shared - i need it to be useful

[Nicholas Wilson]

On Saturday, 20 October 2018 at 17:06:22 UTC, Stanislav Blinov 
wrote:
> On Saturday, 20 October 2018 at 16:48:05 UTC, Nicholas Wilson 
> wrote:
>> On Saturday, 20 October 2018 at 09:04:17 UTC, Walter Bright 
>> wrote:
>>> by code that believes it is unshared
>>
>> you cannot `@safe`ly modify the memory  through `b`, `a`'s 
>> view of the memory is unchanged in @safe code.
>
> And that's already a bug, because the language can't enforce 
> threadsafe access through `a`, regardless of presence of `b`. 
> Only the programmer can.

  access through `a` is through the owned reference threadsafety 
through a does't mean anything, all _other_ access must ensure 
that the are ordered correctly.

>
>>> and, code that believes it is shared.
>>
>> you cannot have non-atomic access though `b`, `b` has no @safe 
>> view of the memory, unless it is atomic (which by definition 
>> is synchronised).
>
> Synchronized with what? You still have `a`, which isn't 
> `shared` and doesn't require any atomic access or 
> synchronization.

Synchronized w.r.t any writes to that memory, e.g. from `a`.

>At this point it doesn't matter if it's an int
> or a struct.

Yes.

> As soon as you share `a`, you can't just pretend that reading 
> or writing `a` is safe.

You can if no-one else writes to it, which is the whole point of 
Manu's proposal. Perhaps it should be const shared instead of 
shared but still.