Traits: Defining Shared Behavior

Traits allow us to use another kind of abstraction: they let us abstract over behavior that types can have in common. A trait tells the Rust compiler about functionality a particular type has and might share with other types. In situations where we use generic type parameters, we can use trait bounds to specify, at compile time, that the generic type may be any type that implements a trait and therefore has the behavior we want to use in that situation.

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

Defining a Trait

The behavior of a type 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 in order to define a set of behaviors necessary to accomplish some purpose.

For example, 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 place in the world, and a Tweet that can have at most 140 characters in its content along with metadata like whether it was 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. The behavior we need each struct to have is that it's able to be summarized, and that we can ask for that summary by calling a summary method on an instance. Listing 10-11 shows the definition of a Summarizable trait that expresses this concept:

Filename: lib.rs

pub trait Summarizable {
    fn summary(&self) -> String;
}

Listing 10-11: Definition of a Summarizable trait that consists of the behavior provided by a summary method

We declare a trait with the trait keyword, then the trait's name, in this case Summarizable. Inside curly braces we declare the method signatures that describe the behaviors that types that implement this trait will need to have, in this case fn summary(&self) -> String. After the method signature, instead of providing an implementation within curly braces, we put a semicolon. Each type that implements this trait must then provide its own custom behavior for the body of the method, but the compiler will enforce that any type that has the Summarizable trait will have the method summary defined for it with this signature exactly.

A trait can have multiple methods in its body, with the method signatures listed one per line and each line ending in a semicolon.

Implementing a Trait on a Type

Now that we've defined the Summarizable trait, we can implement it on the types in our media aggregator that we want to have this behavior. Listing 10-12 shows an implementation of the Summarizable trait on the NewsArticle struct that uses the headline, the author, and the location to create the return value of summary. For the Tweet struct, we've chosen to define summary as the username followed by the whole text of the tweet, assuming that tweet content is already limited to 140 characters.

Filename: lib.rs

# pub trait Summarizable {
#     fn summary(&self) -> String;
# }
#
pub struct NewsArticle {
    pub headline: String,
    pub location: String,
    pub author: String,
    pub content: String,
}

impl Summarizable for NewsArticle {
    fn summary(&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 Summarizable for Tweet {
    fn summary(&self) -> String {
        format!("{}: {}", self.username, self.content)
    }
}

Listing 10-12: Implementing the Summarizable trait on the NewsArticle and Tweet types

Implementing a trait on a type is similar to implementing methods that aren't related to a trait. The difference is after impl, we put the trait name that we want to implement, then say for and the name of the type that we want to implement the trait for. Within the impl block, we put the method signatures that the trait definition has defined, but instead of putting a semicolon after each signature, we put curly braces and fill in the method body with the specific behavior that we want the methods of the trait to have for the particular type.

Once we've implemented the trait, we can call the methods on instances of NewsArticle and Tweet in the same manner that we call methods that aren't part of a trait:

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.summary());

This will print 1 new tweet: horse_ebooks: of course, as you probably already know, people.

Note that because we've defined the Summarizable trait and the NewsArticle and Tweet types all in the same lib.rs in Listing 10-12, they're all in the same scope. If this lib.rs is for a crate we've called aggregator, and someone else wants to use our crate's functionality plus implement the Summarizable trait on their WeatherForecast struct, their code would need to import the Summarizable trait into their scope first before they could implement it, like in Listing 10-13:

Filename: lib.rs

extern crate aggregator;

use aggregator::Summarizable;

struct WeatherForecast {
    high_temp: f64,
    low_temp: f64,
    chance_of_precipitation: f64,
}

impl Summarizable for WeatherForecast {
    fn summary(&self) -> String {
        format!("The high will be {}, and the low will be {}. The chance of
        precipitation is {}%.", self.high_temp, self.low_temp,
        self.chance_of_precipitation)
    }
}

Listing 10-13: Bringing the Summarizable trait from our aggregator crate into scope in another crate

This code also assumes Summarizable is a public trait, which it is because we put the pub keyword before trait in Listing 10-11.

One restriction to note with trait implementations: we may implement a trait on a type as long as either the trait or the type are local to our crate. In other words, we aren't allowed to implement external traits on external types. We can't implement the Display trait on Vec, for example, since both Display and Vec are defined in the standard library. We are allowed to implement standard library traits like Display on a custom type like Tweet as part of our aggregator crate functionality. We could also implement Summarizable on Vec in our aggregator crate, since we've defined Summarizable there. This restriction is part of what's called the orphan rule, which you can look up if you're interested in type theory. Briefly, it's called the orphan rule because the parent type is not present. Without this rule, two crates could implement the same trait for the same type, and the two implementations would conflict: Rust wouldn't know which implementation to use. Because Rust enforces the orphan rule, other people's code can't break your code and vice versa.

Default Implementations

Sometimes it's useful to have default behavior for some or all of the methods in a trait, instead of making every implementation on every type define custom behavior. When we implement the trait on a particular type, we can choose to keep or override each method's default behavior.

Listing 10-14 shows how we could have chosen to specify a default string for the summary method of the Summarize trait instead of only choosing to only define the method signature like we did in Listing 10-11:

Filename: lib.rs

pub trait Summarizable {
    fn summary(&self) -> String {
        String::from("(Read more...)")
    }
}

Listing 10-14: Definition of a Summarizable trait with a default implementation of the summary method

If we wanted to use this default implementation to summarize instances of NewsArticle instead of defining a custom implementation like we did in Listing 10-12, we would specify an empty impl block:

impl Summarizable for NewsArticle {}

Even though we're no longer choosing to define the summary method on NewsArticle directly, since the summary method has a default implementation and we specified that NewsArticle implements the Summarizable trait, we can still call the summary method on an instance of NewsArticle:

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.summary());

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

Changing the Summarizable trait to have a default implementation for summary does not require us to change anything about the implementations of Summarizable on Tweet in Listing 10-12 or WeatherForecast in Listing 10-13: the syntax for overriding a default implementation is exactly the same as the syntax for implementing a trait method that doesn't have a default implementation.

