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.

  1. use std::error::Error;
  2. use std::fs;
  3. use std::io::Read;
  5. fn read_count(path: &str) -> Result<i32, Box<dyn Error>> {
  6.     let mut count_str = String::new();
  7.     fs::File::open(path)?.read_to_string(&mut count_str)?;
  8.     let count: i32 = count_str.parse()?;
  9.     Ok(count)
  10. }
  12. fn main() {
  13.     fs::write("count.dat", "1i3").unwrap();
  14.     match read_count("count.dat") {
  15.         Ok(count) => println!("Count: {count}"),
  16.         Err(err) => println!("Error: {err}"),
  17.     }
  18. }

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.