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Unit-like struct

Get to know the fieldless unit-like struct.

Unit-like structs

A struct with no fields is called a unit-like struct. It holds no data and takes 0 bytes in memory; it's used to carry a behaviour (trait) or to mark a state at the type level:

struct Sessiz;

let s = Sessiz;
println!("{}", std::mem::size_of::<Sessiz>());   // 0

The instance is created with just the name, without braces.

Marker types

Unit-like structs work as "labels" in the type system. For example, whether a connection is open or closed can be shown with a type instead of a runtime bool:

struct Acik;
struct Kapali;

struct Kapi<Durum> { ad: String, durum: Durum }

let k = Kapi { ad: String::from("giriş"), durum: Kapali };

You'll see this "typestate" pattern with generics in later lessons. For now, what matters is: no fields means no cost.

Practice Measure the sizes

Define struct Sessiz; and struct Renk(u8, u8, u8);. Print the size of both with std::mem::size_of, separated by a space.

Starter code

main.rs
fn main() {
    // buraya yaz
}

Sample solution

main.rs
struct Sessiz;
struct Renk(u8, u8, u8);

fn main() {
    println!("{} {}", std::mem::size_of::<Sessiz>(), std::mem::size_of::<Renk>());
}

Expected output

0 3

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What is <code>std::mem::size_of::&lt;Renk&gt;()</code>? (<code>struct Renk(u8, u8, u8);</code>)

  • 3 Correct answer
  • 0
  • 8
  • 24

Explanation: Three u8 fields are 1 byte each; no padding is needed.

2 A struct without fields

Question 
struct Sessiz____

let s = Sessiz;
Answer 
struct Sessiz;

let s = Sessiz;

Explanation: A unit-like struct ends with a semicolon; no braces are needed.

3 Struct and its size

Term Meaning
struct Sessiz; 0 bytes
struct Renk(u8, u8, u8); 3 bytes
struct Tek(u32); 4 bytes
struct Cift(u64, u64); 16 bytes

All lessons in Beginner

1 Rust basics

  1. What is Rust?
  2. Project with Cargo
  3. Your first program
  4. println! and formatting
  5. Comments
  6. Summary - Rust basics

2 Variables

  1. Variables with let
  2. Mutability with mut
  3. Constants with const
  4. Static variables
  5. Shadowing
  6. Summary - Variables

3 Data types

  1. Integers
  2. Floating point
  3. Bool and char
  4. Tuple
  5. Array
  6. Summary - Data types

4 Strings

  1. &str vs String
  2. Creating a String
  3. Concatenating strings
  4. format! macro
  5. String length
  6. Summary - Strings

5 Project: Calculator

  1. Project skeleton
  2. Reading input
  3. Computing
  4. Error handling
  5. Loop and exit

6 Functions

  1. Defining with fn
  2. Parameters
  3. Return value
  4. Expressions and statements
  5. Early return
  6. Summary - Functions

7 Control flow

  1. if-else
  2. match basics
  3. loop
  4. while loop
  5. for loop
  6. Summary - Control flow

8 Ownership

  1. What is ownership?
  2. Move semantics
  3. Copy trait
  4. Functions and ownership
  5. Return and ownership
  6. Summary - Ownership

9 Borrowing

  1. References
  2. Mutable reference
  3. Borrowing rules
  4. Dangling reference
  5. References and functions
  6. Summary - Borrowing

10 Slices

  1. String slices
  2. Slice syntax
  3. Array slices
  4. Slices and functions
  5. Range types
  6. Summary - Slices

11 Project: Guessing game

  1. Game skeleton
  2. Reading input
  3. Comparing numbers
  4. Random number
  5. Loop and exit

12 Structs

  1. Struct definition
  2. Creating an instance
  3. Tuple struct
  4. Unit-like struct
  5. Field update
  6. Summary - Structs

13 Methods

  1. impl block
  2. self parameter
  3. &mut self
  4. Associated functions
  5. Multiple impl blocks
  6. Summary - Methods

14 Enums

  1. Enum definition
  2. Variants
  3. Variants with data
  4. Option<T>
  5. Option instead of null
  6. Summary - Enums

15 Pattern matching

  1. match patterns
  2. Value binding
  3. if let
  4. while let
  5. _ wildcard
  6. Summary - Pattern matching

16 Project: To-do CLI

  1. Task model
  2. Adding
  3. Listing
  4. Marking done
  5. Reading commands

17 Vectors

  1. Vec<T>
  2. Push and pop
  3. Index and get
  4. Looping a vector
  5. with_capacity
  6. Summary - Vectors

18 Advanced strings

  1. String and &str conversion
  2. chars and bytes
  3. Slicing strings
  4. contains and replace
  5. split and join
  6. Summary - Advanced strings

19 Error handling

  1. Result<T, E>
  2. panic!
  3. ? operator
  4. match and Result
  5. unwrap and expect
  6. Summary - Error handling

20 Beginner final

  1. Vowel counter
  2. Average calculator
  3. Bank account
  4. Temperature converter