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.
// This function takes ownership of a box and destroys it fn eat_box_i32(boxed_i32: Box<i32>) { println!("Destroying box that contains {}", boxed_i32); } // This function borrows an i32 fn borrow_i32(borrowed_i32: &i32) { println!("This int is: {}", borrowed_i32); } fn main() { // Create a boxed i32 in the heap, and a i32 on the stack // Remember: numbers can have arbitrary underscores added for readability // 5_i32 is the same as 5i32 let boxed_i32 = Box::new(5_i32); let stacked_i32 = 6_i32; // Borrow the contents of the box. Ownership is not taken, // so the contents can be borrowed again. borrow_i32(&boxed_i32); borrow_i32(&stacked_i32); { // Take a reference to the data contained inside the box let _ref_to_i32: &i32 = &boxed_i32; // Error! // Can't destroy `boxed_i32` while the inner value is borrowed later in scope. eat_box_i32(boxed_i32); // FIXME ^ Comment out this line // Attempt to borrow `_ref_to_i32` after inner value is destroyed borrow_i32(_ref_to_i32); // `_ref_to_i32` goes out of scope and is no longer borrowed. } // `boxed_i32` can now give up ownership to `eat_box` and be destroyed eat_box_i32(boxed_i32); }