Mutable References in Functions

Snippet

Mutable References in Functions

Primitive arguments are passed by value (copied). The function receives a copy, so the original remains unchanged. To modify the original, pass a mutable reference: &mut num, or return the new value.

snippet.rs
rust
fn increment(mut value: i32) {
    value += 1;
}
 
fn main() {
    let mut num = 5;
    increment(num);
    println!("{}", num);
}

Breakdown

fn increment(mut value: i32)

Parameters receive a copy of the value. mut allows local modification inside the function.

num stays 5

The original num in main is unaffected because a copy was passed.

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Mutable References in Functions

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