Low-Lock Singletons In D

[Steven Schveighoffer]

On Tue, 07 May 2013 10:33:13 -0400, David Nadlinger <see at klickverbot.at>  
wrote:

> On Tuesday, 7 May 2013 at 06:50:16 UTC, Mehrdad wrote:
>> As far as I can see, there shouldn't be a need for any other handshake  
>> in this example.
>>
>> As long as the object is fully initialized before _static is written to  
>> (easy enough with just a memory barrier), there is no penalty for  
>> subsequent reads whatsoever.
>>
>> Right?
>
> The issue is that the write to _static might never appear on the other  
> threads, thus leading to multiple instances being created - even though  
> it is atomic in the sense that you never end up reading a pointer with  
> e.g. only half of the bytes updated.

I don't think that is an issue.  The write to _static is protected by a  
lock, which should present a consistent view of it inside the lock.

Mehrdad corrected his version right away that you need to check the value  
inside the lock again.  This essentially is classic double-checked  
locking, with one check being a virtual table lookup.

-Steve