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Billy Chan
Billy Chan

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The Question Mark `?` Operator in Rust

The question mark ? operator is used for early exit an function with return type that's compatible with the value of the ? is used on. Such as Err(e) but not many people know it can be used on None as well.

The ? operator is commonly used with Result<T, E> to propagate error up the call chain. We can define a unified error enum to host all errors. Remember to take advantage of Rust's conversion method From<T> to cast foreign errors into the unified error enum. That way, we take advantage of ? operator, it will perform the conversion behind the scene.

// Unified `Error` enum
#[derive(Debug)]
enum Error {
    ParseIntError(std::num::ParseIntError),
    ParseFloatError(std::num::ParseFloatError),
}

impl std::fmt::Display for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::ParseIntError(e) => write!(f, "ParseIntError: {}", e.to_string()),
            Self::ParseFloatError(e) => write!(f, "ParseFloatError: {}", e.to_string()),
        }
    }
}

impl std::error::Error for Error {}

// Convert `std::num::ParseIntError` to `Error`
impl From<std::num::ParseIntError> for Error {
    fn from(err: std::num::ParseIntError) -> Self {
        Self::ParseIntError(err)
    }
}

// Convert `std::num::ParseFloatError` to `Error`
impl From<std::num::ParseFloatError> for Error {
    fn from(err: std::num::ParseFloatError) -> Self {
        Self::ParseFloatError(err)
    }
}

fn main() -> Result<(), Error> {
    // Parse an integer and unwrap it, or throw `Error::ParseIntError`
    let _: i32 = "123".parse()?;
    let _: i32 = "not_an_integer".parse()?;

    // Parse a float and unwrap it, or throw `Error::ParseFloatError`
    let _: f64 = "not_a_number".parse()?;

    Ok(())
}

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The ? operator could also perform early return on Option<T> return type.

fn get_first_char_of_second_word(phrase: &str) -> Option<char> {
    // Return `None` if the phrase consist of a single word
    phrase.split(" ").skip(1).next()?.chars().next()
}

fn main() {
    assert_eq!(get_first_char_of_second_word("Hello World"), Some('W'));
    assert_eq!(get_first_char_of_second_word("Hello?"), None);
}
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