Borrowing

Most of the time, we’d like to access data without taking ownership over it. To accomplish this, Rust uses a borrowing mechanism. Instead of passing objects by value (T), objects can be passed by reference (&T).

The compiler statically guarantees (via its borrow checker) that references always point to valid objects. That is, while references to an object exist, the object cannot be destroyed.

1. // This function takes ownership of a box and destroys it
2. fn eat_box_i32(boxed_i32: Box<i32>) {
3. println!("Destroying box that contains {}", boxed_i32);
4. }

6. // This function borrows an i32
7. fn borrow_i32(borrowed_i32: &i32) {
8. println!("This int is: {}", borrowed_i32);
9. }

11. fn main() {
12. // Create a boxed i32, and a stacked i32
13. let boxed_i32 = Box::new(5_i32);
14. let stacked_i32 = 6_i32;

16. // Borrow the contents of the box. Ownership is not taken,
17. // so the contents can be borrowed again.
18. borrow_i32(&boxed_i32);
19. borrow_i32(&stacked_i32);

21. {
22. // Take a reference to the data contained inside the box
23. let _ref_to_i32: &i32 = &boxed_i32;

25. // Error!
26. // Can't destroy `boxed_i32` while the inner value is borrowed.
27. eat_box_i32(boxed_i32);
28. // FIXME ^ Comment out this line

30. // `_ref_to_i32` goes out of scope and is no longer borrowed.
31. }

33. // `boxed_i32` can now give up ownership to `eat_box` and be destroyed
34. eat_box_i32(boxed_i32);
35. }