On Monday, 10 December 2012 at 13:37:46 UTC, Thiez wrote:
On Monday, 10 December 2012 at 12:45:16 UTC, Dan wrote:
That would be an infinite loop. If you have a compile time
cycle you would likely need your own custom dups anyway, as
you are doing low level and heap allocating already. But for
the simpler cases without cycles, if dup encounters a pointer
it creates a new instance on the heap (if compilation is
possible) and dup's into it.
Wouldn't it be better to do a cycle detection here? I imagine
such a thing could be done quite easily by adding every pointer
in the original struct to an associative array along with its
(new) copy. Then, whenever you encounter a new pointer, you can
check if it is already in the AA, and if so use the copy you
made before. Of course this has some overhead compared to your
suggestion, but it seems to me it would be safe in all cases,
which makes more sense with a 'general dup'.
I see a few possibilities:
(0) Do nothing and caveat coder
(1) Compile time check for possibility of cycle and if exists do
not compile. This way the scenario you mention, which may or may
not really have instance cycles, would not even compile. This may
be overly aggressive.
(2) Compile time check for possibility of cycle and runtime
checks ensuring there are none.
(3) Disallow dup on structs with embedded pointers (excepting
array and associative array). Similar to (0) but now basic
structs with pointers to other basic structs and no chance of
cycles would not be dupable.
I'm fine with any of these because I figure if you are allocating
your own objects you probably want to write your own postblit and
dup. But, you are correct, cycle detection at runtime would be
better - assuming there was no runtime performance hit for the
case when cycles are not possible which is known at compile time.
I'm not so sure it is as easy as a simple AA, though.
-------
struct S {
struct Guts {
}
Guts guts;
}
S s;
-------
In this case both &s and &s.guts have the same address. So you
might want to have an AA per type? But where would those exist?
On the stack? If so how would you pass it through to all the
recursive calls?
I'm not saying it is not doable - I just think it may be a pretty
big effort.
Thanks,
Dan
