Understanding Structs in Rust

Introduction

Structs (structures) are one of Rust's fundamental building blocks for creating custom data types. Unlike tuples, which group related data anonymously, structs give each piece of data a meaningful name. This makes them ideal for creating complex data structures where the relationship between components should be explicit and self-documenting.

Defining Structs

Basic Struct Definition

struct User {
    username: String,
    email: String,
    sign_in_count: u64,
    active: bool,
}

Note: Unlike C/C++, Rust struct definitions:

Don't need a semicolon after the closing brace
Can't include instance declarations
Use commas to separate fields

Creating Struct Instances

Standard Instantiation

let user1 = User {
    email: String::from("[email protected]"),
    username: String::from("someusername123"),
    active: true,
    sign_in_count: 1,
};

Field Init Shorthand

When variables have the same name as struct fields:

fn build_user(email: String, username: String) -> User {
    User {
        email,      // same as email: email
        username,   // same as username: username
        active: true,
        sign_in_count: 1,
    }
}

Struct Update Syntax

Create a new instance from an existing one:

let user2 = User {
    email: String::from("[email protected]"),
    ..user1    // copy remaining fields from user1
};

Note: The ..user1 must come last and cannot have a comma after it.

Tuple Structs

Tuple structs are a hybrid between tuples and structs. They have a name but their fields don't:

struct Color(i32, i32, i32);    // RGB
struct Point(f64, f64, f64);    // 3D coordinate

let black = Color(0, 0, 0);
let origin = Point(0.0, 0.0, 0.0);

// Access fields like tuples
println!("First color value: {}", black.0);

When to Use Tuple Structs

When naming the struct adds meaning but field names would be redundant
When you want different types for tuples that have the same field types
For simple wrappers around single values (newtype pattern)

Unit Structs

Unit structs have no fields. They're useful for:

Implementing traits on some type without storing data
Creating type-level markers

struct UnitStruct;

// Usage example
let unit = UnitStruct;

Ownership and Structs

Field Ownership

Structs take ownership of their fields by default:

struct Person {
    name: String,    // Person owns this String
    age: u32,       // Copy types don't need ownership management
}

Using References in Structs

To store references, you need lifetime annotations (covered in a later chapter):

struct PersonReference<'a> {
    name: &'a str,    // Reference to a string slice
    age: u32,
}

Methods and Associated Functions

Defining Methods

Methods are functions associated with a struct:

struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    // Method - takes &self as first parameter
    fn area(&self) -> u32 {
        self.width * self.height
    }
    
    // Method with parameters
    fn can_hold(&self, other: &Rectangle) -> bool {
        self.width > other.width && self.height > other.height
    }
}

// Usage
let rect = Rectangle { width: 30, height: 50 };
println!("Area: {}", rect.area());

Associated Functions

Functions associated with a struct but don't take self as a parameter:

impl Rectangle {
    // Associated function (like a static method)
    fn square(size: u32) -> Rectangle {
        Rectangle {
            width: size,
            height: size,
        }
    }
}

// Usage
let square = Rectangle::square(20);

Debug Output

To print struct contents for debugging:

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

let rect = Rectangle { width: 30, height: 50 };

// Print with debug formatting
println!("rect is {:?}", rect);

// Pretty print with debug formatting
println!("rect is {:#?}", rect);

Best Practices

1. Field Naming

// Good
struct Rectangle {
    width: u32,
    height: u32,
}

// Less clear
struct Rectangle {
    w: u32,
    h: u32,
}

2. Method Organization

impl Rectangle {
    // Constructor-like associated functions first
    fn new(width: u32, height: u32) -> Rectangle {
        Rectangle { width, height }
    }
    
    // Core methods next
    fn area(&self) -> u32 {
        self.width * self.height
    }
    
    // Utility methods last
    fn describe(&self) -> String {
        format!("{}x{} rectangle", self.width, self.height)
    }
}

3. Multiple impl Blocks

You can split methods across multiple impl blocks for better organization:

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

impl Rectangle {
    // Utility methods
    fn describe(&self) -> String {
        format!("{}x{} rectangle", self.width, self.height)
    }
}

Common Patterns

1. Builder Pattern

struct Server {
    host: String,
    port: u16,
    secure: bool,
}

impl Server {
    fn builder() -> ServerBuilder {
        ServerBuilder::default()
    }
}

struct ServerBuilder {
    host: Option<String>,
    port: Option<u16>,
    secure: bool,
}

impl ServerBuilder {
    fn default() -> ServerBuilder {
        ServerBuilder {
            host: None,
            port: None,
            secure: false,
        }
    }
    
    fn host(mut self, host: String) -> ServerBuilder {
        self.host = Some(host);
        self
    }
    
    fn port(mut self, port: u16) -> ServerBuilder {
        self.port = Some(port);
        self
    }
    
    fn secure(mut self, secure: bool) -> ServerBuilder {
        self.secure = secure;
        self
    }
    
    fn build(self) -> Option<Server> {
        match (self.host, self.port) {
            (Some(host), Some(port)) => Some(Server {
                host,
                port,
                secure: self.secure,
            }),
            _ => None,
        }
    }
}

2. New Type Pattern

// Create a new type for type safety
struct Meters(f64);
struct Kilometers(f64);

impl Meters {
    fn to_kilometers(&self) -> Kilometers {
        Kilometers(self.0 / 1000.0)
    }
}

Performance Considerations

Memory Layout
- Structs are stored contiguously in memory
- Field order can affect memory usage due to padding
- Consider ordering fields by size for optimal memory usage
Stack vs Heap
- Small structs are typically stored on the stack
- Strings and other heap-allocated types in structs use the heap
- Consider using references for large structs passed to functions

Common Pitfalls

1. Partial Moves

struct Person {
    name: String,
    age: u32,
}

let p = Person {
    name: String::from("Alice"),
    age: 30,
};

let name = p.name;    // Moves name out of p
// println!("{}", p.name);  // Error: value partially moved
println!("{}", p.age);     // OK: age wasn't moved

2. Forgetting Debug Derive

struct Point {    // Won't compile if you try to print with {:?}
    x: i32,
    y: i32,
}

#[derive(Debug)]  // Fixed!
struct Point {
    x: i32,
    y: i32,
}

Next Steps

After mastering structs:

Learn about enums and pattern matching
Study traits and how to implement them
Explore generic types with structs
Understand lifetimes in struct definitions

Remember: Structs are fundamental to organizing data in Rust. They provide a way to create meaningful abstractions and are the building blocks for more complex data structures and types.

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