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.

Exercise 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.

Show example solution

struct Sessiz;
struct Renk(u8, u8, u8);

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

Test yourself

What is <code>std::mem::size_of::<Renk>()</code>? (<code>struct Renk(u8, u8, u8);</code>)
- 3
- 0
- 8
- 24
Show answer

3 — Three u8 fields are 1 byte each; no padding is needed.

A struct without fields

struct Sessiz____

let s = Sessiz;

Show answer

struct Sessiz;

let s = Sessiz;

Struct and its size

struct Sessiz;

0 bytes
struct Renk(u8, u8, u8);

3 bytes
struct Tek(u32);

4 bytes
struct Cift(u64, u64);

16 bytes

Next lesson: Field update

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