A few questions regarding GC.malloc

[Steven Schveighoffer via Digitalmars-d-learn]

On 9/26/14 3:24 PM, monarch_dodra wrote:
> On Friday, 26 September 2014 at 18:03:40 UTC, Steven Schveighoffer wrote:
>> On 9/25/14 6:03 PM, Sean Kelly wrote:
>>> On Thursday, 25 September 2014 at 21:43:53 UTC, monarch_dodra wrote:
>>>> On Thursday, 25 September 2014 at 20:58:29 UTC, Gary Willoughby wrote:
>>>>> What does BlkAttr.FINALIZE do when used in the GC.malloc call?
>>>>
>>>> I have no idea. I think its for classes though, since we (currently)
>>>> don't finalize structs anyways.
>>>
>>> Yes it's for memory blocks containing class instances.  It basically
>>> tells the GC to call Object.~this() when collecting the block.
>>
>> Just to add to Sean's statement, don't use this flag. It will crash
>> the runtime, unless you have properly set up the classinfo pointer :)
>> It does NOT work on structs, though I think there is a PR in the works
>> to have structs destroyed from the GC.
>
> Kind of like APPENDABLE I guess, since it only works if you correctly
> setup the appendable data, and correctly slice at the correct offset,
> both of which are implementation defined.

Well, kind of yes.

But the difference here is that if APPENDABLE is set, and you never 
actually append to an array in that block, then you are going to be 
fine. Even if you didn't set it up, it's most likely that you will not 
encounter a problem, because whatever is there is likely not the right 
size (and that simply results in a reallocation without touching 
anything). Even if you do manage to use appending on that block, and the 
data that happens to be where the allocated size is matches the current 
slice end, it will corrupt some data (or not, the smaller block sizes 
keep the append data at the end of the block). This could potentially 
crash, or it could do nothing.

If FINALIZE is set, when the GC collects the memory it WILL crash if you 
didn't set up some sort of mock classinfo.

I don't recommend using either, but FINALIZE is a much worse outcome IMO :)

-Steve