## Traits: Defining Shared Behavior

A *trait* defines the 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 type 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 crate named `aggregator` 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’ll request that
summary by calling a `summarize` method on an instance. Listing 10-12 shows the
definition of a public `Summary` trait that expresses this behavior.

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-12/src/lib.rs}}
```

<span class="caption">Listing 10-12: A `Summary` trait that consists of the
behavior provided by a `summarize` method</span>

Here, we declare a trait using the `trait` keyword and then the trait’s name,
which is `Summary` in this case. We also declare the trait as `pub` so that
crates depending on this crate can make use of this trait too, as we’ll see in
a few examples. 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 signatures of the `Summary` trait’s methods,
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 the tweet content is
already limited to 280 characters.

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-13/src/lib.rs:here}}
```

<span class="caption">Listing 10-13: Implementing the `Summary` trait on the
`NewsArticle` and `Tweet` types</span>

Implementing a trait on a type is similar to implementing regular methods. The
difference is that after `impl`, we put the trait name 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.

Now that the library has implemented the `Summary` trait on `NewsArticle` and
`Tweet`, users of the crate can call the trait methods on instances of
`NewsArticle` and `Tweet` in the same way we call regular methods. The only
difference is that the user must bring the trait into scope as well as the
types. Here’s an example of how a binary crate could use our `aggregator`
library crate:

```rust,ignore
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/no-listing-01-calling-trait-method/src/main.rs}}
```

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

Other crates that depend on the `aggregator` crate can also bring the `Summary`
trait into scope to implement `Summary` on their own types. One restriction to
note is that we can implement a trait on a type only if either the trait or the
type, or both, are 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 both defined in the standard library and aren’t
local to our `aggregator` crate. This restriction is part of a property 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.

In Listing 10-14, we 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.

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-14/src/lib.rs:here}}
```

<span class="caption">Listing 10-14: Defining a `Summary` trait with a default
implementation of the `summarize` method</span>

To use a default implementation to summarize instances of `NewsArticle`, 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:

```rust,ignore
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/no-listing-02-calling-default-impl/src/main.rs:here}}
```

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

Creating a default implementation 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:

```rust,noplayground
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/no-listing-03-default-impl-calls-other-methods/src/lib.rs:here}}
```

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

```rust,ignore
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/no-listing-03-default-impl-calls-other-methods/src/lib.rs:impl}}
```

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. Here’s what
that looks like:

```rust,ignore
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/no-listing-03-default-impl-calls-other-methods/src/main.rs:here}}
```

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. We’ll use the
`Summary` trait we implemented on the `NewsArticle` and `Tweet` types in
Listing 10-13 to 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 use the `impl Trait` syntax, like this:

```rust,ignore
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/no-listing-04-traits-as-parameters/src/lib.rs:here}}
```

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`.

<!-- Old headings. Do not remove or links may break. -->
<a id="fixing-the-largest-function-with-trait-bounds"></a>

#### Trait Bound Syntax

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

```rust,ignore
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, while the fuller trait bound syntax can express more complexity in other
cases. For example, we can have two parameters that implement `Summary`. Doing
so with the `impl Trait` syntax looks like this:

```rust,ignore
pub fn notify(item1: &impl Summary, item2: &impl Summary) {
```

Using `impl Trait` is appropriate if we want this function to allow `item1` and
`item2` to have different types (as long as both types implement `Summary`). If
we want to force both parameters to have the same type, however, we must use a
trait bound, like this:

```rust,ignore
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 as well as `summarize` on `item`: we specify in the `notify`
definition that `item` must implement both `Display` and `Summary`. We can do
so using the `+` syntax:

```rust,ignore
pub fn notify(item: &(impl Summary + Display)) {
```

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

```rust,ignore
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:

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

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

```rust,ignore
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/no-listing-07-where-clause/src/lib.rs:here}}
```

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:

```rust,ignore
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/no-listing-05-returning-impl-trait/src/lib.rs:here}}
```

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 need to know that.

The ability to specify a return type only 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:

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/no-listing-06-impl-trait-returns-one-type/src/lib.rs:here}}
```

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”][using-trait-objects-that-allow-for-values-of-different-types]<!--
ignore --> section of Chapter 17.

### 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 10-15 always implements the
`new` function to return a new instance of `Pair<T>` (recall from the
[“Defining Methods”][methods]<!-- ignore --> section of Chapter 5 that `Self`
is a type alias for the type of the `impl` block, which in this case is
`Pair<T>`). But in the next `impl` block, `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.

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-15/src/lib.rs}}
```

<span class="caption">Listing 10-15: Conditionally implementing methods on a
generic type depending on trait bounds</span>

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 used extensively 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:

```rust,ignore
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`:

```rust
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 which didn’t define the method. 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.

[using-trait-objects-that-allow-for-values-of-different-types]: ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types
[methods]: ch05-03-method-syntax.html#defining-methods
