The compiler is capable of providing basic implementations for some traits via
the #[derive]attribute. These traits can still be
manually implemented if a more complex behavior is required.
The following is a list of the "derivable" traits:
Default,
to create an empty instance of a data type.
Zero, to
create a zero instance of a numeric data type.
Debug, to
format a value using the {:?} formatter.
// `Centimeters`, a tuple struct that can be compared
#[derive(PartialEq, PartialOrd)]
struct Centimeters(f64);
// `Inches`, a tuple struct that can be printed
#[derive(Debug)]
struct Inches(i32);
impl Inches {
fn to_centimeters(&self) -> Centimeters {
let &Inches(inches) = self;
Centimeters(inches as f64 * 2.54)
}
}
// `Seconds`, a tuple struct no additional attributes
struct Seconds(i32);
fn main() {
let _one_second = Seconds(1);
// Error: `Seconds` can't be printed; it doesn't implement the `Debug` trait
//println!("One second looks like: {:?}", _one_second);
// TODO ^ Try uncommenting this line
// Error: `Seconds` can't be compared; it doesn't implement the `PartialEq` trait
//let _this_is_true = (_one_second == _one_second);
// TODO ^ Try uncommenting this line
let foot = Inches(12);
println!("One foot equals {:?}", foot);
let meter = Centimeters(100.0);
let cmp =
if foot.to_centimeters() < meter {
"smaller"
} else {
"bigger"
};
println!("One foot is {} than one meter.", cmp);
}