Advanced Traits - The Rust Programming Language

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## Advanced Traits

We first covered traits in the “Defining Shared Behavior with Traits” section in Chapter 10, but we didn’t discuss the more advanced details. Now that you know more about Rust, we can get into the nitty-gritty.

### Defining Traits with Associated Types

Associated types connect a type placeholder with a trait such that the trait method definitions can use these placeholder types in their signatures. The implementor of a trait will specify the concrete type to be used instead of the placeholder type for the particular implementation. That way, we can define a trait that uses some types without needing to know exactly what those types are until the trait is implemented.

We’ve described most of the advanced features in this chapter as being rarely needed. Associated types are somewhere in the middle: They’re used more rarely than features explained in the rest of the book but more commonly than many of the other features discussed in this chapter.

One example of a trait with an associated type is the `Iterator` trait that the standard library provides. The associated type is named `Item` and stands in for the type of the values the type implementing the `Iterator` trait is iterating over. The definition of the `Iterator` trait is as shown in Listing 20-13.

    pub trait Iterator {
        type Item;

        fn next(&mut self) -> Option<Self::Item>;
    }

Listing 20-13: The definition of the `Iterator` trait that has an associated type `Item`

The type `Item` is a placeholder, and the `next` method’s definition shows that it will return values of type `Option<Self::Item>`. Implementors of the `Iterator` trait will specify the concrete type for `Item`, and the `next` method will return an `Option` containing a value of that concrete type.

Associated types might seem like a similar concept to generics, in that the latter allow us to define a function without specifying what types it can handle. To examine the difference between the two concepts, we’ll look at an implementation of the `Iterator` trait on a type named `Counter` that specifies the `Item` type is `u32`:

Filename: src/lib.rs

    struct Counter {
        count: u32,
    }

    impl Counter {
        fn new() -> Counter {
            Counter { count: 0 }
        }
    }

    impl Iterator for Counter {
        type Item = u32;

        fn next(&mut self) -> Option<Self::Item> {
            // --snip--
            if self.count < 5 {
                self.count += 1;
                Some(self.count)
            } else {
                None
            }
        }
    }

This syntax seems comparable to that of generics. So, why not just define the `Iterator` trait with generics, as shown in Listing 20-14?

    pub trait Iterator<T> {
        fn next(&mut self) -> Option<T>;
    }

Listing 20-14: A hypothetical definition of the `Iterator` trait using generics

The difference is that when using generics, as in Listing 20-14, we must annotate the types in each implementation; because we can also implement `Iterator<String> for Counter` or any other type, we could have multiple implementations of `Iterator` for `Counter`. In other words, when a trait has a generic parameter, it can be implemented for a type multiple times, changing the concrete types of the generic type parameters each time. When we use the `next` method on `Counter`, we would have to provide type annotations to indicate which implementation of `Iterator` we want to use.

With associated types, we don’t need to annotate types, because we can’t implement a trait on a type multiple times. In Listing 20-13 with the definition that uses associated types, we can choose what the type of `Item` will be only once because there can be only one `impl Iterator for Counter`. We don’t have to specify that we want an iterator of `u32` values everywhere we call `next` on `Counter`.

Associated types also become part of the trait’s contract: Implementors of the trait must provide a type to stand in for the associated type placeholder. Associated types often have a name that describes how the type will be used, and documenting the associated type in the API documentation is a good practice.

### Using Default Generic Parameters and Operator Overloading

When we use generic type parameters, we can specify a default concrete type for the generic type. This eliminates the need for implementors of the trait to specify a concrete type if the default type works. You specify a default type when declaring a generic type with the `<PlaceholderType=ConcreteType>` syntax.

A great example of a situation where this technique is useful is with operator overloading, in which you customize the behavior of an operator (such as `+`) in particular situations.

