comprehensive-rust/src/error-handling/error.md

---
minutes: 5
---

# 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.

```rust,editable
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!("Count: {count}"),
        Err(err) => println!("Error: {err}"),
    }
}
```

<details>

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](https://github.com/rust-lang/rust/issues/103765) only.

</details>