Using Trait Objects That Allow for Values of Different Types
In Chapter 8, we mentioned that one limitation of vectors is that they can
store elements of only one type. We created a workaround in Listing 8-9 where
we defined a SpreadsheetCell enum that had variants to hold integers, floats,
and text. This meant we could store different types of data in each cell and
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.
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
this, we’ll create an example graphical user interface (GUI) tool that iterates
through a list of items, calling a draw method on each one to draw it to the
screen—a common technique for GUI tools. We’ll create a library crate called
gui that contains the structure of a GUI library. This crate might include
some types for people to use, such as Button or TextField. In addition,
gui users will want to create their own types that can be drawn: for
instance, one programmer might add an Image and another might add a
SelectBox.
We won’t implement a fully fledged GUI library for this example but will show
how the pieces would fit together. 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 gui needs to keep track of many values of different types, and it
needs to call a draw method on each of these differently typed values. It
doesn’t need to know exactly what will happen when we call the draw method,
just that the value will have that method available for us to call.
To do this in a language with inheritance, we might define a class named
Component that has a method named draw on it. The other classes, such as
Button, Image, and SelectBox, would inherit from Component and thus
inherit the draw method. They could each override the draw method to define
their custom behavior, but the framework could treat all of the types as if
they were Component instances and call draw on them. But because Rust
doesn’t have inheritance, we need another way to structure the gui library to
allow users to extend it with new types.
Defining a Trait for Common Behavior
To implement the behavior we want gui to have, we’ll define a trait named
Draw that will have one method named draw. Then we can define a vector that
takes a trait object. A trait object points to both an instance of a type
implementing our specified trait and a table used to look up trait methods on
that type at runtime. We create a trait object by specifying some sort of
pointer, such as a & reference or a Box<T> smart pointer, then the dyn
keyword, and then specifying the relevant trait. (We’ll talk about the reason
trait objects must use a pointer in Chapter 20 in the section “Dynamically
Sized Types and the Sized Trait.”) We can
use trait objects in place of a generic or concrete type. Wherever we use a
trait object, Rust’s type system will ensure at compile time that any value
used in that context will implement the trait object’s trait. Consequently, we
don’t need to know all the possible types at compile time.
We’ve mentioned that, in Rust, we refrain from calling structs and enums
“objects” to distinguish them from other languages’ objects. In a struct or
enum, the data in the struct fields and the behavior in impl blocks are
separated, whereas in other languages, the data and behavior combined into one
concept is often labeled an object. However, trait objects are more like
objects in other languages in the sense that they combine data and behavior.
But trait objects differ from traditional objects 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.
Listing 18-3 shows how to define a trait named Draw with one method named
draw:
pub trait Draw {
fn draw(&self);
}
Draw traitThis 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
Screen that holds a vector named components. This vector is of type
Box<dyn Draw>, which is a trait object; it’s a stand-in for any type inside
a Box that implements the Draw trait.
pub trait Draw {
fn draw(&self);
}
pub struct Screen {
pub components: Vec<Box<dyn Draw>>,
}
Screen struct with a components field holding a vector of trait objects that implement the Draw traitOn the Screen struct, we’ll define a method named run that will call the
draw method on each of its components, as shown in Listing 18-5:
pub trait Draw {
fn draw(&self);
}
pub struct Screen {
pub components: Vec<Box<dyn Draw>>,
}
impl Screen {
pub fn run(&self) {
for component in self.components.iter() {
component.draw();
}
}
}
run method on Screen that calls the draw method on each componentThis works differently from defining a struct that uses a generic type
parameter with trait bounds. A generic type parameter can only be substituted
with 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 Screen struct using a generic type and a trait bound
as in Listing 18-6:
pub trait Draw {
fn draw(&self);
}
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();
}
}
}
Screen struct and its run method using generics and trait boundsThis restricts us to a Screen instance that has a list of components all of
type Button or all of type TextField. 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.
On the other hand, with the method using trait objects, one Screen instance
can hold a Vec<T> that contains a Box<Button> as well as a
Box<TextField>. Let’s look at how this works, and then we’ll talk about the
runtime performance implications.
Implementing the Trait
Now we’ll add some types that implement the Draw trait. We’ll provide the
Button type. Again, actually implementing a GUI library is beyond the scope
of this book, so the draw method won’t have any useful implementation in its
body. To imagine what the implementation might look like, a Button struct
might have fields for width, height, and label, as shown in Listing 18-7:
pub trait Draw {
fn draw(&self);
}
pub struct Screen {
pub components: Vec<Box<dyn Draw>>,
}
impl Screen {
pub fn run(&self) {
for component in self.components.iter() {
component.draw();
}
}
}
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
}
}
Button struct that implements the Draw traitThe width, height, and label fields on Button will differ from the
fields on other components; for example, a TextField type might have those
same fields plus a placeholder field. Each of the types we want to draw on
the screen will implement the Draw trait but will use different code in the
draw method to define how to draw that particular type, as Button has here
(without the actual GUI code, as mentioned). The Button type, for instance,
might have an additional impl block containing methods related to what
happens when a user clicks the button. These kinds of methods won’t apply to
types like TextField.
If someone using our library decides to implement a SelectBox struct that has
width, height, and options fields, they implement the Draw trait on the
SelectBox type as well, as shown in Listing 18-8:
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
}
}
fn main() {}
gui and implementing the Draw trait on a SelectBox structOur library’s user can now write their main function to create a Screen
instance. To the Screen instance, they can add a SelectBox and a Button
by putting each in a Box<T> to become a trait object. They can then call the
run method on the Screen instance, which will call draw on each of the
components. Listing 18-9 shows this implementation:
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
}
}
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();
}
When we wrote the library, we didn’t know that someone might add the
SelectBox type, but our Screen implementation was able to operate on the
new type and draw it because SelectBox implements the Draw trait, which
means it implements the draw method.
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 duck
typing 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 run on Screen
in Listing 18-5, run 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 Button
or a SelectBox, it just calls the draw method on the component. By
specifying Box<dyn Draw> as the type of the values in the components
vector, we’ve defined Screen to need values that we can call the draw
method on.
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.
For example, Listing 18-10 shows what happens if we try to create a Screen
with a String as a component:
use gui::Screen;
fn main() {
let screen = Screen {
components: vec![Box::new(String::from("Hi"))],
};
screen.run();
}
We’ll get this error because String doesn’t implement the Draw trait:
$ 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
This error lets us know that either we’re passing something to Screen we
didn’t mean to pass and so should pass a different type or we should implement
Draw on String so that Screen is able to call draw on it.
Trait Objects Perform Dynamic Dispatch
Recall in the “Performance of Code Using Generics” section 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 static dispatch, which is when the compiler knows what method you’re calling at compile time. This is opposed to dynamic dispatch, 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 figure out which method to call.
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 dyn compatibility. 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.