Clarification on migration to value types and boxed vs unboxed representations

Richard Warburton richard.warburton at gmail.com
Tue Jan 6 02:36:16 UTC 2015


Hi,

Thanks for your reply John - that clarified a lot.

> Its probably worth also noting at this point that about 18 months ago the
> > LJC ran a hackday on the IBM Packed Objects proposal which has some
> overlap
> > with value types. One of the things that people found most confusing was
> > that packed objects had a transparent way of returning what was called a
> > "proxy object". So that's like a boxed version of the packed object. It
> was
> > not at all obvious when looking at the code that you wrote as to whether
> > you were referring to the packed or the proxy object. I suppose part of
> > that was not being able to easily refer to a spec - but part of it is
> just
> > the general opaqueness of implicit conversions in general.
>
> Key question:  How did users experience the difference between the packed
> and the proxy version?
>
> In that design it might have been because of side effects viewed through a
> surprising alias (since packed values are manipulated by-reference).  Value
> types as proposed do not have that set of surprises, because they are
> immutable.
>

No - I think the confusion was that you couldn't look at the code and
obviously see what was a reference and what was packed when thinking about
the object graph. This applied to reading from values as much as writing to
them. I appreciate that mutability adds its own set of confusions but IMHO
this wasn't one of them.

> Having a similar situation with value types would be a real shame. Would
> it
> > be possible to clarify when value types become converted to their boxed
> > equivalents?
>
> If you cast a value to Object you get a box.  Just like casting an int to
> an Object.  ("Codes like a class, works like an int.")  The syntax for
> naming the box-type for a value V is TBD; there are not many use cases for
> it.  (Bad bikeshed color of the moment:  V&Object.)


That's re-assuring to know. I guess of more concern to me is that
re-reading once I assign the value to an interface there will be boxing
(right?), and this seems a lot trickier to spot than the explicit casting
case. I'll try to keep an eye out for this situation.

regards,

  Richard Warburton

  http://insightfullogic.com
  @RichardWarburto <http://twitter.com/richardwarburto>


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