Rust
Error trait
Define your own error by implementing std::error::Error.
The Error trait
std::error::Error is the common interface of error types. For a type to count as an error, it must implement Debug and Display; Error's methods have default bodies:
use std::error::Error;
use std::fmt;
#[derive(Debug)]
struct YasHatasi(i32);
impl fmt::Display for YasHatasi {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "geçersiz yaş: {}", self.0)
}
}
impl Error for YasHatasi {}
Display is the message for the user; Debug is developer detail.
Returning with Box<dyn Error>
Your own error type can be converted into Box<dyn Error>; thanks to Display, it prints with {}:
fn yas(n: i32) -> Result<u8, Box<dyn Error>> {
if !(0..=150).contains(&n) {
return Err(Box::new(YasHatasi(n)));
}
Ok(n as u8)
}
match yas(-5) {
Ok(y) => println!("{y}"),
Err(e) => println!("{e}"), // geçersiz yaş: -5
}
Exercise Age check
Write the YasHatasi type above (Debug, Display, Error) and the yas function. Print the result of yas(30) with {:?} and the error of yas(-5) with {} on separate lines.
Show example solution
use std::error::Error;
use std::fmt;
#[derive(Debug)]
struct YasHatasi(i32);
impl fmt::Display for YasHatasi {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "geçersiz yaş: {}", self.0)
}
}
impl Error for YasHatasi {}
fn yas(n: i32) -> Result<u8, Box<dyn Error>> {
if !(0..=150).contains(&n) {
return Err(Box::new(YasHatasi(n)));
}
Ok(n as u8)
}
fn main() {
println!("{:?}", yas(30).ok());
if let Err(e) = yas(-5) {
println!("{}", e);
}
}
Test yourself
-
What is the advantage of the <code>Box<dyn Error></code> return type?
- It's faster
- Distinguishing error kinds with match gets easier
- It doesn't panic
- It can return any type implementing Error via ?
Show answer
It can return any type implementing Error via ? — You gain flexibility but lose the concrete type; if the caller wants to act by type it needs a downcast.
-
Implement Error
impl ____ for YasHatasi {}
Show answer
impl Error for YasHatasi {}
-
Trait and role
- Display
- User message
- Debug
- Developer detail
- Error
- Common error interface
- Box<dyn Error>
- Any error
Next lesson: thiserror
