Traits: Defining Shared Behavior - The Rust Programming Language

## Traits: Defining Shared Behavior

A trait tells the Rust compiler about functionality a particular type has and can share with other types. We can use traits to define shared behavior in an abstract way. We can use trait bounds to specify that a generic can be any type that has certain behavior.

> Note: Traits are similar to a feature often called interfaces in other languages, although with some differences.

### Defining a Trait

A type’s behavior consists of the methods we can call on that type. Different types share the same behavior if we can call the same methods on all of those types. Trait definitions are a way to group method signatures together to define a set of behaviors necessary to accomplish some purpose.

For example, let’s say we have multiple structs that hold various kinds and amounts of text: a `NewsArticle` struct that holds a news story filed in a particular location and a `Tweet` that can have at most 280 characters along with metadata that indicates whether it was a new tweet, a retweet, or a reply to another tweet.

We want to make a media aggregator library that can display summaries of data that might be stored in a `NewsArticle` or `Tweet` instance. To do this, we need a summary from each type, and we need to request that summary by calling a `summarize` method on an instance. Listing 10-12 shows the definition of a `Summary` trait that expresses this behavior.

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    pub trait Summary {
        fn summarize(&self) -> String;
    }
    #}

Listing 10-12: A `Summary` trait that consists of the behavior provided by a `summarize` method

Here, we declare a trait using the `trait` keyword and then the trait’s name, which is `Summary` in this case. Inside the curly brackets, we declare the method signatures that describe the behaviors of the types that implement this trait, which in this case is `fn summarize(&self) -> String`.

After the method signature, instead of providing an implementation within curly brackets, we use a semicolon. Each type implementing this trait must provide its own custom behavior for the body of the method. The compiler will enforce that any type that has the `Summary` trait will have the method `summarize` defined with this signature exactly.

A trait can have multiple methods in its body: the method signatures are listed one per line and each line ends in a semicolon.

### Implementing a Trait on a Type

Now that we’ve defined the desired behavior using the `Summary` trait, we can implement it on the types in our media aggregator. Listing 10-13 shows an implementation of the `Summary` trait on the `NewsArticle` struct that uses the headline, the author, and the location to create the return value of `summarize`. For the `Tweet` struct, we define `summarize` as the username followed by the entire text of the tweet, assuming that tweet content is already limited to 280 characters.

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub trait Summary {
    #     fn summarize(&self) -> String;
    # }
    #
    pub struct NewsArticle {
        pub headline: String,
        pub location: String,
        pub author: String,
        pub content: String,
    }

    impl Summary for NewsArticle {
        fn summarize(&self) -> String {
            format!("{}, by {} ({})", self.headline, self.author, self.location)
        }
    }

    pub struct Tweet {
        pub username: String,
        pub content: String,
        pub reply: bool,
        pub retweet: bool,
    }

    impl Summary for Tweet {
        fn summarize(&self) -> String {
            format!("{}: {}", self.username, self.content)
        }
    }
    #}

Listing 10-13: Implementing the `Summary` trait on the `NewsArticle` and `Tweet` types

Implementing a trait on a type is similar to implementing regular methods. The difference is that after `impl`, we put the trait name that we want to implement, then use the `for` keyword, and then specify the name of the type we want to implement the trait for. Within the `impl` block, we put the method signatures that the trait definition has defined. Instead of adding a semicolon after each signature, we use curly brackets and fill in the method body with the specific behavior that we want the methods of the trait to have for the particular type.

After implementing the trait, we can call the methods on instances of `NewsArticle` and `Tweet` in the same way we call regular methods, like this:

    let tweet = Tweet {
        username: String::from("horse_ebooks"),
        content: String::from("of course, as you probably already know, people"),
        reply: false,
        retweet: false,
    };

    println!("1 new tweet: {}", tweet.summarize());

This code prints `1 new tweet: horse_ebooks: of course, as you probably already know, people`.

