Creating an instance

Create a struct instance and access its fields.

Creating instances

When creating an instance, every field must be given a value; order doesn't matter. If the variable name equals the field name, the field init shorthand is used:

fn yeni_oyuncu(ad: String) -> Oyuncu {
    Oyuncu {
        ad,          // ad: ad
        puan: 0,
        aktif: true,
    }
}

A missing field is a "missing field" compile error; Rust doesn't assign default values automatically.

Printing with Debug

By default structs can't be printed with {} or {:?}. Once #[derive(Debug)] is added, {:?} gives one-line output and {:#?} indented output:

#[derive(Debug)]
struct Oyuncu { ad: String, puan: u32, aktif: bool }

let o = yeni_oyuncu(String::from("Ali"));
println!("{:?}", o);
// Oyuncu { ad: "Ali", puan: 0, aktif: true }

dbg!(&o) writes to stderr together with the file and line.

Exercise Builder function

Define Oyuncu { ad: String, puan: u32, aktif: bool } with #[derive(Debug)]. Let fn yeni_oyuncu(ad: String) -> Oyuncu set puan: 0 and aktif: true using the field shorthand. Create one for "Ali" and print it with {:?}.

Show example solution

#[derive(Debug)]
struct Oyuncu {
    ad: String,
    puan: u32,
    aktif: bool,
}

fn yeni_oyuncu(ad: String) -> Oyuncu {
    Oyuncu {
        ad,
        puan: 0,
        aktif: true,
    }
}

fn main() {
    let o = yeni_oyuncu(String::from("Ali"));
    println!("{:?}", o);
}

Test yourself

Does <code>Oyuncu { ad, puan: 0 }</code> (without aktif) compile?
- Yes, aktif stays undefined
- It only warns
- No: the field aktif is missing
- Yes, aktif becomes false
Show answer

No: the field aktif is missing — All fields must be initialized. If you want defaults, use #[derive(Default)] and ..Default::default().

Derive Debug

#[derive(____)]
struct Oyuncu {
    ad: String,
}

Show answer

#[derive(Debug)]
struct Oyuncu {
    ad: String,
}

Syntax and meaning

Oyuncu { ad, .. }

Field with shorthand
#[derive(Debug)]

{:?} support
{:#?}

Indented output
dbg!(&o)

Writes to stderr

Next lesson: Tuple struct

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