Hands-on Rust: Chapter 7 Invalid Camera Method from Chapter 5

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That’s a fun one. :slight_smile: I’ve written a few functions with mutable, non-reference parameters - usually when porting directly from C++ and its ilk. It’s a good way to let the function modify the parameter without side-effects - that is, the changes to the parameter never leave the function.

Say you had a function:

fn do_stuff(mut n: i32) {
   while n > 0 {
      // Do something with n here!
      n -= 1;
   }
}

When you call do_stuff, it modifies n inside the function - but the original n that is passed in never changes. n is either a copy or a move depending upon the type - but it is now local to the function, just as if you’d typed let mut n = 5 as the first line of the function. So when you call it:

let mut n = 5;
do_stuff(n);
println!("{}", n);

You still get the output of 5, because n is copied (i32 copies by default - most primitives do). Whereas if you’d use a mutable reference, you’d get:

fn do_stuff(n: &mut i32) {
   while n > 0 {
      // Do something with n here!
      n -= 1;
   }
}

..
let mut n = 5;
do_stuff(&mut n);
println!("{}", n);

You will get the output 0, because the original n was modified.

I often think of mutable borrow parameters as “out” parameters (some other languages use that terminology) - because they are effectively returning something, even if they are doing it through a parameter rather than a return statement.

Hope that helps! (Sorry about the edits; I wrote it on my phone the first time, and Rust isn’t well suited to mobile keyboards)

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