Methods

Rust allows you to associate functions with your new types. You do this with an impl block:

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
#[derive(Debug)]
struct CarRace {
    name: String,
    laps: Vec<i32>,
}

impl CarRace {
    // No receiver, a static method
    fn new(name: &str) -> Self {
        Self { name: String::from(name), laps: Vec::new() }
    }

    // Exclusive borrowed read-write access to self
    fn add_lap(&mut self, lap: i32) {
        self.laps.push(lap);
    }

    // Shared and read-only borrowed access to self
    fn print_laps(&self) {
        println!("Recorded {} laps for {}:", self.laps.len(), self.name);
        for (idx, lap) in self.laps.iter().enumerate() {
            println!("Lap {idx}: {lap} sec");
        }
    }

    // Exclusive ownership of self (covered later)
    fn finish(self) {
        let total: i32 = self.laps.iter().sum();
        println!("Race {} is finished, total lap time: {}", self.name, total);
    }
}

fn main() {
    let mut race = CarRace::new("Monaco Grand Prix");
    race.add_lap(70);
    race.add_lap(68);
    race.print_laps();
    race.add_lap(71);
    race.print_laps();
    race.finish();
    // race.add_lap(42);
}

The self arguments specify the "receiver" - the object the method acts on. There are several common receivers for a method:

• &self: borrows the object from the caller using a shared and immutable reference. The object can be used again afterwards.
• &mut self: borrows the object from the caller using a unique and mutable reference. The object can be used again afterwards.
• self: takes ownership of the object and moves it away from the caller. The method becomes the owner of the object. The object will be dropped (deallocated) when the method returns, unless its ownership is explicitly transmitted. Complete ownership does not automatically mean mutability.
• mut self: same as above, but the method can mutate the object.
• No receiver: this becomes a static method on the struct. Typically used to create constructors that are called new by convention.

Key Points:

• It can be helpful to introduce methods by comparing them to functions.
  • Methods are called on an instance of a type (such as a struct or enum), the first parameter represents the instance as self.
  • Developers may choose to use methods to take advantage of method receiver syntax and to help keep them more organized. By using methods we can keep all the implementation code in one predictable place.
  • Note that methods can also be called like associated functions by explicitly passing the receiver in, e.g. CarRace::add_lap(&mut race, 20).
• Point out the use of the keyword self, a method receiver.
  • Show that it is an abbreviated term for self: Self and perhaps show how the struct name could also be used.
  • Explain that Self is a type alias for the type the impl block is in and can be used elsewhere in the block.
  • Note how self is used like other structs and dot notation can be used to refer to individual fields.
  • This might be a good time to demonstrate how the &self differs from self by trying to run finish twice.
  • Beyond variants on self, there are also special wrapper types allowed to be receiver types, such as Box<Self>.