I want to add a Phobos module with template mixins for common idioms.

[Diggory]

On Saturday, 18 May 2013 at 16:58:19 UTC, Idan Arye wrote:
> OK, I implemented everything and made a pull request: 
> https://github.com/D-Programming-Language/phobos/pull/1294

Nice, but the singleton implementation seems somewhat 
over-complicated, and the low-lock singleton is broken, possibly 
as a result of the former problem.

You need two variables of type T (assuming T is the target 
class). One should be __gshared, the other thread-local. When 
"instance()" is called, first check if the thread-local variable 
is non-null, if so just return it. Otherwise enter a synchronized 
block, check if the __gshared variables is null (and if so 
initialise it) then copy its value to the thread-local variable 
and finally return it.

For a "hasInstance" method to make any sense, the caller must be 
able to synchronize on the same object that "instance()" uses to 
synchronize on when it accesses the __gshared variable. Otherwise 
the return value is out of date before it's even been returned.