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<!-- Old headings. Do not remove or links may break. -->
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<p><a id="using-trait-objects-that-allow-for-values-of-different-types"></a></p>
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<h2 id="using-trait-objects-to-abstract-over-shared-behavior"><a class="header" href="#using-trait-objects-to-abstract-over-shared-behavior">Using Trait Objects to Abstract over Shared Behavior</a></h2>
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<p>In Chapter 8, we mentioned that one limitation of vectors is that they can
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store elements of only one type. We created a workaround in Listing 8-9 where
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we defined a <code>SpreadsheetCell</code> enum that had variants to hold integers, floats,
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and text. This meant we could store different types of data in each cell and
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still have a vector that represented a row of cells. This is a perfectly good
|
||
solution when our interchangeable items are a fixed set of types that we know
|
||
when our code is compiled.</p>
|
||
<p>However, sometimes we want our library user to be able to extend the set of
|
||
types that are valid in a particular situation. To show how we might achieve
|
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this, we’ll create an example graphical user interface (GUI) tool that iterates
|
||
through a list of items, calling a <code>draw</code> method on each one to draw it to the
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screen—a common technique for GUI tools. We’ll create a library crate called
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||
<code>gui</code> that contains the structure of a GUI library. This crate might include
|
||
some types for people to use, such as <code>Button</code> or <code>TextField</code>. In addition,
|
||
<code>gui</code> users will want to create their own types that can be drawn: For
|
||
instance, one programmer might add an <code>Image</code>, and another might add a
|
||
<code>SelectBox</code>.</p>
|
||
<p>At the time of writing the library, we can’t know and define all the types
|
||
other programmers might want to create. But we do know that <code>gui</code> needs to keep
|
||
track of many values of different types, and it needs to call a <code>draw</code> method
|
||
on each of these differently typed values. It doesn’t need to know exactly what
|
||
will happen when we call the <code>draw</code> method, just that the value will have that
|
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method available for us to call.</p>
|
||
<p>To do this in a language with inheritance, we might define a class named
|
||
<code>Component</code> that has a method named <code>draw</code> on it. The other classes, such as
|
||
<code>Button</code>, <code>Image</code>, and <code>SelectBox</code>, would inherit from <code>Component</code> and thus
|
||
inherit the <code>draw</code> method. They could each override the <code>draw</code> method to define
|
||
their custom behavior, but the framework could treat all of the types as if
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||
they were <code>Component</code> instances and call <code>draw</code> on them. But because Rust
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||
doesn’t have inheritance, we need another way to structure the <code>gui</code> library to
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||
allow users to create new types compatible with the library.</p>
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||
<h3 id="defining-a-trait-for-common-behavior"><a class="header" href="#defining-a-trait-for-common-behavior">Defining a Trait for Common Behavior</a></h3>
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<p>To implement the behavior that we want <code>gui</code> to have, we’ll define a trait
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named <code>Draw</code> that will have one method named <code>draw</code>. Then, we can define a
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vector that takes a trait object. A <em>trait object</em> points to both an instance
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||
of a type implementing our specified trait and a table used to look up trait
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||
methods on that type at runtime. We create a trait object by specifying some
|
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sort of pointer, such as a reference or a <code>Box<T></code> smart pointer, then the
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<code>dyn</code> keyword, and then specifying the relevant trait. (We’ll talk about the
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||
reason trait objects must use a pointer in <a href="ch20-03-advanced-types.html#dynamically-sized-types-and-the-sized-trait">“Dynamically Sized Types and the
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<code>Sized</code> Trait”</a><!-- ignore --> in Chapter 20.) We can use
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trait objects in place of a generic or concrete type. Wherever we use a trait
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object, Rust’s type system will ensure at compile time that any value used in
