EC
Box<dyn Error>
RustERRORNotableError Handling

Boxed dynamic error

Quick Answer

Use Box<dyn Error> for simple programs; prefer typed errors or anyhow in libraries/larger apps.

What this means

A heap-allocated trait object wrapping any type that implements std::error::Error. Commonly used as a catch-all return type for functions that can fail in multiple ways.

Why it happens
  1. 1fn main() returns Result<(), Box<dyn Error>>
  2. 2Multiple distinct error types must be unified without a custom enum

Fix

Return Box<dyn Error> from main

Return Box<dyn Error> from main
use std::fs;
use std::num::ParseIntError;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let s = fs::read_to_string("num.txt")?;
    let n: i32 = s.trim().parse()?;
    println!("number: {}", n);
    Ok(())
}

Why this works

? automatically converts any Error into Box<dyn Error> via the From impl.

Code examples
Type aliasrust
type Result<T> = std::result::Result<T, Box<dyn std::error::Error>>;
Sources

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