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 Interesting {
    /// integers and floats default to zero
    cardinality: usize,
    /// bool defaults to false
    really: bool,
    /// collections default to empty
    see_also: Vec<Interesting>
}

fn interestingness(i: &Interesting) -> usize {
    if i.really {
        i.cardinality
    } else {
        // we can use `Option`'s `unwrap_or_default()` method here because our
        // `Interesting` struct implements `Default`.
        i.see_also.first().unwrap_or_default().cardinality
    }
}

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()