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that context will implement the trait object’s trait. Consequently, we don’t
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need to know all the possible types at compile time.</p>
|
||
<p>We’ve mentioned that, in Rust, we refrain from calling structs and enums
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||
“objects” to distinguish them from other languages’ objects. In a struct or
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enum, the data in the struct fields and the behavior in <code>impl</code> blocks are
|
||
separated, whereas in other languages, the data and behavior combined into one
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||
concept is often labeled an object. Trait objects differ from objects in other
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||
languages in that we can’t add data to a trait object. Trait objects aren’t as
|
||
generally useful as objects in other languages: Their specific purpose is to
|
||
allow abstraction across common behavior.</p>
|
||
<p>Listing 18-3 shows how to define a trait named <code>Draw</code> with one method named
|
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<code>draw</code>.</p>
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||
<figure class="listing" id="listing-18-3">
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||
<span class="file-name">Filename: src/lib.rs</span>
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||
<pre><code class="language-rust noplayground">pub trait Draw {
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||
fn draw(&self);
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}</code></pre>
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||
<figcaption><a href="#listing-18-3">Listing 18-3</a>: Definition of the <code>Draw</code> trait</figcaption>
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||
</figure>
|
||
<p>This syntax should look familiar from our discussions on how to define traits
|
||
in Chapter 10. Next comes some new syntax: Listing 18-4 defines a struct named
|
||
<code>Screen</code> that holds a vector named <code>components</code>. This vector is of type
|
||
<code>Box<dyn Draw></code>, which is a trait object; it’s a stand-in for any type inside a
|
||
<code>Box</code> that implements the <code>Draw</code> trait.</p>
|
||
<figure class="listing" id="listing-18-4">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground"><span class="boring">pub trait Draw {
|
||
</span><span class="boring"> fn draw(&self);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>pub struct Screen {
|
||
pub components: Vec<Box<dyn Draw>>,
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-18-4">Listing 18-4</a>: Definition of the <code>Screen</code> struct with a <code>components</code> field holding a vector of trait objects that implement the <code>Draw</code> trait</figcaption>
|
||
</figure>
|
||
<p>On the <code>Screen</code> struct, we’ll define a method named <code>run</code> that will call the
|
||
<code>draw</code> method on each of its <code>components</code>, as shown in Listing 18-5.</p>
|
||
<figure class="listing" id="listing-18-5">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground"><span class="boring">pub trait Draw {
|
||
</span><span class="boring"> fn draw(&self);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">pub struct Screen {
|
||
</span><span class="boring"> pub components: Vec<Box<dyn Draw>>,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>impl Screen {
|
||
pub fn run(&self) {
|
||
for component in self.components.iter() {
|
||
component.draw();
|
||
}
|
||
}
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-18-5">Listing 18-5</a>: A <code>run</code> method on <code>Screen</code> that calls the <code>draw</code> method on each component</figcaption>
|
||
</figure>
|
||
<p>This works differently from defining a struct that uses a generic type
|
||
parameter with trait bounds. A generic type parameter can be substituted with
|
||
only one concrete type at a time, whereas trait objects allow for multiple
|
||
concrete types to fill in for the trait object at runtime. For example, we
|
||
could have defined the <code>Screen</code> struct using a generic type and a trait bound,
|
||
as in Listing 18-6.</p>
|
||
<figure class="listing" id="listing-18-6">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground"><span class="boring">pub trait Draw {
|
||
</span><span class="boring"> fn draw(&self);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>pub struct Screen<T: Draw> {
|
||
pub components: Vec<T>,
|
||
}
|
||
|
||
impl<T> Screen<T>
|
||
where
|
||
T: Draw,
|
||
{
|
||
pub fn run(&self) {
|
||
for component in self.components.iter() {
|
||
component.draw();
|
||
}
|
||
}
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-18-6">Listing 18-6</a>: An alternate implementation of the <code>Screen</code> struct and its <code>run</code> method using generics and trait bounds</figcaption>
|
||
</figure>
|
||
<p>This restricts us to a <code>Screen</code> instance that has a list of components all of
|
||
type <code>Button</code> or all of type <code>TextField</code>. If you’ll only ever have homogeneous
|
||
collections, using generics and trait bounds is preferable because the
|
||
definitions will be monomorphized at compile time to use the concrete types.</p>
|
