Borrowing - Rust By Example
Description: A description

# 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, and a stacked i32
        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);
    }