Rust doesn’t allow you to create your own operators or overload arbitrary operators. But you can overload the operations and corresponding traits listed in `std::ops` by implementing the traits associated with the operator. For example, in Listing 20-15, we overload the `+` operator to add two `Point` instances together. We do this by implementing the `Add` trait on a `Point` struct.

Filename: src/main.rs

    use std::ops::Add;

    #[derive(Debug, Copy, Clone, PartialEq)]
    struct Point {
        x: i32,
        y: i32,
    }

    impl Add for Point {
        type Output = Point;

        fn add(self, other: Point) -> Point {
            Point {
                x: self.x + other.x,
                y: self.y + other.y,
            }
        }
    }

    fn main() {
        assert_eq!(
            Point { x: 1, y: 0 } + Point { x: 2, y: 3 },
            Point { x: 3, y: 3 }
        );
    }

Listing 20-15: Implementing the `Add` trait to overload the `+` operator for `Point` instances

The `add` method adds the `x` values of two `Point` instances and the `y` values of two `Point` instances to create a new `Point`. The `Add` trait has an associated type named `Output` that determines the type returned from the `add` method.

The default generic type in this code is within the `Add` trait. Here is its definition:

    #![allow(unused)]
    fn main() {
    trait Add<Rhs=Self> {
        type Output;

        fn add(self, rhs: Rhs) -> Self::Output;
    }
    }

This code should look generally familiar: a trait with one method and an associated type. The new part is `Rhs=Self`: This syntax is called default type parameters. The `Rhs` generic type parameter (short for “right-hand side”) defines the type of the `rhs` parameter in the `add` method. If we don’t specify a concrete type for `Rhs` when we implement the `Add` trait, the type of `Rhs` will default to `Self`, which will be the type we’re implementing `Add` on.

When we implemented `Add` for `Point`, we used the default for `Rhs` because we wanted to add two `Point` instances. Let’s look at an example of implementing the `Add` trait where we want to customize the `Rhs` type rather than using the default.

We have two structs, `Millimeters` and `Meters`, holding values in different units. This thin wrapping of an existing type in another struct is known as the newtype pattern, which we describe in more detail in the “Implementing External Traits with the Newtype Pattern” section. We want to add values in millimeters to values in meters and have the implementation of `Add` do the conversion correctly. We can implement `Add` for `Millimeters` with `Meters` as the `Rhs`, as shown in Listing 20-16.

Filename: src/lib.rs

    use std::ops::Add;

    struct Millimeters(u32);
    struct Meters(u32);

    impl Add<Meters> for Millimeters {
        type Output = Millimeters;

        fn add(self, other: Meters) -> Millimeters {
            Millimeters(self.0 + (other.0 * 1000))
        }
    }

Listing 20-16: Implementing the `Add` trait on `Millimeters` to add `Millimeters` and `Meters`

To add `Millimeters` and `Meters`, we specify `impl Add<Meters>` to set the value of the `Rhs` type parameter instead of using the default of `Self`.

You’ll use default type parameters in two main ways:

1. To extend a type without breaking existing code
2. To allow customization in specific cases most users won’t need

The standard library’s `Add` trait is an example of the second purpose: Usually, you’ll add two like types, but the `Add` trait provides the ability to customize beyond that. Using a default type parameter in the `Add` trait definition means you don’t have to specify the extra parameter most of the time. In other words, a bit of implementation boilerplate isn’t needed, making it easier to use the trait.

The first purpose is similar to the second but in reverse: If you want to add a type parameter to an existing trait, you can give it a default to allow extension of the functionality of the trait without breaking the existing implementation code.

### Disambiguating Between Identically Named Methods

Nothing in Rust prevents a trait from having a method with the same name as another trait’s method, nor does Rust prevent you from implementing both traits on one type. It’s also possible to implement a method directly on the type with the same name as methods from traits.

When calling methods with the same name, you’ll need to tell Rust which one you want to use. Consider the code in Listing 20-17 where we’ve defined two traits, `Pilot` and `Wizard`, that both have a method called `fly`. We then implement both traits on a type `Human` that already has a method named `fly` implemented on it. Each `fly` method does something different.