Default implementations are allowed to call the 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 implementers to specify a small part of it. We could choose to have the Summarizable trait also have an author_summary method whose implementation is required, then a summary method that has a default implementation that calls the author_summary method:

pub trait Summarizable {
    fn author_summary(&self) -> String;

    fn summary(&self) -> String {
        format!("(Read more from {}...)", self.author_summary())
    }
}

In order to use this version of Summarizable, we're only required to define author_summary when we implement the trait on a type:

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

Once we define author_summary, we can call summary on instances of the Tweet struct, and the default implementation of summary will call the definition of author_summary that we've provided.

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.summary());

This will print 1 new tweet: (Read more from @horse_ebooks...).

Note that it is not possible to call the default implementation from an overriding implementation.

Trait Bounds

Now that we've defined traits and implemented those traits on types, we can use traits with generic type parameters. We can constrain generic types so that rather than being any type, the compiler will ensure that the type will be limited to those types that implement a particular trait and thus have the behavior that we need the types to have. This is called specifying trait bounds on a generic type.

For example, in Listing 10-12, we implemented the Summarizable trait on the types NewsArticle and Tweet. We can define a function notify that calls the summary method on its parameter item, which is of the generic type T. To be able to call summary on item without getting an error, we can use trait bounds on T to specify that item must be of a type that implements the Summarizable trait:

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

Trait bounds go with the declaration of the generic type parameter, after a colon and within the angle brackets. Because of the trait bound on T, we can call notify and pass in any instance of NewsArticle or Tweet. The external code from Listing 10-13 that's using our aggregator crate can call our notify function and pass in an instance of WeatherForecast, since Summarizable is implemented for WeatherForecast as well. Code that calls notify with any other type, like a String or an i32, won't compile, since those types do not implement Summarizable.

We can specify multiple trait bounds on a generic type by using +. If we needed to be able to use display formatting on the type T in a function as well as the summary method, we can use the trait bounds T: Summarizable + Display. This means T can be any type that implements both Summarizable and Display.

For functions that have multiple generic type parameters, each generic has its own trait bounds. Specifying lots of trait bound information in the angle brackets between a function's name and its parameter list can get hard to read, so there's an alternate syntax for specifying trait bounds that lets us move them to a where clause after the function signature. So instead of:

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

We can write this instead with a where clause:

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

This is less cluttered and makes this function's signature look more similar to a function without lots of trait bounds, in that the function name, parameter list, and return type are close together.

Fixing the largest Function with Trait Bounds

So any time you want to use behavior defined by a trait on a generic, you need to specify that trait in the generic type parameter's type bounds. We can now fix the definition of the largest function that uses a generic type parameter from Listing 10-5! When we set that code aside, we were getting this error:

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

In the body of largest we wanted to be able to compare two values of type T using the greater-than operator. That operator is defined as a default method on the standard library trait std::cmp::PartialOrd. So in order to be able to use the greater-than operator, we need to specify PartialOrd in the trait bounds for T so that the largest function will work on slices of any type that can be compared. We don't need to bring PartialOrd into scope because it's in the prelude.

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

If we try to compile this, we'll get different errors:

error[E0508]: cannot move out of type `[T]`, a non-copy array
 --> src/main.rs:4:23
  |
4 |     let mut largest = list[0];
  |         -----------   ^^^^^^^ cannot move out of here
  |         |
  |         hint: to prevent move, use `ref largest` or `ref mut largest`

error[E0507]: cannot move out of borrowed content
 --> src/main.rs:6:9
  |
6 |     for &item in list.iter() {
  |         ^----
  |         ||
  |         |hint: to prevent move, use `ref item` or `ref mut item`
  |         cannot move out of borrowed content

The key to this error is cannot move out of type [T], a non-copy array. With our non-generic versions of the largest function, we were only trying to find the largest i32 or char. As we discussed 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. When we changed the largest function to be generic, it's now possible that the list parameter could have types in it that don't implement the Copy trait, which means we wouldn't be able to move the value out of list[0] and into the largest variable.

If we only want to be able to call this code with types that are Copy, 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 largest implement both the PartialOrd and Copy traits, like i32 and char:

Filename: src/main.rs

use std::cmp::PartialOrd;

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 numbers = vec![34, 50, 25, 100, 65];

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

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

    let result = largest(&chars);
    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 our largest function to only types that implement the Copy trait, we could specify that T has the trait bound Clone instead of Copy and 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, though, 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 be &T instead of T and change the body of the function to return a reference, we wouldn't need either the Clone or Copy trait bounds and we wouldn't be doing any heap allocations. Try implementing these alternate solutions on your own!

Traits and trait bounds let us write code that uses generic type parameters in order to reduce duplication, but still specify to the compiler exactly what behavior our code needs the generic type to have. Because we've given the trait bound information to the compiler, it can check that all the concrete types used with our code provide the right behavior. In dynamically typed languages, if we tried to call a method on a type that the type didn't implement, we'd get an error at runtime. Rust moves these errors to compile time so that 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 since we've already checked at compile time, which improves performance compared to other languages without having to give up the flexibility of generics.

There's another kind of generics that we've been using without even realizing it called lifetimes. Rather than helping us ensure that a type has the behavior we need it to have, lifetimes help us ensure that references are valid as long as we need them to be. Let's learn how lifetimes do that.