patterns/idioms/default.md

The Default Trait

Description

Many types in Rust have a constructor. However, this is specific to the type; Rust cannot abstract over "everything that has a new() method". To allow this, the Default trait was conceived, which can be used with containers and other generic types (e.g. see Option::unwrap_or_default()). Notably, some containers already implement it where applicable.

Not only do one-element containers like Cow, Box or Arc implement Default for contained Default types, one can automatically #[derive(Default)] for structs whose fields all implement it, so the more types implement Default, the more useful it becomes.

Example

// note that we can simply auto-derive Default here.
#[derive(Default)]
struct MyConfiguration {
    // Option defaults to None
    output: Option<Path>,
    // Vecs default to empty vector
    search_path: Vec<Path>,
    // Duration defaults to zero time
    timeout: Duration,
    // bool defaults to false
    check: bool,
}

impl MyConfiguration {
    // add setters here
}

See also

• The constructor idiom is another way to generate instances that may or may not be "default"
• The Default documentation (scroll down for the list of implementors)
• Option::unwrap_or_default()