Filename: src/main.rs

    trait Pilot {
        fn fly(&self);
    }

    trait Wizard {
        fn fly(&self);
    }

    struct Human;

    impl Pilot for Human {
        fn fly(&self) {
            println!("This is your captain speaking.");
        }
    }

    impl Wizard for Human {
        fn fly(&self) {
            println!("Up!");
        }
    }

    impl Human {
        fn fly(&self) {
            println!("*waving arms furiously*");
        }
    }

    fn main() {}

Listing 20-17: Two traits are defined to have a `fly` method and are implemented on the `Human` type, and a `fly` method is implemented on `Human` directly.

When we call `fly` on an instance of `Human`, the compiler defaults to calling the method that is directly implemented on the type, as shown in Listing 20-18.

Filename: src/main.rs

    trait Pilot {
        fn fly(&self);
    }

    trait Wizard {
        fn fly(&self);
    }

    struct Human;

    impl Pilot for Human {
        fn fly(&self) {
            println!("This is your captain speaking.");
        }
    }

    impl Wizard for Human {
        fn fly(&self) {
            println!("Up!");
        }
    }

    impl Human {
        fn fly(&self) {
            println!("*waving arms furiously*");
        }
    }

    fn main() {
        let person = Human;
        person.fly();
    }

Listing 20-18: Calling `fly` on an instance of `Human`

Running this code will print `*waving arms furiously*`, showing that Rust called the `fly` method implemented on `Human` directly.

To call the `fly` methods from either the `Pilot` trait or the `Wizard` trait, we need to use more explicit syntax to specify which `fly` method we mean. Listing 20-19 demonstrates this syntax.

Filename: src/main.rs

    trait Pilot {
        fn fly(&self);
    }

    trait Wizard {
        fn fly(&self);
    }

    struct Human;

    impl Pilot for Human {
        fn fly(&self) {
            println!("This is your captain speaking.");
        }
    }

    impl Wizard for Human {
        fn fly(&self) {
            println!("Up!");
        }
    }

    impl Human {
        fn fly(&self) {
            println!("*waving arms furiously*");
        }
    }

    fn main() {
        let person = Human;
        Pilot::fly(&person);
        Wizard::fly(&person);
        person.fly();
    }

Listing 20-19: Specifying which trait’s `fly` method we want to call

Specifying the trait name before the method name clarifies to Rust which implementation of `fly` we want to call. We could also write `Human::fly(&person)`, which is equivalent to the `person.fly()` that we used in Listing 20-19, but this is a bit longer to write if we don’t need to disambiguate.

Running this code prints the following:

    $ cargo run
       Compiling traits-example v0.1.0 (file:///projects/traits-example)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.46s
         Running `target/debug/traits-example`
    This is your captain speaking.
    Up!
    *waving arms furiously*

Because the `fly` method takes a `self` parameter, if we had two types that both implement one trait, Rust could figure out which implementation of a trait to use based on the type of `self`.

However, associated functions that are not methods don’t have a `self` parameter. When there are multiple types or traits that define non-method functions with the same function name, Rust doesn’t always know which type you mean unless you use fully qualified syntax. For example, in Listing 20-20, we create a trait for an animal shelter that wants to name all baby dogs Spot. We make an `Animal` trait with an associated non-method function `baby_name`. The `Animal` trait is implemented for the struct `Dog`, on which we also provide an associated non-method function `baby_name` directly.

