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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Mutable data can be mutably borrowed using &mut T. This is called a mutable reference and gives read/write access to the borrower. In contrast, &T borrows the data via an immutable reference, and the borrower can read the data but not modify it:
#[allow(dead_code)]
#[derive(Clone, Copy)]
struct Book {
// `&'static str` is a reference to a string allocated in read only memory
author: &'static str,
title: &'static str,
year: u32,
}
// This function takes a reference to a book
fn borrow_book(book: &Book) {
println!("I immutably borrowed {} - {} edition", book.title, book.year);
}
// This function takes a reference to a mutable book and changes `year` to 2014
fn new_edition(book: &mut Book) {
book.year = 2014;
println!("I mutably borrowed {} - {} edition", book.title, book.year);
}
fn main() {
// Create an immutable Book named `immutabook`
let immutabook = Book {
// string literals have type `&'static str`
author: "Douglas Hofstadter",
title: "Gödel, Escher, Bach",
year: 1979,
};
// Create a mutable copy of `immutabook` and call it `mutabook`
let mut mutabook = immutabook;
// Immutably borrow an immutable object
borrow_book(&immutabook);
// Immutably borrow a mutable object
borrow_book(&mutabook);
// Borrow a mutable object as mutable
new_edition(&mut mutabook);
// Error! Cannot borrow an immutable object as mutable
new_edition(&mut immutabook);
// FIXME ^ Comment out this line
}
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Data can be immutably borrowed any number of times, but while immutably borrowed, the original data can't be mutably borrowed. On the other hand, only one mutable borrow is allowed at a time. The original data can be borrowed again only after the mutable reference has been used for the last time.
struct Point { x: i32, y: i32, z: i32 }
fn main() {
let mut point = Point { x: 0, y: 0, z: 0 };
let borrowed_point = &point;
let another_borrow = &point;
// Data can be accessed via the references and the original owner
println!("Point has coordinates: ({}, {}, {})",
borrowed_point.x, another_borrow.y, point.z);
// Error! Can't borrow `point` as mutable because it's currently
// borrowed as immutable.
// let mutable_borrow = &mut point;
// TODO ^ Try uncommenting this line
// The borrowed values are used again here
println!("Point has coordinates: ({}, {}, {})",
borrowed_point.x, another_borrow.y, point.z);
// The immutable references are no longer used for the rest of the code so
// it is possible to reborrow with a mutable reference.
let mutable_borrow = &mut point;
// Change data via mutable reference
mutable_borrow.x = 5;
mutable_borrow.y = 2;
mutable_borrow.z = 1;
// Error! Can't borrow `point` as immutable because it's currently
// borrowed as mutable.
// let y = &point.y;
// TODO ^ Try uncommenting this line
// Error! Can't print because `println!` takes an immutable reference.
// println!("Point Z coordinate is {}", point.z);
// TODO ^ Try uncommenting this line
// Ok! Mutable references can be passed as immutable to `println!`
println!("Point has coordinates: ({}, {}, {})",
mutable_borrow.x, mutable_borrow.y, mutable_borrow.z);
// The mutable reference is no longer used for the rest of the code so it
// is possible to reborrow
let new_borrowed_point = &point;
println!("Point now has coordinates: ({}, {}, {})",
new_borrowed_point.x, new_borrowed_point.y, new_borrowed_point.z);
}
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