Problem with const correctness

[Dan]

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