Borrowing means accessing a value without taking ownership of it. The owner remains responsible for cleanup, while a reference lets another part of the program use the value during a valid lifetime. section 4.0
Why borrow instead of move?
Suppose check_status(sat_a) takes a CubeSat by value. Ownership moves into the function, and the value is dropped when the function returns. Calling check_status(sat_a) again fails: the variable is still in scope, but its value is no longer available. A reference avoids requesting ownership when the function only needs access. section 4.1.1 section 4.5.1
Choose the access the function needs:
- Use &T for read-only access. section 4.5.1
- Use &mut T for read-write access. Only one read-write borrow can exist at a time. section 4.5.1 section 4.6
- Take ownership when the function needs it; borrowing is not a replacement for every move. section 4.5.1
Send a message without consuming the satellite
A send method can take to: &mut CubeSat and push a message into to.mailbox.messages. The caller passes &mut sat_a, then can call sat_a.recv() afterward. The satellite stays owned by the caller, while the message moves into its mailbox. section 4.5.1
Listing 4.13 Avoiding ownership issues with references
#[derive(Debug)]
struct CubeSat {
id: u64,
mailbox: Mailbox,
}
#[derive(Debug)]
struct Mailbox {
messages: Vec<Message>,
}
type Message = String;
struct GroundStation;
impl GroundStation {
fn send(&self, to: &mut CubeSat, msg: Message) {
to.mailbox.messages.push(msg);
}
}
impl CubeSat {
fn recv(&mut self) -> Option<Message> {
self.mailbox.messages.pop()
}
}
fn main() {
let base = GroundStation {};
let mut sat_a = CubeSat {
id: 0,
mailbox: Mailbox {
messages: vec![],
},
};
println!("t0: {:?}", sat_a);
base.send(&mut sat_a,
Message::from("hello there!")); #1
println!("t1: {:?}", sat_a);
let msg = sat_a.recv();
println!("t2: {:?}", sat_a);
println!("msg: {:?}", msg);
}
Follow the satellite’s mailbox from empty, to holding a message, to empty again after recv(). The complete listing shows the mutable reference at the call site and in the method signature. section 4.5.1
