“?” Operator in Rust

January 3, 2024

The Rust question mark (?) operator is a shorthand that simplifies error handling, particularly when working with functions that return Result types. It streamlines code by replacing explicit match statements used to unwrap Result values.

In the absence of the ? operator, handling a Result typically involves using a match statement, as shown the example below:

Rust

#[derive(Debug)]
enum MathError {
    DivisionByZero,
}

fn div(a: f64, b: f64) -> Result<f64, MathError> {
    if b == 0.0 {
        Err(MathError::DivisionByZero)
    } else {
        Ok(a / b)
    }
}

fn main() {
    let result = div(1.0, 2.0);
    match result {
        Ok(value) => println!("{}", value),
        Err(err) => println!("error: {:?}", err),
    }
}

Here, each Result returned by the div function must be explicitly unpacked using a match statement.

The ? operator provides a more concise way to handle Result. If the value of Result is an Err, it returns from the function immediately. If it’s an Ok, it unwraps the value for further use.

Rust

fn main() -> Result<(), MathError> {
    let result = div(1.0, 2.0)?;
    println!("{}", result);
    Ok(())
}

In this example, div(1.0, 2.0)? will either:

Immediately return Err(MathError::DivisionByZero) if div returns an Err, or
Continue executing with result set to the unwrapped value of Ok(a / b).

Some notes:

Function Signature: The function using ? must return a Result (or Option, in the case of ? used with Option types).
Error Propagation: The ? operator propagates errors up the call stack, allowing higher-level functions to handle them or report them as needed.
Readability: It makes error handling more concise and readable, especially in functions with multiple potential points of failure.

The ? operator in Rust is a powerful tool that simplifies error handling in functions that return Result types. It makes code more concise and readable by eliminating the need for repetitive match statements to unwrap Result values. However, it’s important to understand the context in which it can be used and its implications for error propagation in your code.

Tags	rust