// In this exercise, we want to express the concept of multiple owners via the // `Rc<T>` type. This is a model of our solar system - there is a `Sun` type and // multiple `Planet`s. The planets take ownership of the sun, indicating that // they revolve around the sun. use std::rc::Rc; #[derive(Debug)] struct Sun; #[derive(Debug)] enum Planet { Mercury(Rc<Sun>), Venus(Rc<Sun>), Earth(Rc<Sun>), Mars(Rc<Sun>), Jupiter(Rc<Sun>), Saturn(Rc<Sun>), Uranus(Rc<Sun>), Neptune(Rc<Sun>), } impl Planet { fn details(&self) { println!("Hi from {self:?}!"); } } fn main() { // You can optionally experiment here. } #[cfg(test)] mod tests { use super::*; #[test] fn rc1() { let sun = Rc::new(Sun); println!("reference count = {}", Rc::strong_count(&sun)); // 1 reference let mercury = Planet::Mercury(Rc::clone(&sun)); println!("reference count = {}", Rc::strong_count(&sun)); // 2 references mercury.details(); let venus = Planet::Venus(Rc::clone(&sun)); println!("reference count = {}", Rc::strong_count(&sun)); // 3 references venus.details(); let earth = Planet::Earth(Rc::clone(&sun)); println!("reference count = {}", Rc::strong_count(&sun)); // 4 references earth.details(); let mars = Planet::Mars(Rc::clone(&sun)); println!("reference count = {}", Rc::strong_count(&sun)); // 5 references mars.details(); let jupiter = Planet::Jupiter(Rc::clone(&sun)); println!("reference count = {}", Rc::strong_count(&sun)); // 6 references jupiter.details(); let saturn = Planet::Saturn(Rc::clone(&sun)); println!("reference count = {}", Rc::strong_count(&sun)); // 7 references saturn.details(); let uranus = Planet::Uranus(Rc::clone(&sun)); println!("reference count = {}", Rc::strong_count(&sun)); // 8 references uranus.details(); let neptune = Planet::Neptune(Rc::clone(&sun)); println!("reference count = {}", Rc::strong_count(&sun)); // 9 references neptune.details(); assert_eq!(Rc::strong_count(&sun), 9); drop(neptune); println!("reference count = {}", Rc::strong_count(&sun)); // 8 references drop(uranus); println!("reference count = {}", Rc::strong_count(&sun)); // 7 references drop(saturn); println!("reference count = {}", Rc::strong_count(&sun)); // 6 references drop(jupiter); println!("reference count = {}", Rc::strong_count(&sun)); // 5 references drop(mars); println!("reference count = {}", Rc::strong_count(&sun)); // 4 references drop(earth); println!("reference count = {}", Rc::strong_count(&sun)); // 3 references drop(venus); println!("reference count = {}", Rc::strong_count(&sun)); // 2 references drop(mercury); println!("reference count = {}", Rc::strong_count(&sun)); // 1 reference assert_eq!(Rc::strong_count(&sun), 1); } }

Clone-on-write