Difference between __gshared and shared.

[tcak via Digitalmars-d]

On Wednesday, 8 July 2015 at 10:10:58 UTC, Jonathan M Davis wrote:
> On Wednesday, 8 July 2015 at 09:54:01 UTC, ketmar wrote:
>> On Wed, 08 Jul 2015 09:43:38 +0000, wobbles wrote:
>>
>>> Ok, so we should prioritise using 'shared' over __gshared as 
>>> much as possible. Good to know!
>>
>> only `shared` is PITA...
>
> The primary advantage of shared is that it allows most 
> everything to be thread-local.
>
> Though arguably, shared _should_ be a bit of a pain, since its 
> usage should normally be very restricted. But we do need to 
> revisit shared and figure out what we want/need to do with it. 
> Synchronized classes as described in TDPL were never even 
> implemented (though I contest that they really make shared 
> usable in any kind of sane way; I really don't see how you can 
> do anything other than really basic stuff with shared without 
> requiring that the programmer deal with the locks and casting 
> properly on their own). So, more work needs to be done there 
> even if it's figuring out what shared _isn't_ going to be doing.
>
> Really though, one of the bigger problems is dealing with 
> allocation and deallocation of shared objects and passing 
> objects across threads, since we keep wanting to be able to do 
> stuff like have thread-specific allocators, but the way shared 
> currently works doesn't actually allow for it. :|
>
> Regardless, while I would very much like to see shared properly 
> ironed out, I'm _very_ grateful that thread-local is the 
> default in D. It's just so much saner.
>
> - Jonathan M Davis

I still couldn't have found my answer though. I have three 
different use cases, one is missing in the language.

1. Thread-local object.
2. Shared, but implemented as not to be synchronised.
3. Shared, and implemented to be synchronised.

There is no simple way to design a class, so that you can 
implement it for points 2 and 3. The only way is to use _gshared 
with point one to solve it.

I use shared in many of my classes. Thus I experience different 
situations.