Methods & Associated Functions

Methods are functions with a self receiver in an impl block. Associated functions have no self - constructors like new are the familiar example.

Recipe

struct Rectangle { width: u32, height: u32 }
 
impl Rectangle {
    fn new(width: u32, height: u32) -> Self {
        Self { width, height }
    }
 
    fn area(&self) -> u32 {
        self.width * self.height
    }
}
 
fn main() {
    let r = Rectangle::new(10, 20);
    println!("{}", r.area());
}

When to reach for this: Grouping behavior with data, builder APIs, and encapsulating invariants on struct mutation.

Working Example

#[derive(Debug)]
struct Account {
    balance: i64,
}
 
impl Account {
    fn open(initial: i64) -> Self {
        Self { balance: initial.max(0) }
    }
 
    fn deposit(&mut self, amount: i64) {
        if amount > 0 { self.balance += amount; }
    }
 
    fn balance(&self) -> i64 {
        self.balance
    }
}
 
fn main() {
    let mut a = Account::open(100);
    a.deposit(50);
    println!("{:?}", a.balance());
}

What this demonstrates:

• open is associated function - called as Account::open
• deposit mutates through &mut self
• balance reads through &self
• Invariants enforced inside methods

Deep Dive

Receiver Types

Multiple impl Blocks

impl Rectangle { /* methods */ }
impl Rectangle { /* more methods - merges conceptually */ }

Generic impl

impl<T: std::fmt::Display> Pair<T> {
    fn show(&self) { println!("{}", self.0); }
}

Traits vs Inherent Methods

Inherent methods are type-specific. Trait impls provide shared interfaces (Display, etc.).

Gotchas

• Public fields bypass encapsulation - Callers mutate directly. Fix: Make fields private, expose methods.
• self vs &self choice wrong - Consuming method when borrow suffices forces moves. Fix: Default to &self.
• Method name conflicts with field - self.x() vs self.x - different namespaces but confusing. Fix: Rename for clarity.
• Forgot Self alias - Self refers to implementing type in impl. Fix: Use Self in constructors returning own type.
• Async methods need async fn in trait carefully - Native async traits stabilized in recent Rust. Fix: Follow edition docs for async in traits.

Alternatives

FAQs

Related

• Structs Basics - struct forms
• Traits Basics - trait methods
• Enums - methods on enums
• Deriving Traits - auto impls

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.