Advanced Traits
We covered traits in Chapter 10, but like lifetimes, we didn’t get to all the details. Now that we know more Rust, we can get into the nitty-gritty.
Associated Types
Associated types are a way of associating a type placeholder with a trait such that the trait method definitions can use these placeholder types in their signatures. The implementor of a trait will specify the concrete type to be used in this type’s place for the particular implementation.
We’ve described most of the things in this chapter as being very rare. Associated types are somewhere in the middle; they’re more rare than the rest of the book, but more common than many of the things in this chapter.
An example of a trait with an associated type is the Iterator trait provided
by the standard library. It has an associated type named Item that stands in
for the type of the values that we’re iterating over. We mentioned in Chapter
13 that the definition of the Iterator trait is as shown in Listing 19-20:
# #![allow(unused_variables)] #fn main() { pub trait Iterator { type Item; fn next(&mut self) -> Option<Self::Item>; } #}
Listing 19-20: The definition of the Iterator trait
that has an associated type Item
This says that the Iterator trait has an associated type named Item. Item
is a placeholder type, and the return value of the next method will return
values of type Option<Self::Item>. Implementors of this trait will specify
the concrete type for Item, and the next method will return an Option
containing a value of whatever type the implementor has specified.
Associated Types Versus Generics
When we implemented the Iterator trait on the Counter struct in Listing
13-6, we specified that the Item type was u32:
impl Iterator for Counter {
type Item = u32;
fn next(&mut self) -> Option<Self::Item> {
This feels similar to generics. So why isn’t the Iterator trait defined as
shown in Listing 19-21?
# #![allow(unused_variables)] #fn main() { pub trait Iterator<T> { fn next(&mut self) -> Option<T>; } #}
Listing 19-21: A hypothetical definition of the
Iterator trait using generics
The difference is that with the definition in Listing 19-21, we could also
implement Iterator<String> for Counter, or any other type as well, so that
we’d have multiple implementations of Iterator for Counter. In other words,
when a trait has a generic parameter, we can implement that trait for a type
multiple times, changing the generic type parameters’ concrete types each time.
Then when we use the next method on Counter, we’d have to provide type
annotations to indicate which implementation of Iterator we wanted to use.
With associated types, we can’t implement a trait on a type multiple times.
Using the actual definition of Iterator from Listing 19-20, we can only
choose once what the type of Item will be, since there can only be one impl Iterator for Counter. We don’t have to specify that we want an iterator of
u32 values everywhere that we call next on Counter.
The benefit of not having to specify generic type parameters when a trait uses
associated types shows up in another way as well. Consider the two traits
defined in Listing 19-22. Both are defining a trait having to do with a graph
structure that contains nodes of some type and edges of some type. GGraph is
defined using generics, and AGraph is defined using associated types:
# #![allow(unused_variables)] #fn main() { trait GGraph<Node, Edge> { // methods would go here } trait AGraph { type Node; type Edge; // methods would go here } #}
Listing 19-22: Two graph trait definitions, GGraph
using generics and AGraph using associated types for Node and Edge
Let’s say we wanted to implement a function that computes the distance between
two nodes in any types that implement the graph trait. With the GGraph trait
defined using generics, our distance function signature would have to look
like Listing 19-23:
# #![allow(unused_variables)] #fn main() { # trait GGraph<Node, Edge> {} # fn distance<N, E, G: GGraph<N, E>>(graph: &G, start: &N, end: &N) -> u32 { // ...snip... # 0 } #}
Listing 19-23: The signature of a distance function
that uses the trait GGraph and has to specify all the generic
parameters
Our function would need to specify the generic type parameters N, E, and
G, where G is bound by the trait GGraph that has type N as its Node
type and type E as its Edge type. Even though distance doesn’t need to
know the types of the edges, we’re forced to declare an E parameter, because
we need to to use the GGraph trait and that requires specifying the type for
Edge.
Contrast with the definition of distance in Listing 19-24 that uses the
AGraph trait from Listing 19-22 with associated types:
# #![allow(unused_variables)] #fn main() { # trait AGraph { # type Node; # type Edge; # } # fn distance<G: AGraph>(graph: &G, start: &G::Node, end: &G::Node) -> u32 { // ...snip... # 0 } #}
Listing 19-24: The signature of a distance function
that uses the trait AGraph and the associated type Node
This is much cleaner. We only need to have one generic type parameter, G,
with the trait bound AGraph. Since distance doesn’t use the Edge type at
all, it doesn’t need to be specified anywhere. To use the Node type
associated with AGraph, we can specify G::Node.
