Dynamic Error Types

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()?;

fn main() {
    fs::write("count.dat", "1i3").unwrap();
    match read_count("count.dat") {
        Ok(count) => println!("Count: {count}"),
        Err(err) => println!("Error: {err}"),
This slide should take about 5 minutes.

The read_count function can return std::io::Error (from file operations) or std::num::ParseIntError (from String::parse).

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.