Rust Traits

Introduction

Syntax

• trait Trait { fn method(...) -> ReturnType; ... }
• trait Trait: Bound { fn method(...) -> ReturnType; ... }
• impl Trait for Type { fn method(...) -> ReturnType { ... } ... }
• impl<T> Trait for T where T: Bounds { fn method(...) -> ReturnType { ... } ... }

Remarks

Basics

Creating a Trait

trait Speak {
    fn speak(&self) -> String;
}

Implementing a Trait

struct Person;
struct Dog;

impl Speak for Person {
    fn speak(&self) -> String {
        String::from("Hello.")
    }
}

impl Speak for Dog {
    fn speak(&self) -> String {
        String::from("Woof.")
    }
}

fn main() {
    let person = Person {};
    let dog = Dog {};
    println!("The person says {}", person.speak());
    println!("The dog says {}", dog.speak());
}

Static and Dynamic Dispatch

It is possible to create a function that accepts objects that implement a specific trait.

Static Dispatch

fn generic_speak<T: Speak>(speaker: &T) {
    println!("{0}", speaker.speak());
}

fn main() {
    let person = Person {};
    let dog = Dog {};

    generic_speak(&person);
    generic_speak(&dog);
}

Static dispatch is used here, which means that Rust compiler will generate specialized versions of generic_speak function for both Dog and Person types. This generation of specialized versions of a polymorphic function (or any polymorphic entity) during compilation is called Monomorphization.

Dynamic Dispatch

fn generic_speak(speaker: &Speak) {
    println!("{0}", speaker.speak());
}

fn main() {
    let person = Person {};
    let dog = Dog {};

    generic_speak(&person as &Speak);
    generic_speak(&dog); // gets automatically coerced to &Speak
}

Here, only a single version of generic_speak exists in the compiled binary, and the speak() call is made using a vtable lookup at runtime. Thus, using dynamic dispatch results in faster compilation and smaller size of compiled binary, while being slightly slower at runtime.

Objects of type &Speak or Box<Speak> are called trait objects.

Associated Types

• Use associated type when there is a one-to-one relationship between the type implementing the trait and the associated type.
• It is sometimes also known as the output type, since this is an item given to a type when we apply a trait to it.

Creation

trait GetItems {
    type First;
//  ^~~~ defines an associated type. 
    type Last: ?Sized;
//           ^~~~~~~~ associated types may be constrained by traits as well
    fn first_item(&self) -> &Self::First;
//                           ^~~~~~~~~~~ use `Self::` to refer to the associated type 
    fn last_item(&self) -> &Self::Last;
//                          ^~~~~~~~~~ associated types can be used as function output...
    fn set_first_item(&mut self, item: Self::First);
//                                     ^~~~~~~~~~~  ... input, and anywhere.
}

Implemention

impl<T, U: ?Sized> GetItems for (T, U) {
    type First = T;
    type Last = U;
//              ^~~ assign the associated types
    fn first_item(&self) -> &Self::First { &self.0 }
    fn last_item(&self) -> &Self::Last { &self.1 }
    fn set_first_item(&mut self, item: Self::First) { self.0 = item; }
}

impl<T> GetItems for [T; 3] {
    type First = T;
    type Last = T;
    fn first_item(&self) -> &T { &self[0] }
//                           ^ you could refer to the actual type instead of `Self::First`
    fn last_item(&self) -> &T { &self[2] }
    fn set_first_item(&mut self, item: T) { self[0] = item; }
}

Refering to associated types

If we are sure that a type T implements GetItems e.g. in generics, we could simply use T::First to obtain the associated type.

fn get_first_and_last<T: GetItems>(obj: &T) -> (&T::First, &T::Last) {
//                                               ^~~~~~~~ refer to an associated type
    (obj.first_item(), obj.last_item())
}

Otherwise, you need to explicitly tell the compiler which trait the type is implementing

let array: [u32; 3] = [1, 2, 3];
let first: &<[u32; 3] as GetItems>::First = array.first_item();
//          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [u32; 3] may implement multiple traits which many
//                                        of them provide the `First` associated type.
//                                        thus the explicit "cast" is necessary here.
assert_eq!(*first, 1);

Constraining with associated types

fn clone_first_and_last<T: GetItems>(obj: &T) -> (T::First, T::Last)
    where T::First: Clone, T::Last: Clone
//  ^~~~~ use the `where` clause to constraint associated types by traits
{
    (obj.first_item().clone(), obj.last_item().clone())
}

fn get_first_u32<T: GetItems<First=u32>>(obj: &T) -> u32 {
//                          ^~~~~~~~~~~ constraint associated types by equality
    *obj.first_item()
}

Default methods

trait Speak {
    fn speak(&self) -> String {
        String::from("Hi.")
    }
}

The method will be called by default except if it's overwritten in the impl block.

struct Human;
struct Cat;

impl Speak for Human {}

impl Speak for Cat {
    fn speak(&self) -> String {
        String::from("Meow.")
    }
}

fn main() {
    let human = Human {};
    let cat = Cat {};
    println!("The human says {}", human.speak());
    println!("The cat says {}", cat.speak());
}

Output :

The human says Hi.

The cat says Meow.

Placing a bound on a trait

When defining a new trait it is possible to enforce that types wishing to implement this trait verify a number of constraints or bounds.

Taking an example from the standard library, the DerefMut trait requires that a type first implement its sibling Deref trait:

pub trait DerefMut: Deref {
    fn deref_mut(&mut self) -> &mut Self::Target;
}

This, in turn, enables DerefMut to use the associated type Target defined by Deref.

---

While the syntax might be reminiscent of inheritance:

• it brings in all the associated items (constants, types, functions, ...) of the bound trait
• it enables polymorphism from &DerefMut to &Deref

This is different in nature:

• it is possible to use a lifetime (such as 'static) as a bound
• it is not possible to override the bound trait items (not even the functions)

Thus it is best to think of it as a separate concept.

Multiple bound object types

It's also possible to add multiple object types to a Static Dispatch function.

fn mammal_speak<T: Person + Dog>(mammal: &T) {
    println!("{0}", mammal.speak());
}

fn main() {
    let person = Person {};
    let dog = Dog {};

    mammal_speak(&person);
    mammal_speak(&dog);
}