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Sometimes we want to allow any type of error to be returned without writing our own enum covering all the different possibilities. The std::error::Error trait makes it easy to create a trait object that can contain any error.

use std::error::Error;
use std::fs;
use std::io::Read;

fn read_count(path: &str) -> Result<i32, Box<dyn Error>> {
    let mut count_str = String::new();
    fs::File::open(path)?.read_to_string(&mut count_str)?;
    let count: i32 = count_str.parse()?;
    Ok(count)
}

fn main() {
    fs::write("count.dat", "1i3").unwrap();
    match read_count("count.dat") {
        Ok(count) => println!("개수: {count}"),
        Err(err) => println!("오류: {err}"),
    }
}

This slide should take about 5 minutes.

read_count 함수는 std::io::Error(파일 작업에서) 또는 std::num::ParseIntError(String::parse에서)를 반환할 수 있습니다.

Boxing errors saves on code, but gives up the ability to cleanly handle different error cases differently in the program. As such it’s generally not a good idea to use Box<dyn Error> in the public API of a library, but it can be a good option in a program where you just want to display the error message somewhere.

Make sure to implement the std::error::Error trait when defining a custom error type so it can be boxed. But if you need to support the no_std attribute, keep in mind that the std::error::Error trait is currently compatible with no_std in nightly only.

Comprehensive Rust 🦀

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