What does the ampersand mean in a Rust type?

Question

I've seen this code in the Rust documentation:

fn eat(&self) {     println!("{} is done eating.", self.name); } 

what does the & in &self mean?

Answer 1

This means you'll be passing in a reference to the object, as opposed to moving the object itself. It's important to distinguish this because if your function looked like:

fn eat(self) {     println!("{} is done eating.", self.name); } 

and you tried calling it then using the variable after, you'd get an error

object = Foo::new(); object.eat(); object.something(); // error, because you moved object in eat 

because when you don't specify &, rust moves the value into the function and your original binding no longer has ownership. check out this minimal example I created (playground version):

struct Foo {     x : u32 }  impl Foo {      fn eat(self) {         println!("eating");     }      fn something(&self) {         println!("else");     }  }  fn main() {     println!("Hello, world!");      let g = Foo { x: 5 };     g.eat();     g.something();  // if this comes before eat, no errors because we arent moving } 

Now switch something to be called before eat. Because something only takes a reference, g still has ownership and you can continue on. eat on the other hand moves g and you no longer can use g.