Supertrait
- Rust course
- Advanced
- Lesson 63/81
- 4 min
Constrain a trait with another trait using a supertrait.
Supertraits
A trait can depend on another trait: trait A: B means "every type implementing A must also implement B". A's default methods can use B's methods:
use std::fmt::Display;
trait Raporlanir: Display {
fn rapor(&self) -> String {
format!("[rapor] {self}") // thanks to Display
}
}
struct Sicaklik(f64);
impl Display for Sicaklik {
fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
write!(f, "{}°C", self.0)
}
}
impl Raporlanir for Sicaklik {}This isn't inheritance: the supertrait's implementation is written separately; only its existence is required.
Why is it useful?
Supertraits gather the abilities a trait needs in one place. For example std::error::Error: Debug + Display; every error type is automatically printable. Eq: PartialEq, Ord: Eq + PartialOrd and Copy: Clone are other examples in the standard library.
fn hepsini_raporla(v: &[&dyn Raporlanir]) {
for x in v {
println!("{}", x.rapor()); // Display is reachable too
}
}Practice Reportable temperature
Write the Raporlanir trait above and the Sicaklik type (Display: {}°C). Print the result of Sicaklik(21.5).rapor().
Starter code
use std::fmt::{self, Display};
fn main() {
// buraya yaz
}Sample solution
use std::fmt::{self, Display};
trait Raporlanir: Display {
fn rapor(&self) -> String {
format!("[rapor] {self}")
}
}
struct Sicaklik(f64);
impl Display for Sicaklik {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "{}°C", self.0)
}
}
impl Raporlanir for Sicaklik {}
fn main() {
println!("{}", Sicaklik(21.5).rapor());
}Expected output
[rapor] 21.5°C
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What happens if <code>impl Raporlanir for X {}</code> is written for a type not implementing Display?
- E0277: X doesn't implement Display Correct answer
- Display is generated automatically
- It works
- A warning
Explanation: The supertrait bound is a precondition of the subtrait implementation.
2 Supertrait
trait Raporlanir____ {
fn rapor(&self) -> String {
format!("[rapor] {self}")
}
}trait Raporlanir: Display {
fn rapor(&self) -> String {
format!("[rapor] {self}")
}
}Explanation: trait A: B requires implementers of A to implement B too.
3 Trait and its supertrait
| Term | Meaning |
|---|---|
Error |
Debug + Display |
Eq |
PartialEq |
Copy |
Clone |
Ord |
Eq + PartialOrd |