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// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! Thread-local reference-counted boxes (the `Rc<T>` type). //! //! The `Rc<T>` type provides shared ownership of an immutable value. //! Destruction is deterministic, and will occur as soon as the last owner is //! gone. It is marked as non-sendable because it avoids the overhead of atomic //! reference counting. //! //! The `downgrade` method can be used to create a non-owning `Weak<T>` pointer //! to the box. A `Weak<T>` pointer can be upgraded to an `Rc<T>` pointer, but //! will return `None` if the value has already been dropped. //! //! For example, a tree with parent pointers can be represented by putting the //! nodes behind strong `Rc<T>` pointers, and then storing the parent pointers //! as `Weak<T>` pointers. //! //! # Examples //! //! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`. //! We want to have our `Gadget`s point to their `Owner`. We can't do this with //! unique ownership, because more than one gadget may belong to the same //! `Owner`. `Rc<T>` allows us to share an `Owner` between multiple `Gadget`s, //! and have the `Owner` remain allocated as long as any `Gadget` points at it. //! //! ```rust //! use rc::Rc; //! //! struct Owner { //! name: String //! // ...other fields //! } //! //! struct Gadget { //! id: i32, //! owner: Rc<Owner> //! // ...other fields //! } //! //! fn main() { //! // Create a reference counted Owner. //! let gadget_owner : Rc<Owner> = Rc::new( //! Owner { name: String::from("Gadget Man") } //! ); //! //! // Create Gadgets belonging to gadget_owner. To increment the reference //! // count we clone the `Rc<T>` object. //! let gadget1 = Gadget { id: 1, owner: gadget_owner.clone() }; //! let gadget2 = Gadget { id: 2, owner: gadget_owner.clone() }; //! //! drop(gadget_owner); //! //! // Despite dropping gadget_owner, we're still able to print out the name //! // of the Owner of the Gadgets. This is because we've only dropped the //! // reference count object, not the Owner it wraps. As long as there are //! // other `Rc<T>` objects pointing at the same Owner, it will remain //! // allocated. Notice that the `Rc<T>` wrapper around Gadget.owner gets //! // automatically dereferenced for us. //! println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name); //! println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name); //! //! // At the end of the method, gadget1 and gadget2 get destroyed, and with //! // them the last counted references to our Owner. Gadget Man now gets //! // destroyed as well. //! } //! ``` //! //! If our requirements change, and we also need to be able to traverse from //! Owner → Gadget, we will run into problems: an `Rc<T>` pointer from Owner //! → Gadget introduces a cycle between the objects. This means that their //! reference counts can never reach 0, and the objects will remain allocated: a //! memory leak. In order to get around this, we can use `Weak<T>` pointers. //! These pointers don't contribute to the total count. //! //! Rust actually makes it somewhat difficult to produce this loop in the first //! place: in order to end up with two objects that point at each other, one of //! them needs to be mutable. This is problematic because `Rc<T>` enforces //! memory safety by only giving out shared references to the object it wraps, //! and these don't allow direct mutation. We need to wrap the part of the //! object we wish to mutate in a `RefCell`, which provides *interior //! mutability*: a method to achieve mutability through a shared reference. //! `RefCell` enforces Rust's borrowing rules at runtime. Read the `Cell` //! documentation for more details on interior mutability. //! //! ```rust #![cfg_attr(feature = "unstable", doc = "# #![feature(rc_weak)]")] //! use rc::Rc; //! use rc::Weak; //! use std::cell::RefCell; //! //! struct Owner { //! name: String, //! gadgets: RefCell<Vec<Weak<Gadget>>> //! // ...other fields //! } //! //! struct Gadget { //! id: i32, //! owner: Rc<Owner> //! // ...other fields //! } //! //! fn main() { //! // Create a reference counted Owner. Note the fact that we've put the //! // Owner's vector of Gadgets inside a RefCell so that we can mutate it //! // through a shared reference. //! let gadget_owner : Rc<Owner> = Rc::new( //! Owner { //! name: "Gadget Man".to_string(), //! gadgets: RefCell::new(Vec::new()) //! } //! ); //! //! // Create Gadgets belonging to gadget_owner as before. //! let gadget1 = Rc::new(Gadget{id: 1, owner: gadget_owner.clone()}); //! let gadget2 = Rc::new(Gadget{id: 2, owner: gadget_owner.clone()}); //! //! // Add the Gadgets to their Owner. To do this we mutably borrow from //! // the RefCell holding the Owner's Gadgets. //! gadget_owner.gadgets.borrow_mut().push(gadget1.clone().downgrade()); //! gadget_owner.gadgets.borrow_mut().push(gadget2.clone().downgrade()); //! //! // Iterate over our Gadgets, printing their details out //! for gadget_opt in gadget_owner.gadgets.borrow().iter() { //! //! // gadget_opt is a Weak<Gadget>. Since weak pointers can't guarantee //! // that their object is still allocated, we need to call upgrade() //! // on them to turn them into a strong reference. This returns an //! // Option, which contains a reference to our object if it still //! // exists. //! let gadget = gadget_opt.upgrade().unwrap(); //! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name); //! } //! //! // At the end of the method, gadget_owner, gadget1 and gadget2 get //! // destroyed. There are now no strong (`Rc<T>`) references to the gadgets. //! // Once they get destroyed, the Gadgets get destroyed. This zeroes the //! // reference count on Gadget Man, they get destroyed as well. //! } //! ``` #[cfg(feature = "unstable")] pub use std::rc::*; #[cfg(not(feature = "unstable"))] pub use stable_rc::*; #[cfg(not(feature = "unstable"))] mod stable_rc;