Using Trait Objects that Allow for Values of Different Types
In Chapter 8, we mentioned that one limitation of vectors is that they can only
store elements of one type. We created a workaround in Listing 8-10 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 gets compiled.
Sometimes, however, we want the user of our library 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 tool that
iterates through a list of items, calling a draw
method on each one to drawn
it to the screen; a common technique for GUI tools. We’re going to create a
library crate containing the structure of a GUI library called rust_gui
. This
crate might include some types for people to use, such as Button
or
TextField
. On top of these, users of rust_gui
will want to create their own
types that can be drawn on the screen: for instance, one programmer might add
an Image
, 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 will want to create. What we do
know is that rust_gui
needs to keep track of a bunch of values that are of
different types, and it needs to be able 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 like
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 Rust doesn’t have
inheritance, so we need another way.
Defining a Trait for Common Behavior
To implement the behavior we want rust_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 an instance of a
type that implements the trait we specify. We create a trait object by
specifying some sort of pointer, such as a &
reference or a Box<T>
smart
pointer, and then specifying the relevant trait (we’ll talk about the reason
trait objects have to use a pointer in Chapter 19 in the section on Dynamically
Sized Types). 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.
This way 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 is
separated, whereas in other languages the data and behavior combined into one
concept is often labeled an object. Trait objects, though, are more like
objects in other languages, in the sense that they combine both data and
behavior. However, 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 17-3 shows how to define a trait named Draw
with one method named
draw
:
Filename: src/lib.rs
# #![allow(unused_variables)] #fn main() { pub trait Draw { fn draw(&self); } #}
This should look familiar from our discussions on how to define traits in
Chapter 10. Next comes something new: Listing 17-4 defines a struct named
Screen
that holds a vector named components
. This vector is of type
Box<Draw>
, which is a trait object: it’s a stand-in for any type inside a
Box
that implements the Draw
trait.
Filename: src/lib.rs
# #![allow(unused_variables)] #fn main() { # pub trait Draw { # fn draw(&self); # } # pub struct Screen { pub components: Vec<Box<Draw>>, } #}
On 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 17-5:
Filename: src/lib.rs
# #![allow(unused_variables)] #fn main() { # pub trait Draw { # fn draw(&self); # } # # pub struct Screen { # pub components: Vec<Box<Draw>>, # } # impl Screen { pub fn run(&self) { for component in self.components.iter() { component.draw(); } } } #}
This works differently to 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, while 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 17-6:
Filename: src/lib.rs
# #![allow(unused_variables)] #fn main() { # 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(); } } } #}
This 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 since the
definitions will be monomorphized at compile time to use the concrete types.
With the method using trait objects, on the other hand, one Screen
instance can hold a Vec
that contains a Box<Button>
as well as a
Box<TextField>
. Let’s see how that works, and then talk about the runtime
performance implications.
Implementing the Trait
Now we’ll add some types that implement the Draw
trait. We’re going to
provide the Button
type. Again, actually implementing a GUI library is out of
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 17-7:
Filename: src/lib.rs
# #![allow(unused_variables)] #fn main() { # pub trait Draw { # fn draw(&self); # } # 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 } } #}
The width
, height
, and label
fields on Button
will differ from the
fields on other components, such as a TextField
type that might have those
plus a placeholder
field instead. Each of the types we want to draw on the
screen will implement the Draw
trait, with different code in the draw
method to define how to draw that particular type, like Button
has here
(without the actual GUI code that’s out of scope of this chapter). Button
,
for instance, might have an additional impl
block containing methods related
to what happens if the button is clicked. These kinds of methods won’t apply to
types like TextField
.
