Out parameter

Often in more complex Rust programs we want to have out parameters in a function. With "out" parameters, values assigned are retrieved outside the function.

In Rust we do not have an "out" keyword, but we can pass local variables (like usize) as a mutable reference. Then we dereference the arguments and modify the value.

Example

Here we have a str that contains an ampersand, and several period characters. We want to count these characters in a single function, and retrieve the counts.

fn test_characters(value: &str, ampersand: &mut usize, period: &mut usize) {
    // Use mutable references as out parameters.
    for v in value.chars() {
        match v {
            '&' => *ampersand += 1,
            '.' => *period += 1,
            _ => (),
        }
    }
}

fn main() {
    let data = "A cat & a dog...";
    let mut ampersand = 0;
    let mut period = 0;
    test_characters(&data, &mut ampersand, &mut period);
    // Print final counts.
    println!("{ampersand} {period}");
}
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Multiple return values

Sometimes it is more elegant to return a tuple containing multiple values from a function. This can sometimes make programs more clear.

It is possible to implement "out" parameters in Rust programs with mut references. The syntax is confusing at first, but can be memorized with practice.