Module std::rc

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 Gadgets are owned by a given Owner. We want to have our Gadgets 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 Gadgets, and have the Owner remain allocated as long as any Gadget points at it.

use std::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.

use std::rc::Rc;
use std::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(Rc::downgrade(&gadget1));
    gadget_owner.gadgets.borrow_mut().push(Rc::downgrade(&gadget2));

    // 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.
}

Structs

Rc	A reference-counted pointer type over an immutable value.
Weak	A weak version of Rc<T>.