Repository navigation
Tracking issue for std::ptr::eq, Arc::ptr_eq, and Rc::ptr_eq #36497
Description
Activity
I needed Rc::ptr_eq to avoid a double borrow in a recursive data structure with Rc<RefCell< T >>. Thanks a lot!
@vadixidav It can be implemented outside of
stdby tweaking it a bit. Copy-pasting this into your code can unblock you untilRc::prt_eqis stabilized:pub fn rc_ptr_eq<T: ?Sized>(this: &Rc<T>, other: &Rc<T>) -> bool { let this_ptr: *const T = &*this; let other_ptr: *const T = &*other; this_ptr == other_ptr }
Reacted by Antoine Büsch, Ruud van Asseldonk and Robert GrosseSeem like good/easy APIs to stabilize!
@rfcbot fcp merge
Team member @alexcrichton has proposed to merge this. The next step is review by the rest of the tagged teams:
No concerns currently listed.
Once these reviewers reach consensus, this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!
See this document for info about what commands tagged team members can give me.
FWIW, I just found a good use for this in librsvg's port to Rust.
Librsvg has a Node structure for the tree of SVG objects. The C code keeps around
RsvgNode*values all over the place, including multiple references to the same value.Sometimes there is code like
if (current_node == tree_root)for special-casing... and you see where this is going.The things I expose to the C code are pointers that come from
Box<Rc<Node>>I also exposenode_ref()andnode_unref()functions so that C can get new references to existing nodes and discard them as needed.This means that
RsvgNode *node1 = rsvg_rust_cnode_new (...); /* returns a Box::into_raw (Box::new (Rc::new (Node))) */ RsvgNode *node2 = rsvg_node_ref (node1); assert (node1 == node2);is invalid now. What I want is to
assert (rsvg_node_is_same (node1, node2));and that can be implemented as
pub type RsvgNode = Rc<Node>; #[no_mangle] pub extern fn rsvg_node_is_same (raw_node1: *const RsvgNode, raw_node2: *const RsvgNode) -> bool { if raw_node1.is_null () && raw_node2.is_null () { true } else if !raw_node1.is_null () && !raw_node2.is_null () { let node1: &RsvgNode = unsafe { & *raw_node1 }; let node2: &RsvgNode = unsafe { & *raw_node2 }; Rc::ptr_eq (node1, node2) } else { false } }For now I'm using @SimonSapin's code (it's missing double asterisks in
&**thisand&**other), but it would indeed be nice to have an Rc::ptr_eq() in stable!For now I'm using @SimonSapin's code
I sometimes find convenient to make it less generic (when it’s only used with one
Ttype):fn same_node(a: *const Node, b: *const Node) -> bool { a == b }
This looks the same as
==, but since it expects raw pointer they can be coerced implicitly from references. (And, by the way,(*const T)::as_ref()can be more convenient thanis_null)#[no_mangle] pub extern fn rsvg_node_is_same (raw_node1: *const RsvgNode, raw_node2: *const RsvgNode) -> bool { match (raw_node1.as_ref(), raw_node2.as_ref()) { (Some(node1), Some(node2)) => same_node(&**node1, &**node2), (None, None) => true, _ => false } }
(Of course,
Rc::eqis nicer still.)ping @BurntSushi (checkbox)
🔔 This is now entering its final comment period, as per the review above. 🔔
The final comment period is now complete.
- added a commit that references this issue
on Mar 17, 2017 For future GitHub travelers, investigating
clippy::vtable_address_comparisons:Looks like the methods were stabilized with the
?Sizedbound, while the current impl doesn't quite handle them. Specifically,ptr_eqcan spuriously returnfalsefor identical trait objects. This is being tracked in #69757.This is not unique to these
ptr_eqmethods. Wide pointers to trait objects have been comparable with the==operator forever, with the same spurious results.
Implemented in #35992 as
#[unstable(feature = "ptr_eq")].std::ptr::eqis rust-lang/rfcs#1155.