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digitalmars.D - Does the spec preclude objects being movable by the GC?

reply Ben Davis <entheh cantab.net> writes:
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.
Feb 25 2013
parent reply Ben Davis <entheh cantab.net> writes:
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?
Feb 25 2013
parent "Vladimir Panteleev" <vladimir thecybershadow.net> writes:
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.
Feb 25 2013