shared - i need it to be useful

[Isaac S.]

On Tuesday, 16 October 2018 at 02:26:04 UTC, Manu wrote:
>> I understand your point but I think the current shared (no 
>> implicit conversion) has its uses. It can be quite useful to 
>> have one interface for when an object is shared and one for 
>> when it is not (one with and without the synchronization 
>> cost). Sure, something as trivial as a counter can be 
>> re-implemented in both ways but more complex objects would 
>> easily result in extreme code duplication.
>
> If you can give a single 'use', I'm all ears ;)

My usages are a custom ref counted template and list types (which 
are built on top of the former). The ref counted template will 
initialize naively when not shared but utilize compare-and-set 
and thread yielding if needed when shared (ensureInitialized can 
occur any time after declaration). The list types (which are not 
shared-compatible *yet*) will utilize a read-write mutex only 
when shared (no cost beyond a little unused memory to non-shared 
objects). As such, casting any of the non-shared versions of 
these types to shared would be unsafe.

(Technically the types can be safely casted to shared so long as 
no non-shared
reference to them exists and vice versa.)

>> It's also easy to acknowledge that implicit conversion to 
>> shared has its uses.
> I actually know of many real uses for this case.
(There was a time when I wanted it as well. I've forgotten what 
for by now.)

> That might be fine.
> Like I said in OP, the first point that I think needs to be 
> agreed on,
> is that shared can not read or write members. I think that's a
> pre-requisite for any interesting development.

I will agree that *outsiders* should not be able to read/write 
members of a shared object. I'm not sure whether your design 
means even within a shared function such members cannot be read 
from or written to. If it does, the casting required may get a 
little annoying but isn't unworkable (especially since an 
unshared template can do the cast in an @trusted manner).