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63 lines
1.6 KiB
Markdown
63 lines
1.6 KiB
Markdown
---
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date: 2020-09-29
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id: 13459bc1-66f2-4eed-b987-061bd0fbeb1c
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title: Rc\<T\>
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---
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# Introduction
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In the majority of cases, ownership is clear: you know exactly which
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variable owns a given value. However, there are cases when a single
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value might have multiple owners. For example, in graph data structures,
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multiple edges might point to the same node, and that node is
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conceptually owned by all of the edges that point to it. A node
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shouldn’t be cleaned up unless it doesn’t have any edges pointing to it.
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To enable multiple ownership, Rust has a type called `Rc<T>`, which is
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an abbreviation for reference counting. The `Rc<T>` type keeps track of
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the number of references to a value which determines whether or not a
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value is still in use. If there are zero references to a value, the
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value can be cleaned up without any references becoming invalid.
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# Reason to choose Rc\<T\>
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`Rc<T>` enables multiple owners of the same data;
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[Box\<T\>](20200929135609-box_t) and
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[RefCell\<T\>](20200929152628-refcell_t) have single owners.
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# Example
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This won't compile because b & c both share ownership of a:
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``` rust
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enum List {
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Cons(i32, Box<List>),
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Nil,
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}
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use crate::List::{Cons, Nil};
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fn main() {
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let a = Cons(5, Box::new(Cons(10, Box::new(Nil))));
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let _b = Cons(3, Box::new(a));
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let _c = Cons(4, Box::new(a));
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}
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```
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The solution:
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``` rust
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enum List {
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Cons(i32, Rc<List>),
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Nil,
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}
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use crate::List::{Cons, Nil};
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use std::rc::Rc;
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fn main() {
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let a = Rc::new(Cons(5, Rc::new(Cons(10, Rc::new(Nil)))));
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let _b = Cons(3, Rc::clone(&a));
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let _c = Cons(4, Rc::clone(&a));
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}
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```
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