||
<p>On the other hand, with the method using trait objects, one <code>Screen</code> instance
|
||
can hold a <code>Vec<T></code> that contains a <code>Box<Button></code> as well as a
|
||
<code>Box<TextField></code>. Let’s look at how this works, and then we’ll talk about the
|
||
runtime performance implications.</p>
|
||
<h3 id="implementing-the-trait"><a class="header" href="#implementing-the-trait">Implementing the Trait</a></h3>
|
||
<p>Now we’ll add some types that implement the <code>Draw</code> trait. We’ll provide the
|
||
<code>Button</code> type. Again, actually implementing a GUI library is beyond the scope
|
||
of this book, so the <code>draw</code> method won’t have any useful implementation in its
|
||
body. To imagine what the implementation might look like, a <code>Button</code> struct
|
||
might have fields for <code>width</code>, <code>height</code>, and <code>label</code>, as shown in Listing 18-7.</p>
|
||
<figure class="listing" id="listing-18-7">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground"><span class="boring">pub trait Draw {
|
||
</span><span class="boring"> fn draw(&self);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">pub struct Screen {
|
||
</span><span class="boring"> pub components: Vec<Box<dyn Draw>>,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl Screen {
|
||
</span><span class="boring"> pub fn run(&self) {
|
||
</span><span class="boring"> for component in self.components.iter() {
|
||
</span><span class="boring"> component.draw();
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>pub struct Button {
|
||
pub width: u32,
|
||
pub height: u32,
|
||
pub label: String,
|
||
}
|
||
|
||
impl Draw for Button {
|
||
fn draw(&self) {
|
||
// code to actually draw a button
|
||
}
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-18-7">Listing 18-7</a>: A <code>Button</code> struct that implements the <code>Draw</code> trait</figcaption>
|
||
</figure>
|
||
<p>The <code>width</code>, <code>height</code>, and <code>label</code> fields on <code>Button</code> will differ from the
|
||
fields on other components; for example, a <code>TextField</code> type might have those
|
||
same fields plus a <code>placeholder</code> field. Each of the types we want to draw on
|
||
the screen will implement the <code>Draw</code> trait but will use different code in the
|
||
<code>draw</code> method to define how to draw that particular type, as <code>Button</code> has here
|
||
(without the actual GUI code, as mentioned). The <code>Button</code> type, for instance,
|
||
might have an additional <code>impl</code> block containing methods related to what
|
||
happens when a user clicks the button. These kinds of methods won’t apply to
|
||
types like <code>TextField</code>.</p>
|
||
<p>If someone using our library decides to implement a <code>SelectBox</code> struct that has
|
||
<code>width</code>, <code>height</code>, and <code>options</code> fields, they would implement the <code>Draw</code> trait
|
||
on the <code>SelectBox</code> type as well, as shown in Listing 18-8.</p>
|
||
<figure class="listing" id="listing-18-8">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore">use gui::Draw;
|
||
|
||
struct SelectBox {
|
||
width: u32,
|
||
height: u32,
|
||
options: Vec<String>,
|
||
}
|
||
|
||
impl Draw for SelectBox {
|
||
fn draw(&self) {
|
||
// code to actually draw a select box
|
||
}
|
||
}
|
||
<span class="boring">
|
||
</span><span class="boring">fn main() {}</span></code></pre>
|
||
<figcaption><a href="#listing-18-8">Listing 18-8</a>: Another crate using <code>gui</code> and implementing the <code>Draw</code> trait on a <code>SelectBox</code> struct</figcaption>
|
||
</figure>
|
||
<p>Our library’s user can now write their <code>main</code> function to create a <code>Screen</code>
|
||
instance. To the <code>Screen</code> instance, they can add a <code>SelectBox</code> and a <code>Button</code>
|
||
by putting each in a <code>Box<T></code> to become a trait object. They can then call the
|
||
<code>run</code> method on the <code>Screen</code> instance, which will call <code>draw</code> on each of the
|
||
components. Listing 18-9 shows this implementation.</p>
|
||
<figure class="listing" id="listing-18-9">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore"><span class="boring">use gui::Draw;
|
||
</span><span class="boring">
|
||
</span><span class="boring">struct SelectBox {
|
||
</span><span class="boring"> width: u32,
|
||
</span><span class="boring"> height: u32,
|
||
</span><span class="boring"> options: Vec<String>,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl Draw for SelectBox {
|
||
</span><span class="boring"> fn draw(&self) {
|
||
</span><span class="boring"> // code to actually draw a select box
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>use gui::{Button, Screen};
|
||
|
||
fn main() {
|
||
let screen = Screen {
|
||
components: vec![
|
||
Box::new(SelectBox {
|
||
width: 75,
|
||
height: 10,
|
||
options: vec![
|
||
String::from("Yes"),
|
||
String::from("Maybe"),
|
||
String::from("No"),
|
||
],
|
||
}),
|
||
Box::new(Button {
|
||
width: 50,
|
||
height: 10,
|
||
label: String::from("OK"),
|
||
}),
|
||
],
|
||
};
|
||
|
||
screen.run();
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-18-9">Listing 18-9</a>: Using trait objects to store values of different types that implement the same trait</figcaption>
|
||
</figure>
|
||
