Rust by Example

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) {

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);

}

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