shared - i need it to be useful

[Stanislav Blinov]

On Wednesday, 17 October 2018 at 14:14:56 UTC, Nicholas Wilson 
wrote:
> On Wednesday, 17 October 2018 at 07:24:13 UTC, Stanislav Blinov 
> wrote:
>> On Wednesday, 17 October 2018 at 05:40:41 UTC, Walter Bright 
>> wrote:
>>
>>> When Andrei and I came up with the rules for:
>>>
>>>    mutable
>>>    const
>>>    shared
>>>    const shared
>>>    immutable
>>>
>>> and which can be implicitly converted to what, so far nobody 
>>> has found a fault in those rules...
>>
>> Here's one: shared -> const shared shouldn't be allowed. 
>> Mutable aliasing in single-threaded code is bad enough as it 
>> is.
>
> Could you explain that a bit more? I don't understand it, 
> mutable -> const is half the reason const exists.

Yes, but `shared` is not `const`. This 'might change' rule is 
poisonous for shared data: you essentially create extra work for 
the CPU for very little gain. There's absolutely no reason to 
share a read-only half-constant anyway. Either give immutable, or 
just flat out copy.

> I was thinking that mutable -> shared const as apposed to 
> mutable -> shared would get around the issues that Timon posted.

It wouldn't, as Timon already explained. Writes to not-`shared` 
mutables might just not be propagated beyond the core. Wouldn't 
happen on amd64, of course, but still. It's not about atomicity 
of reads, it just depends on architecture. On some systems you 
have to explicitly synchronize memory.