Note that because we defined the `Summary` trait and the `NewsArticle` and `Tweet` types in the same lib.rs in Listing 10-13, they’re all in the same scope. Let’s say this lib.rs is for a crate we’ve called `aggregator` and someone else wants to use our crate’s functionality to implement the `Summary` trait on a struct defined within their library’s scope. They would need to bring the trait into their scope first. They would do so by specifying `use aggregator::Summary;`, which then would enable them to implement `Summary` for their type. The `Summary` trait would also need to be a public trait for another crate to implement it, which it is because we put the `pub` keyword before `trait` in Listing 10-12.

One restriction to note with trait implementations is that we can implement a trait on a type only if either the trait or the type is local to our crate. For example, we can implement standard library traits like `Display` on a custom type like `Tweet` as part of our `aggregator` crate functionality, because the type `Tweet` is local to our `aggregator` crate. We can also implement `Summary` on `Vec<T>` in our `aggregator` crate, because the trait `Summary` is local to our `aggregator` crate.

But we can’t implement external traits on external types. For example, we can’t implement the `Display` trait on `Vec<T>` within our `aggregator` crate, because `Display` and `Vec<T>` are defined in the standard library and aren’t local to our `aggregator` crate. This restriction is part of a property of programs called coherence, and more specifically the orphan rule, so named because the parent type is not present. This rule ensures that other people’s code can’t break your code and vice versa. Without the rule, two crates could implement the same trait for the same type, and Rust wouldn’t know which implementation to use.

### Default Implementations

Sometimes it’s useful to have default behavior for some or all of the methods in a trait instead of requiring implementations for all methods on every type. Then, as we implement the trait on a particular type, we can keep or override each method’s default behavior.

Listing 10-14 shows how to specify a default string for the `summarize` method of the `Summary` trait instead of only defining the method signature, as we did in Listing 10-12.

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    pub trait Summary {
        fn summarize(&self) -> String {
            String::from("(Read more...)")
        }
    }
    #}

Listing 10-14: Definition of a `Summary` trait with a default implementation of the `summarize` method

To use a default implementation to summarize instances of `NewsArticle` instead of defining a custom implementation, we specify an empty `impl` block with `impl Summary for NewsArticle {}`.

Even though we’re no longer defining the `summarize` method on `NewsArticle` directly, we’ve provided a default implementation and specified that `NewsArticle` implements the `Summary` trait. As a result, we can still call the `summarize` method on an instance of `NewsArticle`, like this:

    let article = NewsArticle {
        headline: String::from("Penguins win the Stanley Cup Championship!"),
        location: String::from("Pittsburgh, PA, USA"),
        author: String::from("Iceburgh"),
        content: String::from("The Pittsburgh Penguins once again are the best
        hockey team in the NHL."),
    };

    println!("New article available! {}", article.summarize());

This code prints `New article available! (Read more...)`.

Creating a default implementation for `summarize` doesn’t require us to change anything about the implementation of `Summary` on `Tweet` in Listing 10-13. The reason is that the syntax for overriding a default implementation is the same as the syntax for implementing a trait method that doesn’t have a default implementation.

Default implementations can call other methods in the same trait, even if those other methods don’t have a default implementation. In this way, a trait can provide a lot of useful functionality and only require implementors to specify a small part of it. For example, we could define the `Summary` trait to have a `summarize_author` method whose implementation is required, and then define a `summarize` method that has a default implementation that calls the `summarize_author` method:

    # #![allow(unused_variables)]
    #fn main() {
    pub trait Summary {
        fn summarize_author(&self) -> String;

        fn summarize(&self) -> String {
            format!("(Read more from {}...)", self.summarize_author())
        }
    }
    #}

To use this version of `Summary`, we only need to define `summarize_author` when we implement the trait on a type:

    impl Summary for Tweet {
        fn summarize_author(&self) -> String {
            format!("@{}", self.username)
        }
    }

After we define `summarize_author`, we can call `summarize` on instances of the `Tweet` struct, and the default implementation of `summarize` will call the definition of `summarize_author` that we’ve provided. Because we’ve implemented `summarize_author`, the `Summary` trait has given us the behavior of the `summarize` method without requiring us to write any more code.

    let tweet = Tweet {
        username: String::from("horse_ebooks"),
        content: String::from("of course, as you probably already know, people"),
        reply: false,
        retweet: false,
    };

    println!("1 new tweet: {}", tweet.summarize());

This code prints `1 new tweet: (Read more from @horse_ebooks...)`.

