Lesson 13 of 20

6 mins read

Structs, Associated Functions & Methods (impl)

In Plain English

Structs allow you to name and package multiple related values into a cohesive custom type. Methods are defined inside impl blocks and take &self as their first parameter, while associated functions (like Self::new) act as constructors without &self.

The Mental Model

"A struct is like an architect’s blueprint for a house specifying dimensions and materials. An impl block is the user manual explaining what that house can do (like house.turn_on_lights())."

Deep Dive: How It Works

Struct Definition: struct User { username: String, active: bool }

Methods with &self: Borrow the struct instance immutably to read its fields.

Associated Functions (Self::new): Functions associated with the type but not an instance (called with Type::function()).

Syntax Blueprint

impl Rectangle { fn area(&self) -> u32 { self.width * self.height } }

Define methods inside impl blocks with &self to access fields.

Core Rules to Remember

Custom Types: Groups heterogeneous fields under a meaningful domain name.

&self Borrowing: Methods borrow the instance without consuming ownership.

Constructors: Idiomatic Type::new(...) associated functions build instances cleanly.

Common Beginner Traps & How to Fix Them

Mistake:Forgetting &self in method signatures.

Why it happens: Writing fn area(self) consumes ownership of the instance, preventing you from using it again.

How to fix: Use &self for read-only methods and &mut self for mutating methods.

Pro Tip: Add #[derive(Debug)] above your struct to enable automatic {:?} debug formatting support.

Live Interactive Example

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Rectangle Struct with Methods

Rust 1.78 / rustc

Output Console

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How it works: Demonstrates struct instantiation via associated function and calling &self method.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Implement a BankAccount struct

Create struct BankAccount { balance: i32 }
Implement impl BankAccount with fn new(balance: i32) -> Self { Self { balance } } and fn get_balance(&self) -> i32 { self.balance }
In main(), let acc = BankAccount::new(500); print "Account Balance: $500".

Sandbox Output

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