Default in Rust

A canonical empty value, plus struct update syntax to change one field.

#[derive(Debug)]
struct Settings {
    theme: String,
    editor_mode: bool,
    time_limit: u32,
}

impl Default for Settings {
    fn default() -> Self {
        Settings {
            theme: "default".to_string(),
            editor_mode: false,
            time_limit: 60,
        }
    }
}

fn main() {
    println!("{:?}", Settings::default());

    let mine = Settings {
        theme: "monokai".to_string(),
        ..Default::default()
    };
    println!("{:?}", mine);
}

How it works

1. #[derive(Default)] uses each field's own default.
2. A hand-written impl sets the values you actually want.
3. ..Default::default() fills in the rest of a literal.

Keywords and builtins used here

• Default
• Self
• Settings
• String
• bool
• default
• false
• fn
• for
• impl
• let
• main
• struct
• u32

The run, in numbers

Lines

26

Characters to type

377

Tokens

105

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 60 seconds.

Type this snippet

Step 3 of 4 in Standard traits, step 10 of 19 in Traits & generics.

← Previous Next →