On 2011-06-06 04:17, Jacob Carlborg wrote:
> On 2011-06-06 01:38, Jonathan M Davis wrote:
> > On 2011-06-05 10:24, Jose Armando Garcia wrote:
> >> The problem is that std.log is using the UTC() (indirectly) in shared
> >> static this() for the module. At that point the static ctr for UTC
> >> didn't get to execute. I first changed std.datetime to instantiate UTC
> >> in a shared static ctr for UTC but that still didn't work (I assume
> >> that is a bug in dmd). So I had to move the instantiation to a share
> >> static ctr for the module.
> >>
> >> In my branch the follow code doesn't assert. Note that I was selfish
> >> and only fixed UTC but std.datetime has this problem in all the
> >> singletons.
> >>
> >> import std.datetime;
> >>
> >> shared static this()
> >> {
> >>
> >> assert(UTC() !is null); // this is with my fix.
> >> assert(LocalTime() is null); // I didn't fix LocalTime!
> >>
> >> }
> >>
> >> Having said that what is D's idiom/recommended way of constructing
> >> singletons? Maybe phobos should create shared/global singleton with
> >> the following idiom:
> >>
> >> shared(T) singleton(T)() if(is(T == class))
> >> {
> >>
> >> shared static T one;
> >> if(one is null) cas(&one, null, new shared(T));
> >>
> >> return one;
> >>
> >> }
> >
> > Phobos has no idiom or policies for dealing with singletons. In, fact I
> > think that std.datetime is probably the only module that even has any at
> > this point (most Phobos modules have functions primarily rather than
> > types which may or may not be singletons).
> >
> > In this particular case, because the singletons are immutable, changing
> > them to shared makes a lot of sense, but in the general case, it
> > probably wouldn't (at which point it would be a singleton per thread
> > rather than globally). So, I'll take a look at making the approriate
> > changes in std.datetime for all of its singletons.
> >
> > But if you could, I'd really appreciate it if you could create a small
> > test case (which has nothing to do with std.datetime), which shows the
> > bug with regards to the shared module constructor, since that needs to
> > be fixed. Having to move the variable into the module's scope like that
> > is ugly and shouldn't be necessary at all. I guess that std.datetime
> > will be stuck for the moment, but the bug does need to be properly
> > reported and fixed.
> >
> > - Jonathan M Davis
>
> I'm pretty sure someone added a singleton template to Phobos. I can't
> find it though, maybe it's been removed, or it was never added in the
> first place. At least there was talk about a design pattern module, or
> similar, on this newsgroup with a singleton implementation.
Yeah. I think that a template of some variety for singletons was discussed in
the newsgroup at some point, but I'm not aware of anything of the sort ever
getting into Phobos, and I don't remember any of the details at this point.
- Jonathan M Davis
