I wish all qualifiers were revisited with an eye for simplification

Manu turkeyman at gmail.com
Tue Aug 4 12:52:01 UTC 2020

On Tue, Aug 4, 2020 at 5:40 PM Sebastiaan Koppe via Digitalmars-d <
digitalmars-d at puremagic.com> wrote:

> On Monday, 3 August 2020 at 21:56:34 UTC, Manu wrote:
> > Shared recently received a `-preview` which makes it really
> > mean something;
> > this is what shared means:
> > 1. The data is shared; therefore, it is invalid to read or
> > write that
> > memory.
> > 2. The reason this is useful as an attribute, is because you
> > are able to
> > attribute methods. Ability to author a set of threadsafe
> > methods and
> > clearly distinguish them from non-threadsafe methods is a
> > powerful
> > advantage over C++.
> For some reason I often end up with multiple threads and the
> coordination that comes with it. Shared has been very helpful for
> me and I am using no. 2 with good success.
> There is just one thing about shared I don't understand. If I
> design my object such that the non-shared methods are to be used
> thread-local and the shared methods from any thread, it follows
> that I should be able to call said shared methods from both a
> shared and non-shared instance of that object.
> Often I workaround it be introducing a non shared method that
> forwards to the shared method by means of casting.

Yes, this is a thing I talked at great length 1-2 years ago.
If you take shared to mean "is thread-safe", then my idea was that
not-shared -> shared implicit conversion should be possible.
What I often do is this:

struct Thing
  ref shared(Thing) implSharedCast() { return *cast(shared)&this; }
  alias implSharedCast this;

If that were an implicit conversion, that implies a slight change of
meaning of shared (to one that I consider immensely more useful), but it's
more challenging for the compiler to prove with confidence, and there's a
lot of resistance to this change.
In the mean-time, until the shared usage patterns are more well-proven, I
recommend you try to use the 'alias this' pattern I show above, and report
on any issues you encounter using this scheme. If no issues are identified
with extensive real-world usage data, I will push again for that implicit
conversion rule.
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