardless of how many times it actually appears.
--
                                        Dan

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Dan Sugalski                          even samurai
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