所有權和功能呼叫

在編寫函式時，大多數關於所有權的問題都會出現。當你指定型別的函式的引數，你可以選擇如何將該值傳入如果你只需要只讀訪問，你可以採取一個不變的參考：

fn foo(x: &String) {
    // foo is only authorized to read x's contents, and to create
    // additional immutable references to it if it so desires.
    let y = *x; // ERROR, cannot move when not owned
    x.push_str("foo"); // ERROR, cannot mutate with immutable reference
    println!("{}", x.len()); // reading OK
    foo(x); // forwarding reference OK
}

如果 foo 需要修改引數，它應該採用一個獨有的，可變的引用：

fn foo(x: &mut String) {
    // foo is still not responsible for dropping x before returning,
    // nor is it allowed to. however, foo may modify the String.
    let x2 = *x; // ERROR, cannot move when not owned
    x.push_str("foo"); // mutating OK
    drop(*x); // ERROR, cannot drop value when not owned
    println!("{}", x.len()); // reading OK
}

如果你沒有指定 & 或 &mut，你就是說該函式將獲得引數的所有權。這意味著 foo 現在也負責丟棄 x。

fn foo(x: String) {
    // foo may do whatever it wishes with x, since no-one else has
    // access to it. once the function terminates, x will be dropped,
    // unless it is moved away when calling another function.
    let mut x2 = x; // moving OK
    x2.push_str("foo"); // mutating OK
    let _ = &mut x2; // mutable borrow OK
    let _ = &x2; // immutable borrow OK (note that &mut above is dropped)
    println!("{}", x2.len()); // reading OK
    drop(x2); // dropping OK
}