<p>When we wrote the library, we didn’t know that someone might add the
|
||
<code>SelectBox</code> type, but our <code>Screen</code> implementation was able to operate on the
|
||
new type and draw it because <code>SelectBox</code> implements the <code>Draw</code> trait, which
|
||
means it implements the <code>draw</code> method.</p>
|
||
<p>This concept—of being concerned only with the messages a value responds to
|
||
rather than the value’s concrete type—is similar to the concept of <em>duck
|
||
typing</em> in dynamically typed languages: If it walks like a duck and quacks like
|
||
a duck, then it must be a duck! In the implementation of <code>run</code> on <code>Screen</code> in
|
||
Listing 18-5, <code>run</code> doesn’t need to know what the concrete type of each
|
||
component is. It doesn’t check whether a component is an instance of a <code>Button</code>
|
||
or a <code>SelectBox</code>, it just calls the <code>draw</code> method on the component. By
|
||
specifying <code>Box<dyn Draw></code> as the type of the values in the <code>components</code>
|
||
vector, we’ve defined <code>Screen</code> to need values that we can call the <code>draw</code>
|
||
method on.</p>
|
||
<p>The advantage of using trait objects and Rust’s type system to write code
|
||
similar to code using duck typing is that we never have to check whether a
|
||
value implements a particular method at runtime or worry about getting errors
|
||
if a value doesn’t implement a method but we call it anyway. Rust won’t compile
|
||
our code if the values don’t implement the traits that the trait objects need.</p>
|
||
<p>For example, Listing 18-10 shows what happens if we try to create a <code>Screen</code>
|
||
with a <code>String</code> as a component.</p>
|
||
<figure class="listing" id="listing-18-10">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore does_not_compile">use gui::Screen;
|
||
|
||
fn main() {
|
||
let screen = Screen {
|
||
components: vec![Box::new(String::from("Hi"))],
|
||
};
|
||
|
||
screen.run();
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-18-10">Listing 18-10</a>: Attempting to use a type that doesn’t implement the trait object’s trait</figcaption>
|
||
</figure>
|
||
<p>We’ll get this error because <code>String</code> doesn’t implement the <code>Draw</code> trait:</p>
|
||
<pre><code class="language-console">$ cargo run
|
||
Compiling gui v0.1.0 (file:///projects/gui)
|
||
error[E0277]: the trait bound `String: Draw` is not satisfied
|
||
--> src/main.rs:5:26
|
||
|
|
||
5 | components: vec![Box::new(String::from("Hi"))],
|
||
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Draw` is not implemented for `String`
|
||
|
|
||
= help: the trait `Draw` is implemented for `Button`
|
||
= note: required for the cast from `Box<String>` to `Box<dyn Draw>`
|
||
|
||
For more information about this error, try `rustc --explain E0277`.
|
||
error: could not compile `gui` (bin "gui") due to 1 previous error
|
||
</code></pre>
|
||
<p>This error lets us know that either we’re passing something to <code>Screen</code> that we
|
||
didn’t mean to pass and so should pass a different type, or we should implement
|
||
<code>Draw</code> on <code>String</code> so that <code>Screen</code> is able to call <code>draw</code> on it.</p>
|
||
<!-- Old headings. Do not remove or links may break. -->
|
||
<p><a id="trait-objects-perform-dynamic-dispatch"></a></p>
|
||
<h3 id="performing-dynamic-dispatch"><a class="header" href="#performing-dynamic-dispatch">Performing Dynamic Dispatch</a></h3>
|
||
<p>Recall in <a href="ch10-01-syntax.html#performance-of-code-using-generics">“Performance of Code Using
|
||
Generics”</a><!-- ignore --> in Chapter 10 our
|
||
discussion on the monomorphization process performed on generics by the
|
||
compiler: The compiler generates nongeneric implementations of functions and
|
||
methods for each concrete type that we use in place of a generic type
|
||
parameter. The code that results from monomorphization is doing <em>static
|
||
dispatch</em>, which is when the compiler knows what method you’re calling at
|
||
compile time. This is opposed to <em>dynamic dispatch</em>, which is when the compiler
|
||
can’t tell at compile time which method you’re calling. In dynamic dispatch
|
||
cases, the compiler emits code that at runtime will know which method to call.</p>
|
||
<p>When we use trait objects, Rust must use dynamic dispatch. The compiler doesn’t
|
||
know all the types that might be used with the code that’s using trait objects,
|
||
so it doesn’t know which method implemented on which type to call. Instead, at
|
||
runtime, Rust uses the pointers inside the trait object to know which method to
|
||
call. This lookup incurs a runtime cost that doesn’t occur with static dispatch.
|
||
Dynamic dispatch also prevents the compiler from choosing to inline a method’s
|
||
code, which in turn prevents some optimizations, and Rust has some rules about
|
||
where you can and cannot use dynamic dispatch, called <em>dyn compatibility</em>. Those
|
||
rules are beyond the scope of this discussion, but you can read more about them
|
||
<a href="https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility">in the reference</a><!-- ignore -->. However, we did get extra
|
||
flexibility in the code that we wrote in Listing 18-5 and were able to support
|
||
in Listing 18-9, so it’s a trade-off to consider.</p>
|
||
|
||
</main>
|
||
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