`Default` is a Rust standard trait for constructing a type’s usual starting value without arguments. Many standard types already implement it: an integer can default to `0`, a `String` to an empty string, and a `bool` to `false`. For your own type, choose values that represent its common or base state. section 15.1
fn main() {
let default_i8: i8 = Default::default();
let default_str: String = Default::default();
let default_bool: bool = Default::default();
println!("'{default_i8}', '{default_str}', '{default_bool}'");
}
These three calls to `Default::default()` produce `0`, an empty string, and `false`. Each variable’s declared type determines which default is constructed. section 15.1
Choose a default for your own type
Implement `Default` when the starting value needs a deliberate choice: ```rust struct DeskMoney(u32); impl Default for DeskMoney { fn default() -> Self { Self(50) } } ``` Now `DeskMoney::default()` contains `50`, not `0`. A default need not mean “empty”; it should mean what makes sense for the type. section 21.1 section 15.1
When every field already implements `Default`, `#[derive(Default)]` can generate the implementation for a struct. To set just one field differently, use struct update syntax with `..Default::default()` for the rest. section 15.1
#[derive(Debug, Default)]
struct Size {
height: f64,
length: f64,
width: f64,
}
fn main() {
let only_height = Size {
height: 1.0, #1
..Default::default() #2
};
println!("{only_height:?}");
}
The `Size` example sets `height` to `1.0` while its other fields receive their defaults, `0.0`. section 15.1
An enum can derive `Default` by marking its chosen variant with `#[default]`. A calculator, for example, starts with a zero total, an empty number string, and an `Add` operation. section 14.3.2
Implementing the trait also lets the type work with code that requires `Default`. `std::mem::take`, for instance, returns a value while leaving its default behind; taking a `DeskMoney(500)` leaves `DeskMoney(50)`. Defaults can also provide a starting point for changes made through a builder pattern. section 21.1 section 15.1