Note that it isn’t possible to call the default implementation from an overriding implementation of that same method.

### Traits as Parameters

Now that you know how to define and implement traits, we can explore how to use traits to define functions that accept many different types.

For example, in Listing 10-13, we implemented the `Summary` trait on the `NewsArticle` and `Tweet` types. We can define a `notify` function that calls the `summarize` method on its `item` parameter, which is of some type that implements the `Summary` trait. To do this, we can use the `impl Trait` syntax, like this:

    pub fn notify(item: impl Summary) {
        println!("Breaking news! {}", item.summarize());
    }

Instead of a concrete type for the `item` parameter, we specify the `impl` keyword and the trait name. This parameter accepts any type that implements the specified trait. In the body of `notify`, we can call any methods on `item` that come from the `Summary` trait, such as `summarize`. We can call `notify` and pass in any instance of `NewsArticle` or `Tweet`. Code that calls the function with any other type, such as a `String` or an `i32`, won’t compile because those types don’t implement `Summary`.

#### Trait Bound Syntax

The `impl Trait` syntax works for straightforward cases but is actually syntax sugar for a longer form, which is called a trait bound; it looks like this:

    pub fn notify<T: Summary>(item: T) {
        println!("Breaking news! {}", item.summarize());
    }

This longer form is equivalent to the example in the previous section but is more verbose. We place trait bounds with the declaration of the generic type parameter after a colon and inside angle brackets.

The `impl Trait` syntax is convenient and makes for more concise code in simple cases. The trait bound syntax can express more complexity in other cases. For example, we can have two parameters that implement `Summary`. Using the `impl Trait` syntax looks like this:

    pub fn notify(item1: impl Summary, item2: impl Summary) {

If we wanted this function to allow `item1` and `item2` to have different types, using `impl Trait` would be appropriate (as long as both types implement `Summary`). If we wanted to force both parameters to have the same type, that’s only possible to express using a trait bound, like this:

    pub fn notify<T: Summary>(item1: T, item2: T) {

The generic type `T` specified as the type of the `item1` and `item2` parameters constrains the function such that the concrete type of the value passed as an argument for `item1` and `item2` must be the same.

#### Specifying Multiple Trait Bounds with the `+` Syntax

We can also specify more than one trait bound. Say we wanted `notify` to use display formatting on `item` as well as the `summarize` method: we specify in the `notify` definition that `item` must implement both `Display` and `Summary`. We can do so using the `+` syntax:

    pub fn notify(item: impl Summary + Display) {

The `+` syntax is also valid with trait bounds on generic types:

    pub fn notify<T: Summary + Display>(item: T) {

With the two trait bounds specified, the body of `notify` can call `summarize` and use `{}` to format `item`.

#### Clearer Trait Bounds with `where` Clauses

Using too many trait bounds has its downsides. Each generic has its own trait bounds, so functions with multiple generic type parameters can contain lots of trait bound information between the function’s name and its parameter list, making the function signature hard to read. For this reason, Rust has alternate syntax for specifying trait bounds inside a `where` clause after the function signature. So instead of writing this:

    fn some_function<T: Display + Clone, U: Clone + Debug>(t: T, u: U) -> i32 {

we can use a `where` clause, like this:

    fn some_function<T, U>(t: T, u: U) -> i32
        where T: Display + Clone,
              U: Clone + Debug
    {

This function’s signature is less cluttered: the function name, parameter list, and return type are close together, similar to a function without lots of trait bounds.