Filename: src/main.rs

    trait Animal {
        fn baby_name() -> String;
    }

    struct Dog;

    impl Dog {
        fn baby_name() -> String {
            String::from("Spot")
        }
    }

    impl Animal for Dog {
        fn baby_name() -> String {
            String::from("puppy")
        }
    }

    fn main() {
        println!("A baby dog is called a {}", Dog::baby_name());
    }

Listing 20-20: A trait with an associated function and a type with an associated function of the same name that also implements the trait

We implement the code for naming all puppies Spot in the `baby_name` associated function that is defined on `Dog`. The `Dog` type also implements the trait `Animal`, which describes characteristics that all animals have. Baby dogs are called puppies, and that is expressed in the implementation of the `Animal` trait on `Dog` in the `baby_name` function associated with the `Animal` trait.

In `main`, we call the `Dog::baby_name` function, which calls the associated function defined on `Dog` directly. This code prints the following:

    $ cargo run
       Compiling traits-example v0.1.0 (file:///projects/traits-example)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.54s
         Running `target/debug/traits-example`
    A baby dog is called a Spot

This output isn’t what we wanted. We want to call the `baby_name` function that is part of the `Animal` trait that we implemented on `Dog` so that the code prints `A baby dog is called a puppy`. The technique of specifying the trait name that we used in Listing 20-19 doesn’t help here; if we change `main` to the code in Listing 20-21, we’ll get a compilation error.

Filename: src/main.rs

    trait Animal {
        fn baby_name() -> String;
    }

    struct Dog;

    impl Dog {
        fn baby_name() -> String {
            String::from("Spot")
        }
    }

    impl Animal for Dog {
        fn baby_name() -> String {
            String::from("puppy")
        }
    }

    fn main() {
        println!("A baby dog is called a {}", Animal::baby_name());
    }

Listing 20-21: Attempting to call the `baby_name` function from the `Animal` trait, but Rust doesn’t know which implementation to use

Because `Animal::baby_name` doesn’t have a `self` parameter, and there could be other types that implement the `Animal` trait, Rust can’t figure out which implementation of `Animal::baby_name` we want. We’ll get this compiler error:

    $ cargo run
       Compiling traits-example v0.1.0 (file:///projects/traits-example)
    error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
      --> src/main.rs:20:43
       |
     2 |     fn baby_name() -> String;
       |     ------------------------- `Animal::baby_name` defined here
    ...
    20 |     println!("A baby dog is called a {}", Animal::baby_name());
       |                                           ^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
       |
    help: use the fully-qualified path to the only available implementation
       |
    20 |     println!("A baby dog is called a {}", <Dog as Animal>::baby_name());
       |                                           +++++++       +

    For more information about this error, try `rustc --explain E0790`.
    error: could not compile `traits-example` (bin "traits-example") due to 1 previous error

To disambiguate and tell Rust that we want to use the implementation of `Animal` for `Dog` as opposed to the implementation of `Animal` for some other type, we need to use fully qualified syntax. Listing 20-22 demonstrates how to use fully qualified syntax.

Filename: src/main.rs

    trait Animal {
        fn baby_name() -> String;
    }

    struct Dog;

    impl Dog {
        fn baby_name() -> String {
            String::from("Spot")
        }
    }

    impl Animal for Dog {
        fn baby_name() -> String {
            String::from("puppy")
        }
    }

    fn main() {
        println!("A baby dog is called a {}", <Dog as Animal>::baby_name());
    }

Listing 20-22: Using fully qualified syntax to specify that we want to call the `baby_name` function from the `Animal` trait as implemented on `Dog`

We’re providing Rust with a type annotation within the angle brackets, which indicates we want to call the `baby_name` method from the `Animal` trait as implemented on `Dog` by saying that we want to treat the `Dog` type as an `Animal` for this function call. This code will now print what we want:

    $ cargo run
       Compiling traits-example v0.1.0 (file:///projects/traits-example)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.48s
         Running `target/debug/traits-example`
    A baby dog is called a puppy

In general, fully qualified syntax is defined as follows:

    <Type as Trait>::function(receiver_if_method, next_arg, ...);

For associated functions that aren’t methods, there would not be a `receiver`: There would only be the list of other arguments. You could use fully qualified syntax everywhere that you call functions or methods. However, you’re allowed to omit any part of this syntax that Rust can figure out from other information in the program. You only need to use this more verbose syntax in cases where there are multiple implementations that use the same name and Rust needs help to identify which implementation you want to call.

