Forwarding Parameter References to Functions in Rust

Asked Apr 11 '17 at 18:34

Active Apr 11 '17 at 18:38

Viewed 392 times

I'm having trouble figuring out what's happening when I forward a reference argument to another function in Rust that also expects a reference argument.

Basically, I'm confused about when a value is a reference (that needs to be de-referenced with *) and when it's just a value that can be used directly. For example,

fn second(s: &String) {
    println!("Got string '{}'", *s);  // Can also use s 
}

fn first(s: &String) {
    second(s);                        // Can also use &s
}

fn main() {
    let s = String::from("Hello, World!");
    first(&s);
}

As shown above, any of the four total combinations of passing s from first into second and printing s inside second works. My intuition says that s inside first is a reference and thus &s would be a double reference, and that s inside second is a reference that needs to be de-referenced, but it doesn't seem to actually matter. What's going on here?

edited Apr 11 '17 at 18:36

Shepmaster

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asked Apr 11 '17 at 18:34

AUD_FOR_IUV

You've actually asked two separate questions; I've linked to appropriate duplicates. – Shepmaster Apr 11 '17 at 18:38
2

TL;DR: `println!` automatically takes a reference, and `Deref` coercions convert `&&String` to a `&String` for you. – Shepmaster Apr 11 '17 at 18:39
2

Also relevant: [Why is it discouraged to accept a reference to a String (&String) or Vec (&Vec) as a function argument?](http://stackoverflow.com/q/40006219/155423) – Shepmaster Apr 11 '17 at 18:40
Thanks for the help! – AUD_FOR_IUV Apr 11 '17 at 18:43

Forwarding Parameter References to Functions in Rust

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