# Rust by Example

## 13.3 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.
eat_box_i32(boxed_i32);
// FIXME ^ Comment out this line

// _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);
}