### Using Supertraits

Sometimes you might write a trait definition that depends on another trait: For a type to implement the first trait, you want to require that type to also implement the second trait. You would do this so that your trait definition can make use of the associated items of the second trait. The trait your trait definition is relying on is called a supertrait of your trait.

For example, let’s say we want to make an `OutlinePrint` trait with an `outline_print` method that will print a given value formatted so that it’s framed in asterisks. That is, given a `Point` struct that implements the standard library trait `Display` to result in `(x, y)`, when we call `outline_print` on a `Point` instance that has `1` for `x` and `3` for `y`, it should print the following:

    **********
    *        *
    * (1, 3) *
    *        *
    **********

In the implementation of the `outline_print` method, we want to use the `Display` trait’s functionality. Therefore, we need to specify that the `OutlinePrint` trait will work only for types that also implement `Display` and provide the functionality that `OutlinePrint` needs. We can do that in the trait definition by specifying `OutlinePrint: Display`. This technique is similar to adding a trait bound to the trait. Listing 20-23 shows an implementation of the `OutlinePrint` trait.

Filename: src/main.rs

    use std::fmt;

    trait OutlinePrint: fmt::Display {
        fn outline_print(&self) {
            let output = self.to_string();
            let len = output.len();
            println!("{}", "*".repeat(len + 4));
            println!("*{}*", " ".repeat(len + 2));
            println!("* {output} *");
            println!("*{}*", " ".repeat(len + 2));
            println!("{}", "*".repeat(len + 4));
        }
    }

    fn main() {}

Listing 20-23: Implementing the `OutlinePrint` trait that requires the functionality from `Display`

Because we’ve specified that `OutlinePrint` requires the `Display` trait, we can use the `to_string` function that is automatically implemented for any type that implements `Display`. If we tried to use `to_string` without adding a colon and specifying the `Display` trait after the trait name, we’d get an error saying that no method named `to_string` was found for the type `&Self` in the current scope.

Let’s see what happens when we try to implement `OutlinePrint` on a type that doesn’t implement `Display`, such as the `Point` struct:

Filename: src/main.rs

    use std::fmt;

    trait OutlinePrint: fmt::Display {
        fn outline_print(&self) {
            let output = self.to_string();
            let len = output.len();
            println!("{}", "*".repeat(len + 4));
            println!("*{}*", " ".repeat(len + 2));
            println!("* {output} *");
            println!("*{}*", " ".repeat(len + 2));
            println!("{}", "*".repeat(len + 4));
        }
    }

    struct Point {
        x: i32,
        y: i32,
    }

    impl OutlinePrint for Point {}

    fn main() {
        let p = Point { x: 1, y: 3 };
        p.outline_print();
    }

We get an error saying that `Display` is required but not implemented:

    $ cargo run
       Compiling traits-example v0.1.0 (file:///projects/traits-example)
    error[E0277]: `Point` doesn't implement `std::fmt::Display`
      --> src/main.rs:20:23
       |
    20 | impl OutlinePrint for Point {}
       |                       ^^^^^ the trait `std::fmt::Display` is not implemented for `Point`
       |
    note: required by a bound in `OutlinePrint`
      --> src/main.rs:3:21
       |
     3 | trait OutlinePrint: fmt::Display {
       |                     ^^^^^^^^^^^^ required by this bound in `OutlinePrint`

    error[E0277]: `Point` doesn't implement `std::fmt::Display`
      --> src/main.rs:24:7
       |
    24 |     p.outline_print();
       |       ^^^^^^^^^^^^^ the trait `std::fmt::Display` is not implemented for `Point`
       |
    note: required by a bound in `OutlinePrint::outline_print`
      --> src/main.rs:3:21
       |
     3 | trait OutlinePrint: fmt::Display {
       |                     ^^^^^^^^^^^^ required by this bound in `OutlinePrint::outline_print`
     4 |     fn outline_print(&self) {
       |        ------------- required by a bound in this associated function

