passing __FILE__, __MODULE__, etc... with varadic types

[Jonathan M Davis]

On Friday, July 19, 2013 19:23:50 JS wrote:
> On Friday, 19 July 2013 at 15:32:25 UTC, Jonathan M Davis wrote:
> > On Friday, July 19, 2013 11:06:26 JS wrote:
> >> I would like to pass to all my templates the file and module
> >> locations where they are used(this goes into a debugging
> >> system I
> >> have come up with).
> >> 
> >> The problem is, with varadic types being passed I can't do
> >> this:
> >> 
> >> template T!(T..., string file = __FILE__)
> >> 
> >> doesn't work.
> >> 
> >> I think there is no way around except to explicitly pass
> >> __FILE__... which would be a mess?
> > 
> > As long as you're using a templated function and thus can use
> > IFTI (implicit
> > function template instantation) instead of giving the template
> > arguments
> > explicitly, you don't have to put the template parameters with
> > default
> > arguments last. You'd just have to put it after the variadic
> > parameter if you
> > intended to give the template arguments explicitly. So, as long
> > as you don't
> > have to give the template arguments explicitly, you're fine. If
> > you need to
> > give them explicitly though, I think that you're stuck.
> > 
> > - Jonathan M Davis
> 
> I don't at all see how this could possibly work. The order of
> parameters is crucial. With a type tuple it may be different but
> I just tried and it didn't work:
> 
> template A(string f = __FILE__, T...)
> {
> pragma(msg, f);
> enum A = T.stringof;
> }
> 
> called with A!(int, double) and got an error about argument type
> mismatch.
> 
> So unless you are talking about something else or there is some
> "trick" involved I don't think this works...

As I said, it works if you're using IFTI (Implict Function Template 
Instantation) but not if you're giving the template arguments explicitly. So, 
if you're dealing with a templated function such as

auto foo(string file = __FILE__, T...)(T args) {...}

then it works. But if you're dealing with a template which is not a function 
(so you have to instantiate it explicitly), or you have to instantiate the 
templated function explicitly for any reason, then you'll be forced to give 
the file name as the first argument. So, you have a way out with many templated 
functions, but for other templates, you still have the problem that you've 
been running into.

- Jonathan M Davis