Challenge

[Jonathan M Davis via Digitalmars-d]

On Tuesday, October 04, 2016 14:24:59 Manu via Digitalmars-d wrote:
> On 4 October 2016 at 12:30, Jonathan M Davis via Digitalmars-d
>
> <digitalmars-d at puremagic.com> wrote:
> > On Tuesday, October 04, 2016 11:13:36 Manu via Digitalmars-d wrote:
> >> I'm feeling John's solution is a little bit simpler. But nice work,
> >> thanks!
> >
> > So, it is. LOL. I'd actually glanced over that post while I was in the
> > middle of getting my version to work, and I read it too quickly, because I
> > understood that it had just solved the property problem and that it didn't
> > work for all cases. I'll have to update my PR. Though his code does make
> > the mistake of doing
> >
> > mixin(`alias mem = T.` ~ member ~ `;`);
> >
> > rather than doing something like
> >
> > alias mem = AliasSeq!(__traits(getMember, T, member))[0];
> >
> > which means that there are some cases where it won't work properly. The
> > core logic is simpler though, which is definitely a plus.
>
> Make that change in your PR :)

I already updated it and was actually able to make it slightly simpler than
John's example (as far as I can tell, FunctionTypeOf is only needed in the
case where the address is taken).

> I think the PR is important. It's not obvious how to do this, and it's
> very useful. I was astonished it's not already there.

Yeah. I ran into a need for something similar recently, but my
implementation at the time wasn't as thorough, since it just used offsetof
to do the check (though in my case, I think that was enough). Getting it
completely right is surprisingly difficult.

I was also surprised that while we have quite a few __traits for functions,
they're severely lacking for variables (e.g. I was looking for the variable
equivalent of __traits(isStaticFunction, ...), and there is no such beast).
For that matter, even testing whether something is a variable is
surprisingly difficult.

- Jonathan M Davis