    For more information about this error, try `rustc --explain E0277`.
    error: could not compile `traits-example` (bin "traits-example") due to 2 previous errors

To fix this, we implement `Display` on `Point` and satisfy the constraint that `OutlinePrint` requires, like so:

Filename: src/main.rs

    trait OutlinePrint: fmt::Display {
        fn outline_print(&self) {
            let output = self.to_string();
            let len = output.len();
            println!("{}", "*".repeat(len + 4));
            println!("*{}*", " ".repeat(len + 2));
            println!("* {output} *");
            println!("*{}*", " ".repeat(len + 2));
            println!("{}", "*".repeat(len + 4));
        }
    }

    struct Point {
        x: i32,
        y: i32,
    }

    impl OutlinePrint for Point {}

    use std::fmt;

    impl fmt::Display for Point {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "({}, {})", self.x, self.y)
        }
    }

    fn main() {
        let p = Point { x: 1, y: 3 };
        p.outline_print();
    }

Then, implementing the `OutlinePrint` trait on `Point` will compile successfully, and we can call `outline_print` on a `Point` instance to display it within an outline of asterisks.

### Implementing External Traits with the Newtype Pattern

In the “Implementing a Trait on a Type” section in Chapter 10, we mentioned the orphan rule that states we’re only allowed to implement a trait on a type if either the trait or the type, or both, are local to our crate. It’s possible to get around this restriction using the newtype pattern, which involves creating a new type in a tuple struct. (We covered tuple structs in the “Creating Different Types with Tuple Structs” section in Chapter 5.) The tuple struct will have one field and be a thin wrapper around the type for which we want to implement a trait. Then, the wrapper type is local to our crate, and we can implement the trait on the wrapper. Newtype is a term that originates from the Haskell programming language. There is no runtime performance penalty for using this pattern, and the wrapper type is elided at compile time.

As an example, let’s say we want to implement `Display` on `Vec<T>`, which the orphan rule prevents us from doing directly because the `Display` trait and the `Vec<T>` type are defined outside our crate. We can make a `Wrapper` struct that holds an instance of `Vec<T>`; then, we can implement `Display` on `Wrapper` and use the `Vec<T>` value, as shown in Listing 20-24.

Filename: src/main.rs

    use std::fmt;

    struct Wrapper(Vec<String>);

    impl fmt::Display for Wrapper {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "[{}]", self.0.join(", "))
        }
    }

    fn main() {
        let w = Wrapper(vec![String::from("hello"), String::from("world")]);
        println!("w = {w}");
    }

Listing 20-24: Creating a `Wrapper` type around `Vec<String>` to implement `Display`

The implementation of `Display` uses `self.0` to access the inner `Vec<T>` because `Wrapper` is a tuple struct and `Vec<T>` is the item at index 0 in the tuple. Then, we can use the functionality of the `Display` trait on `Wrapper`.

The downside of using this technique is that `Wrapper` is a new type, so it doesn’t have the methods of the value it’s holding. We would have to implement all the methods of `Vec<T>` directly on `Wrapper` such that the methods delegate to `self.0`, which would allow us to treat `Wrapper` exactly like a `Vec<T>`. If we wanted the new type to have every method the inner type has, implementing the `Deref` trait on the `Wrapper` to return the inner type would be a solution (we discussed implementing the `Deref` trait in the “Treating Smart Pointers Like Regular References” section in Chapter 15). If we didn’t want the `Wrapper` type to have all the methods of the inner type—for example, to restrict the `Wrapper` type’s behavior—we would have to implement just the methods we do want manually.

This newtype pattern is also useful even when traits are not involved. Let’s switch focus and look at some advanced ways to interact with Rust’s type system.