Rust book - Rc

• A reference counting pointer allows multiple owners of a single heap-allocated data. 1 value / Multiple readers

• The reference counting happens at compile time, and the compiler determines which variable does the last use of a reference, and makes that variable the owner of the heap data. Then the regular rules of ownership are applied.

• Implementation of Cons list using Rc

enum List {
	Cons(i32, Rc<List>),
	Nil
}

fn main() {
	let shared_list = Rc::new(List::Cons(10, Rc::new(Nil)));
	let list_a = Cons(2, Rc::clone(&shared_list));
	let list_b Cons(5, Rc::clone(&shared_list));
}

• The reference counter for a given Rc instance is updated whenever Rc::clone is called with a reference of that instance. Alternatively, shared_list.clone() could be use to increment the counter, but the current convention is to use Rc::clone.

• Rc::clone doesn’t create a deep copy of the reference, it only updates the reference counter and returns a clone of a pointer to the data on the heap.\

• Rc implements Drop trait for decreasing the ref count.

Questions

How a Rc is different than a regular reference?\ A regular reference can only have a single owner. Rcs, on the other hand, can have multiple owners