Rc<T>
- Rust course
- Intermediate
- Lesson 31/115
- 4 min
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)); // 2Rc::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)); // 1Rc 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.
Practice 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.
Starter code
use std::rc::Rc;
fn main() {
// buraya yaz
}Sample 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);
}Expected output
3 2 paylaşılan
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Can the value inside an <code>Rc<T></code> be changed?
- Not directly; Rc only gives shared (&T) access Correct answer
- Yes, every owner can change it
- Only the first owner
- Yes, with mut Rc
Explanation: If several owners changed it at once, the borrowing rules would break. Use Rc<RefCell<T>> to change it.
2 Share ownership
let a = Rc::new(String::from("x"));
let b = Rc::____(&a);let a = Rc::new(String::from("x"));
let b = Rc::clone(&a);Explanation: Rc::clone increments the counter and gives a new owner of the same data.
3 Operation and counter
| Term | Meaning |
|---|---|
Rc::new(x) |
1 |
+ Rc::clone(&a) |
2 |
+ one more copy |
3 |
When all are dropped |
Value is freed |