Error trait
- Rust course
- Intermediate
- Lesson 65/115
- 4 min
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
}Practice 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.
Starter code
use std::error::Error;
use std::fmt;
fn main() {
// buraya yaz
}Sample 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);
}
}Expected output
Some(30) geçersiz yaş: -5
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 ? Correct answer
Explanation: You gain flexibility but lose the concrete type; if the caller wants to act by type it needs a downcast.
2 Implement Error
impl ____ for YasHatasi {}impl Error for YasHatasi {}Explanation: If Display and Debug are in place, Error is implemented with an empty impl.
3 Trait and role
| Term | Meaning |
|---|---|
Display |
User message |
Debug |
Developer detail |
Error |
Common error interface |
Box<dyn Error> |
Any error |