On Friday, 27 February 2015 at 18:24:27 UTC, Andrei Alexandrescu
wrote:
DIP74's function call protocol for RCOs has the caller insert
opAddRef for each RCO passed by value. Then the callee has the
responsibility to call opRelease (or defer that to another
entity). This choice of protocol mimics the
constructor/destructor protocol and probably shows our C++ bias.
However, ARC does not do that. Instead, it implicitly assumes
the callee is a borrower of the reference. Only if the callee
wants to copy the parameter to a member or a global (i.e. save
it beyond the duration of the call), a new call to retain() (=
opAddRef) is inserted. That way, functions that only need to
look at the object but not store it incur no reference call
overhead.
So I was thinking of changing DIP74 as follows:
* Caller does NOT insert an opAddRef for byval RCOs
* Callee does NOT insert an opRelease for its byval RCO
parameters
It seems everything will just work with this change (including
all move scenarios), but it is simple enough to make me worry
I'm missing something. Thoughts?
I think it's fine. I couldn't even figure out the original motive
for wanting to add those calls -- I thought it must have
something to do with threads or exceptions or something, but even
then I couldn't figure it out. Any reference argument will, by
definition, outlive its function -- it can't possibly die within
the function itself, since the caller still thinks it's a valid
reference.
Another thing is that local references in general need not
participate in reference counting. They will retain and release
the reference automatically when they go in and out of scope. I'm
really no expert (except that I like to study and think and by
thinking become somewhat expert it appears), but if all ARC could
be confined to global/heap <=> global/heap copies, you'd get the
most efficient code. And I'm not trying to advertise a reference
tracking system :-), but the real hiccup is that global reference
can go *through* the stack and land back at a global... and you
would need to keep track of that.