### Returning Types that Implement Traits

We can also use the `impl Trait` syntax in the return position to return a value of some type that implements a trait, as shown here:

    fn returns_summarizable() -> impl Summary {
        Tweet {
            username: String::from("horse_ebooks"),
            content: String::from("of course, as you probably already know, people"),
            reply: false,
            retweet: false,
        }
    }

By using `impl Summary` for the return type, we specify that the `returns_summarizable` function returns some type that implements the `Summary` trait without naming the concrete type. In this case, `returns_summarizable` returns a `Tweet`, but the code calling this function doesn’t know that.

The ability to return a type that is only specified by the trait it implements is especially useful in the context of closures and iterators, which we cover in Chapter 13. Closures and iterators create types that only the compiler knows or types that are very long to specify. The `impl Trait` syntax lets you concisely specify that a function returns some type that implements the `Iterator` trait without needing to write out a very long type.

However, you can only use `impl Trait` if you’re returning a single type. For example, this code that returns either a `NewsArticle` or a `Tweet` with the return type specified as `impl Summary` wouldn’t work:

    fn returns_summarizable(switch: bool) -> impl Summary {
        if switch {
            NewsArticle {
                headline: String::from("Penguins win the Stanley Cup Championship!"),
                location: String::from("Pittsburgh, PA, USA"),
                author: String::from("Iceburgh"),
                content: String::from("The Pittsburgh Penguins once again are the best
                hockey team in the NHL."),
            }
        } else {
            Tweet {
                username: String::from("horse_ebooks"),
                content: String::from("of course, as you probably already know, people"),
                reply: false,
                retweet: false,
            }
        }
    }

Returning either a `NewsArticle` or a `Tweet` isn’t allowed due to restrictions around how the `impl Trait` syntax is implemented in the compiler. We’ll cover how to write a function with this behavior in the “Using Trait Objects That Allow for Values of Different Types” section of Chapter 17.

### Fixing the `largest` Function with Trait Bounds

Now that you know how to specify the behavior you want to use using the generic type parameter’s bounds, let’s return to Listing 10-5 to fix the definition of the `largest` function that uses a generic type parameter! Last time we tried to run that code, we received this error:

    error[E0369]: binary operation `>` cannot be applied to type `T`
     --> src/main.rs:5:12
      |
    5 |         if item > largest {
      |            ^^^^^^^^^^^^^^
      |
      = note: an implementation of `std::cmp::PartialOrd` might be missing for `T`

In the body of `largest` we wanted to compare two values of type `T` using the greater than (`>`) operator. Because that operator is defined as a default method on the standard library trait `std::cmp::PartialOrd`, we need to specify `PartialOrd` in the trait bounds for `T` so the `largest` function can work on slices of any type that we can compare. We don’t need to bring `PartialOrd` into scope because it’s in the prelude. Change the signature of `largest` to look like this:

    fn largest<T: PartialOrd>(list: &[T]) -> T {

This time when we compile the code, we get a different set of errors:

    error[E0508]: cannot move out of type `[T]`, a non-copy slice
     --> src/main.rs:2:23
      |
    2 |     let mut largest = list[0];
      |                       ^^^^^^^
      |                       |
      |                       cannot move out of here
      |                       help: consider using a reference instead: `&list[0]`

    error[E0507]: cannot move out of borrowed content
     --> src/main.rs:4:9
      |
    4 |     for &item in list.iter() {
      |         ^----
      |         ||
      |         |hint: to prevent move, use `ref item` or `ref mut item`
      |         cannot move out of borrowed content

The key line in this error is `cannot move out of type [T], a non-copy slice`. With our non-generic versions of the `largest` function, we were only trying to find the largest `i32` or `char`. As discussed in the “Stack-Only Data: Copy” section in Chapter 4, types like `i32` and `char` that have a known size can be stored on the stack, so they implement the `Copy` trait. But when we made the `largest` function generic, it became possible for the `list` parameter to have types in it that don’t implement the `Copy` trait. Consequently, we wouldn’t be able to move the value out of `list[0]` and into the `largest` variable, resulting in this error.

