Unless I'm mistaken, this page:

http://dlang.org/garbage.html

states that it's safe to use a union to share storage with a pointer, e.g.

union U { void* ptr; int value }

But that wouldn't be safe if the GC ever moved objects. Should the page be tweaked? Especially as further down on that page, it says that a moving garbage collector can be implemented if one follows the rules.

On 25/02/2013 23:15, Ben Davis wrote:
> Unless I'm mistaken, this page:
>
> http://dlang.org/garbage.html
>
> states that it's safe to use a union to share storage with a pointer, e.g.
>
> union U { void* ptr; int value }
>
> But that wouldn't be safe if the GC ever moved objects. Should the page
> be tweaked? Especially as further down on that page, it says that a
> moving garbage collector can be implemented if one follows the rules.

Actually I suppose it's possible for any object affected by that union to be pinned. Is that the intention?

On Monday, 25 February 2013 at 23:17:16 UTC, Ben Davis wrote:
> On 25/02/2013 23:15, Ben Davis wrote:
>> Unless I'm mistaken, this page:
>>
>> http://dlang.org/garbage.html
>>
>> states that it's safe to use a union to share storage with a pointer, e.g.
>>
>> union U { void* ptr; int value }
>>
>> But that wouldn't be safe if the GC ever moved objects. Should the page
>> be tweaked? Especially as further down on that page, it says that a
>> moving garbage collector can be implemented if one follows the rules.
>
> Actually I suppose it's possible for any object affected by that union to be pinned. Is that the intention?

As I understand it, there will always be uncertain references, such as those found on the stack (esp. stack of C functions) and data segment. Such uncertain references will pin objects they point at, however ideally this would only apply to a small percentage of heap objects. A moving GC in D would only move those objects for which it can be certain that it knows how to safely update all existing references to the object.