The `Default` Trait

The Default trait produces a default value for a type.

rust,editable

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
#[derive(Debug, Default)]
struct Derived {
    x: u32,
    y: String,
    z: Implemented,
}

#[derive(Debug)]
struct Implemented(String);

impl Default for Implemented {
    fn default() -> Self {
        Self("John Smith".into())
    }
}

fn main() {
    let default_struct = Derived::default();
    dbg!(default_struct);

    let almost_default_struct =
        Derived { y: "Y is set!".into(), ..Derived::default() };
    dbg!(almost_default_struct);

    let nothing: Option<Derived> = None;
    dbg!(nothing.unwrap_or_default());
}

It can be implemented directly or it can be derived via #[derive(Default)].
A derived implementation will produce a value where all fields are set to their default values.
- This means all types in the struct must implement Default too.
Standard Rust types commonly implement Default with reasonable values (e.g. 0, "", etc).
The partial struct initialization works nicely with default.
The Rust standard library is aware that types can implement Default and provides convenience methods that use it.
The .. syntax is called struct update syntax.

</details>

The `Default` Trait

The Default Trait

The `Default` Trait