[swift-evolution] stored properties in extensions (was: associated objects)

Jay Abbott jay at abbott.me.uk
Sun Oct 9 14:47:07 CDT 2016

I have been thinking further on this and in addition to my previous two
thoughts about implementation, I have had another idea...

3. If there is a bit spare in the object header somewhere (that is
currently always zero), this could be used to signify the presence of an
additional struct that immediately follows after the existing object data.
I *think* that this method would allow binary compatibility with older
modules. Instances that have this bit set would allow stored properties in
extensions. The struct at the end would have one member, a pointer to a
table of additional objects/values, stored properties defined in extensions
could be stored in here, using a hash derived from the
module/protocol/extension/property name (or something better if it exists).

The struct could be very simple as described above or more complex, with
additional features, for example a list of deinit hooks, dynamically added
methods, etc. The struct itself may also be dynamic in size/layout if such
complexity is warranted by size or extensibility concerns. Perhaps it
should start with some flags and its size (size would be fixed and only
increase with revisions so this would double as a 'version' number).

If viable - this would be a much better way to implement this feature than
my previous two ideas. It doesn't require global lookups or additional
levels of indirection beyond accessing the dynamic data/feature itself.

On Mon, 3 Oct 2016 at 04:13 Jay Abbott <jay at abbott.me.uk> wrote:

> Are stored properties in extensions already being discussed elsewhere? Is
> this one of those deferred-but-not-indexed-anywhere subjects?
> I wonder how stored properties could potentially be implemented, I can
> only think of two ways:
> 1. An extra pointer per instance (with resulting ABI compatability
> implications) to hold a collection of the stored items.
> 2. A global lookup for any instance where stored properties have been set.
> I'm not a language implementation expert, or familiar with the swift
> implementation, so there may be other/better ways - I'd like to know if
> there are?
> If not, and option 2 was employed, a little foresight might enable the
> mechanism to be overloaded in the future for other dynamic features too. A
> bit flag (I'm hoping there's a spare bit in an existing flags field
> somewhere?) could indicate whether any feature had caused the object to be
> added to this lookup and deinit could check this bit and make sure the
> object is removed, thus any stored properties are nilled. The lookup value
> could be a struct with one member (extensionStoredProperties), and
> additional members can be added in future for new features.
> I get the impression from the associated objects discussion that perhaps
> there are much better, more optimal, more ingenious, more unknown-by-me
> ways of doing such things, so apologies if this whole idea is way-off the
> mark :D
> Jay
> P.S. Note that stored properties in extensions could enable developers to
> implement their own dynamic features in Swift.. so such desires could be
> satisfied in the short term until they could be done properly in the
> language.
> On Sat, 1 Oct 2016 at 00:49 Chris Lattner via swift-evolution <
> swift-evolution at swift.org> wrote:
> On Sep 30, 2016, at 2:51 PM, Ted F.A. van Gaalen via swift-evolution <
> swift-evolution at swift.org> wrote:
> > Is it possible to have best of (these completely different) both worlds?
> Yes, of course it is.  Your email spends a lot of words trying to form a
> false dichotomy.  Swift can definitely have both awesome dynamic features
> while still having performance, predictability and safety.
> > Would it be possible in Swift to have facilities to generate objects
> > dynamically at runtime? and, if desirable, how can such be implemented?
> Here’s an extant implementation that you can use today:
> https://github.com/Zewo/Reflection
> I’m sure it isn’t ideal, but it proves that it can be done.  When we have
> bandwidth to reevaluate this area from first principles, I’m sure we can
> make improvements on it.
> I will grant you that Smalltalk is a beautiful language in its simplicity,
> but for that simplicity it makes many tradeoffs that we’re not willing to
> make.  We are willing to make the internal implementation of Swift complex
> if that means that we get a beautiful model for programmers - one that
> preserves the virtues of safety-by-default, predictability, performance,
> and joy-to-develop-in.
> The meme of “Swift can never (or will never) support dynamic features” is
> tired, and also wildly inaccurate.
> -Chris
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> swift-evolution at swift.org
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