Use `Weak` to prevent reference cycles

3 files changed, 82 insertions, 0 deletions

diff --git a/tree/.gitignore b/tree/.gitignore
new file mode 100644
index 0000000..e629269
--- /dev/null
+++ b/tree/.gitignore
@@ -0,0 +1,18 @@
+
+# Created by https://www.toptal.com/developers/gitignore/api/rust
+# Edit at https://www.toptal.com/developers/gitignore?templates=rust
+
+### Rust ###
+# Generated by Cargo
+# will have compiled files and executables
+/target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
+Cargo.lock
+
+# These are backup files generated by rustfmt
+**/*.rs.bk
+
+# End of https://www.toptal.com/developers/gitignore/api/rust
+
diff --git a/tree/Cargo.toml b/tree/Cargo.toml
new file mode 100644
index 0000000..717cd24
--- /dev/null
+++ b/tree/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "tree"
+version = "0.1.0"
+authors = ["Jordan Gong <jordan.gong@protonmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/tree/src/main.rs b/tree/src/main.rs
new file mode 100644
index 0000000..2c9f238
--- /dev/null
+++ b/tree/src/main.rs
@@ -0,0 +1,55 @@
+use std::cell::RefCell;
+use std::rc::{Rc, Weak};
+
+#[derive(Debug)]
+struct Node {
+    value: i32,
+    parent: RefCell<Weak<Node>>,
+    children: RefCell<Vec<Rc<Node>>>,
+}
+
+fn main() {
+    let leaf = Rc::new(Node {
+        value: 3,
+        parent: RefCell::new(Weak::new()),
+        children: RefCell::new(vec![]),
+    });
+
+    // println!("leaf parent = {:?}", leaf.parent.borrow().upgrade());
+
+    println!(
+        "leaf strong = {}, weak = {}",
+        Rc::strong_count(&leaf),
+        Rc::weak_count(&leaf),
+    );
+
+    {
+        let branch = Rc::new(Node {
+            value: 5,
+            parent: RefCell::new(Weak::new()),
+            children: RefCell::new(vec![Rc::clone(&leaf)]),
+        });
+
+        *leaf.parent.borrow_mut() = Rc::downgrade(&branch);
+
+        println!(
+            "branch strong = {}, weak = {}",
+            Rc::strong_count(&branch),
+            Rc::weak_count(&branch),
+        );
+
+        println!(
+            "leaf strong = {}, weak = {}",
+            Rc::strong_count(&leaf),
+            Rc::weak_count(&leaf),
+        );
+    }
+
+    println!("leaf parent = {:?}", leaf.parent.borrow().upgrade());
+
+    println!(
+        "leaf strong = {}, weak = {}",
+        Rc::strong_count(&leaf),
+        Rc::weak_count(&leaf),
+    );
+}