To call this code with only those types that implement the `Copy` trait, we can add `Copy` to the trait bounds of `T`! Listing 10-15 shows the complete code of a generic `largest` function that will compile as long as the types of the values in the slice that we pass into the function implement the `PartialOrd` and `Copy` traits, like `i32` and `char` do.

Filename: src/main.rs

    fn largest<T: PartialOrd + Copy>(list: &[T]) -> T {
        let mut largest = list[0];

        for &item in list.iter() {
            if item > largest {
                largest = item;
            }
        }

        largest
    }

    fn main() {
        let number_list = vec![34, 50, 25, 100, 65];

        let result = largest(&number_list);
        println!("The largest number is {}", result);

        let char_list = vec!['y', 'm', 'a', 'q'];

        let result = largest(&char_list);
        println!("The largest char is {}", result);
    }

Listing 10-15: A working definition of the `largest` function that works on any generic type that implements the `PartialOrd` and `Copy` traits

If we don’t want to restrict the `largest` function to the types that implement the `Copy` trait, we could specify that `T` has the trait bound `Clone` instead of `Copy`. Then we could clone each value in the slice when we want the `largest` function to have ownership. Using the `clone` function means we’re potentially making more heap allocations in the case of types that own heap data like `String`, and heap allocations can be slow if we’re working with large amounts of data.

Another way we could implement `largest` is for the function to return a reference to a `T` value in the slice. If we change the return type to `&T` instead of `T`, thereby changing the body of the function to return a reference, we wouldn’t need the `Clone` or `Copy` trait bounds and we could avoid heap allocations. Try implementing these alternate solutions on your own!

### Using Trait Bounds to Conditionally Implement Methods

By using a trait bound with an `impl` block that uses generic type parameters, we can implement methods conditionally for types that implement the specified traits. For example, the type `Pair<T>` in Listing 10-16 always implements the `new` function. But `Pair<T>` only implements the `cmp_display` method if its inner type `T` implements the `PartialOrd` trait that enables comparison and the `Display` trait that enables printing.

    # #![allow(unused_variables)]
    #fn main() {
    use std::fmt::Display;

    struct Pair<T> {
        x: T,
        y: T,
    }

    impl<T> Pair<T> {
        fn new(x: T, y: T) -> Self {
            Self {
                x,
                y,
            }
        }
    }

    impl<T: Display + PartialOrd> Pair<T> {
        fn cmp_display(&self) {
            if self.x >= self.y {
                println!("The largest member is x = {}", self.x);
            } else {
                println!("The largest member is y = {}", self.y);
            }
        }
    }
    #}

Listing 10-16: Conditionally implement methods on a generic type depending on trait bounds

We can also conditionally implement a trait for any type that implements another trait. Implementations of a trait on any type that satisfies the trait bounds are called blanket implementations and are extensively used in the Rust standard library. For example, the standard library implements the `ToString` trait on any type that implements the `Display` trait. The `impl` block in the standard library looks similar to this code:

    impl<T: Display> ToString for T {
        // --snip--
    }

Because the standard library has this blanket implementation, we can call the `to_string` method defined by the `ToString` trait on any type that implements the `Display` trait. For example, we can turn integers into their corresponding `String` values like this because integers implement `Display`:

    # #![allow(unused_variables)]
    #fn main() {
    let s = 3.to_string();
    #}

Blanket implementations appear in the documentation for the trait in the “Implementors” section.

Traits and trait bounds let us write code that uses generic type parameters to reduce duplication but also specify to the compiler that we want the generic type to have particular behavior. The compiler can then use the trait bound information to check that all the concrete types used with our code provide the correct behavior. In dynamically typed languages, we would get an error at runtime if we called a method on a type that the type didn’t implement. But Rust moves these errors to compile time so we’re forced to fix the problems before our code is even able to run. Additionally, we don’t have to write code that checks for behavior at runtime because we’ve already checked at compile time. Doing so improves performance without having to give up the flexibility of generics.

Another kind of generic that we’ve already been using is called lifetimes. Rather than ensuring that a type has the behavior we want, lifetimes ensure that references are valid as long as we need them to be. Let’s look at how lifetimes do that.