On Tuesday, 2 June 2015 at 16:02:56 UTC, Namespace wrote:
Thanks to DIP 25 I think it's time to review this again. I
would implement it (if no one else wants to do it), but there
are still some unanswered questions:
1. Is 'auto ref' still the chosen syntax (I guess so)?
2. Should auto ref for templates act like auto ref for
non-templates (creates a temporary and pass it by ref) or
should the current behaviour stay (duplicate the function 2^N
times)?
3. What's with return ref, should auto ref for non-templates
include return ref (like auto ref for templates)?
4. What's with this constellation:
struct S { }
void ene(S) { }
void mene(ref S) { }
void muh(auto ref S) { }
should 'mene' (ref) interfere with 'muh' (auto ref)?
auto ref with templated functions needs to retain its current
behavior. Changing it would not only break existing code, but it
would lose what we have in terms of perfect forwarding (IIRC,
it's not quite perfect forwarding, but it's close, and we'd be
much farther from it without auto ref).
If we want to have non-templated functions which can accept both
rvalues and lvalues without copying the lvalues, then we need
have a way to mark those parameters so that they're ref and then
have an invisible, temporary variable inserted to hold a copy of
an rvalue so that it can be passed by ref and accepted by the
function as well instead of just accepting lvalues.
If we want that to work with both non-templated and templated
functions, then we need a new syntax. If we're willing to have
them work with just non-templated functions, then we could reuse
auto ref for that (and _maybe_ we could have the compiler
optimize auto ref on templates to mean the same thing when it can
determine that it's safe to do so and thus avoid extra template
instantiations, but I question that that will happen). But then
we only get it for non-templated functions, and auto ref means
somethings slightly different for templated and non-templated
functions, which sucks, but I'm not sure that it's ultimately all
that big a deal.
I _definitely_ think that it would be a huge mistake for ref in
general to accept rvalues. If we did that, then suddenly, ref
would be used everywhere, and you couldn't tell when someone
wanted to actually set the ref argument to something or whether
they were just trying to avoid extraneous copies of lvalues. I'd
much rather have no way to have a parameter accept both rvalues
and lvalues without copying the lvalues with non-templated
functions than have ref accept rvalues.
So, basically, I think that we have three options:
1. Do nothing. If you want a function parameter to accept both
lvalues and rvalues efficently, then either duplicate it with
various overloads to achieve that or templatize it and use auto
ref so that the compiler will do that for you.
2. Extend auto ref so that it works with non-templated functions
by inserting a temporary variable for rvalues so that they can be
passed to the function by ref. Templated functions stay as they
are.
3. Add a new attribute - e.g. @rvalue ref - which inserts a
temporary variable for rvalues so that they can be passed by ref,
and it works with both templated and non-templated functions.
Right now, we seem to be doing #1, and we have never officially
decided whether that's permanent. Andrei in particular has
resisted adding a new attribute, and to some extent, I agree that
that's undesirable, but it _would_ allow us to solve this problem
for both templated and non-templated functions, which we can't
really do otherwise. So, I don't know how reasonable or feasible
#3 is at this point. #2 probably wouldn't be that hard to get in,
but then it only works with non-templated functions, and it
complicates the meaning of auto ref in an already complicated
language.
Honestly, at this point, I don't know how much this issue really
matters. It's annoying that we don't have rvalue references, but
in general, we're living without them just fine, and we're
heading toward templatizing almost everything for ranges anyway,
in which case, the current version of auto ref will work just
fine with most code (though that extraneous template bloat _is_
ugly). So, while I'd like to have rvalue references, I also think
that we can get by just fine without them.
If we _were_ going to add rvalue references, at this point, I'd
probably lean towards a new attribute, because it's cleaner and
more flexible that way, but it _does_ mean adding a new
attribute, and I don't know if that's worth it.
- Jonathan M Davis
