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# Mutability

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
    }

### See also:

`static`