Rc<T>

Learn shared ownership with Rc<T>.

Rc<T>

Sometimes a value needs several owners: for example, many edges share a graph node. Rc<T> (reference counted) keeps the number of owners; when the last owner is dropped, the value is dropped too:

use std::rc::Rc;

let a = Rc::new(String::from("paylaşılan"));
let b = Rc::clone(&a);           // not a deep copy, counter +1
println!("{}", Rc::strong_count(&a));   // 2

Rc::clone only increments the counter, it doesn't copy the data. a.clone() does the same, but writing Rc::clone(&a) shows clearly that it's a cheap operation.

Counter and scope

Each time an Rc copy goes out of scope, the counter goes down:

let a = Rc::new(5);
{
    let _c = Rc::clone(&a);
    println!("{}", Rc::strong_count(&a));   // 2
}
println!("{}", Rc::strong_count(&a));       // 1

Rc is single-threaded; since its counter isn't atomic it can't be sent to another thread (it isn't Send). Sharing across threads needs Arc.

Exercise Shared text

Create Rc::new(String::from("paylaşılan")) and take a copy. In an inner block take one more copy and print the counter; after the block print the counter and the text separated by a space.

Show example solution

use std::rc::Rc;

fn main() {
    let a = Rc::new(String::from("paylaşılan"));
    let _b = Rc::clone(&a);
    {
        let _c = Rc::clone(&a);
        println!("{}", Rc::strong_count(&a));
    }
    println!("{} {}", Rc::strong_count(&a), a);
}

Test yourself

Can the value inside an <code>Rc<T></code> be changed?
- Not directly; Rc only gives shared (&T) access
- Yes, every owner can change it
- Only the first owner
- Yes, with mut Rc
Show answer

Not directly; Rc only gives shared (&T) access — If several owners changed it at once, the borrowing rules would break. Use Rc<RefCell<T>> to change it.

Share ownership

let a = Rc::new(String::from("x"));
let b = Rc::____(&a);

Show answer

let a = Rc::new(String::from("x"));
let b = Rc::clone(&a);

Operation and counter

Rc::new(x)

1
+ Rc::clone(&a)

2
+ one more copy

3
When all are dropped

Value is freed

Next lesson: Arc<T>

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