Someone using our library has decided 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 17-8:
Filename: src/main.rs
extern crate rust_gui;
use rust_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
}
}
The user of our library can now write their main
function to create a
Screen
instance. To this 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 17-9 shows this implementation:
Filename: src/main.rs
use rust_gui::{Screen, Button};
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 would add the
SelectBox
type someday, but our Screen
implementation was able to operate
on the new type and draw it because SelectBox
implements the Draw
type,
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 a concept in dynamically
typed languages called duck typing: 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 17-5, run
doesn’t need to know what the concrete type of each
component is. It doesn’t check to see if a component is an instance of a
Button
or a SelectBox
, it just calls the draw
method on the component. By
specifying Box<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 do something similar to duck typing is that we never have to check that 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 17-10 shows what happens if we try to create a Screen
with a String
as a component:
Filename: src/main.rs
extern crate rust_gui;
use rust_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 rust_gui::Draw
trait:
error[E0277]: the trait bound `std::string::String: rust_gui::Draw` is not satisfied
-->
|
4 | Box::new(String::from("Hi")),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `rust_gui::Draw` is not
implemented for `std::string::String`
|
= note: required for the cast to the object type `rust_gui::Draw`
This lets us know that either we’re passing something to Screen
we didn’t
mean to pass, and we should pass a different type, or implement Draw
on
String
so that Screen
is able to call draw
on it.
Trait Objects Perform Dynamic Dispatch
Recall from Chapter 10 our discussion on the monomorphization process performed by the compiler when we use trait bounds on generics: the compiler generates non-generic 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. Static dispatch is when the compiler knows what method you’re calling at compile time. This is opposed to dynamic dispatch, when the compiler can’t tell at compile time which method you’re calling. In these cases, the compiler emits code that will figure out at runtime which method to call.
When we use trait objects, Rust has to use dynamic dispatch. The compiler doesn’t know all the types that might be used with the code using trait objects, so it doesn’t know which method implemented on which type to call. Instead, Rust uses the pointers inside of the trait object at runtime to know which specific method to call. There’s a runtime cost when this lookup happens, compared to static dispatch. Dynamic dispatch also prevents the compiler from choosing to inline a method’s code which in turn prevents some optimizations. We did get extra flexibility in the code that we wrote and were able to support, though, so it’s a tradeoff to consider.
Object Safety is Required for Trait Objects
Only object safe traits can be made into trait objects. There are some complex rules around all the properties that make a trait object safe, but in practice, there are only two rules that are relevant. A trait is object safe if all of the methods defined in the trait have the following properties:
- The return type isn’t
Self
- There aren’t any generic type parameters
The Self
keyword is an alias for the type we’re implementing traits or
methods on. Object safety is required for trait objects because once you have a
trait object, you no longer know what the concrete type implementing that trait
is. If a trait method returns the concrete Self
type, but a trait object
forgets the exact type that it is, there’s no way that the method can use the
original concrete type that it’s forgotten. Same with generic type parameters
that are filled in with concrete type parameters when the trait is used: the
concrete types become part of the type that implements the trait. When the type
is erased by the use of a trait object, there’s no way to know what types to
fill in the generic type parameters with.
An example of a trait whose methods are not object safe is the standard
library’s Clone
trait. The signature for the clone
method in the Clone
trait looks like this:
# #![allow(unused_variables)] #fn main() { pub trait Clone { fn clone(&self) -> Self; } #}
String
implements the Clone
trait, and when we call the clone
method on
an instance of String
we get back an instance of String
. Similarly, if we
call clone
on an instance of Vec
, we get back an instance of Vec
. The
signature of clone
needs to know what type will stand in for Self
, since
that’s the return type.
The compiler will tell you if you’re trying to do something that violates the
rules of object safety in regards to trait objects. For example, if we had
tried to implement the Screen
struct in Listing 17-4 to hold types that
implement the Clone
trait instead of the Draw
trait, like this:
pub struct Screen {
pub components: Vec<Box<Clone>>,
}
We’ll get this error:
error[E0038]: the trait `std::clone::Clone` cannot be made into an object
-->
|
2 | pub components: Vec<Box<Clone>>,
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::clone::Clone` cannot be
made into an object
|
= note: the trait cannot require that `Self : Sized`
This means you can’t use this trait as a trait object in this way. If you’re interested in more details on object safety, see Rust RFC 255.