Box<dyn Error> in Rust

A boxed trait object holds any error, which suits application code and mains.

use std::error::Error;

fn parse_pair(text: &str) -> Result<(i32, i32), Box<dyn Error>> {
    let (a, b) = text.split_once(',').ok_or("expected a comma")?;
    Ok((a.trim().parse()?, b.trim().parse()?))
}

fn main() {
    println!("{:?}", parse_pair("3, 4"));
    for bad in ["3 4", "3, x"] {
        match parse_pair(bad) {
            Ok(pair) => println!("{:?}", pair),
            Err(e) => println!("{bad:?} -> {e}"),
        }
    }
}

How it works

1. Box<dyn Error> erases the concrete error type.
2. ? boxes anything implementing Error for you.
3. It trades matching on variants for not declaring them.

Keywords and builtins used here

• Box
• Err
• Ok
• Result
• dyn
• fn
• for
• i32
• in
• let
• main
• match
• parse_pair
• str
• use

The run, in numbers

Lines

16

Characters to type

380

Tokens

149

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 85 seconds.

Type this snippet

Step 4 of 5 in Your own error types, step 11 of 15 in Error handling.

← Previous Next →