Trait Objects with Associated Types
You may have been wondering why we didn’t use a trait object in the distance
functions in Listing 19-23 and Listing 19-24. The signature for the distance
function using the generic GGraph trait does get a bit more concise using a
trait object:
# #![allow(unused_variables)] #fn main() { # trait GGraph<Node, Edge> {} # fn distance<N, E>(graph: &GGraph<N, E>, start: &N, end: &N) -> u32 { // ...snip... # 0 } #}
This might be a more fair comparison to Listing 19-24. Specifying the Edge
type is still required, though, which means Listing 19-24 is still preferable
since we don’t have to specify something we don’t use.
It’s not possible to change Listing 19-24 to use a trait object for the graph,
since then there would be no way to refer to the AGraph trait’s associated
type.
It is possible in general to use trait objects of traits that have associated
types, though; Listing 19-25 shows a function named traverse that doesn’t
need to use the trait’s associated types in other arguments. We do, however,
have to specify the concrete types for the associated types in this case. Here,
we’ve chosen to accept types that implement the AGraph trait with the
concrete type of usize as their Node type and a tuple of two usize values
for their Edge type:
# #![allow(unused_variables)] #fn main() { # trait AGraph { # type Node; # type Edge; # } # fn traverse(graph: &AGraph<Node=usize, Edge=(usize, usize)>) { // ...snip... } #}
While trait objects mean that we don’t need to know the concrete type of the
graph parameter at compile time, we do need to constrain the use of the
AGraph trait in the traverse function by the concrete types of the
associated types. If we didn’t provide this constraint, Rust wouldn’t be able
to figure out which impl to match this trait object to.
Operator Overloading and Default Type Parameters
The <PlaceholderType=ConcreteType> syntax is used in another way as well: to
specify the default type for a generic type. A great example of a situation
where this is useful is operator overloading.
Rust does not allow you to create your own operators or overload arbitrary
operators, but the operations and corresponding traits listed in std::ops can
be overloaded by implementing the traits associated with the operator. For
example, Listing 19-25 shows how to overload the + operator by implementing
the Add trait on a Point struct so that we can add two Point instances
together:
Filename: src/main.rs
use std::ops::Add; #[derive(Debug,PartialEq)] struct Point { x: i32, y: i32, } impl Add for Point { type Output = Point; fn add(self, other: Point) -> Point { Point { x: self.x + other.x, y: self.y + other.y, } } } fn main() { assert_eq!(Point { x: 1, y: 0 } + Point { x: 2, y: 3 }, Point { x: 3, y: 3 }); }
Listing 19-25: Implementing the Add trait to overload
the + operator for Point instances
We’ve implemented the add method to add the x values of two Point
instances together and the y values of two Point instances together to
create a new Point. The Add trait has an associated type named Output
that’s used to determine the type returned from the add method.
Let’s look at the Add trait in a bit more detail. Here’s its definition:
# #![allow(unused_variables)] #fn main() { trait Add<RHS=Self> { type Output; fn add(self, rhs: RHS) -> Self::Output; } #}
This should look familiar; it’s a trait with one method and an associated type.
The new part is the RHS=Self in the angle brackets: this syntax is called
default type parameters. RHS is a generic type parameter (short for “right
hand side”) that’s used for the type of the rhs parameter in the add
method. If we don’t specify a concrete type for RHS when we implement the
Add trait, the type of RHS will default to the type of Self (the type
that we’re implementing Add on).
Let’s look at another example of implementing the Add trait. Imagine we have
two structs holding values in different units, Millimeters and Meters. We
can implement Add for Millimeters in different ways as shown in Listing
19-26:
# #![allow(unused_variables)] #fn main() { use std::ops::Add; struct Millimeters(u32); struct Meters(u32); impl Add for Millimeters { type Output = Millimeters; fn add(self, other: Millimeters) -> Millimeters { Millimeters(self.0 + other.0) } } impl Add<Meters> for Millimeters { type Output = Millimeters; fn add(self, other: Meters) -> Millimeters { Millimeters(self.0 + (other.0 * 1000)) } } #}
Listing 19-26: Implementing the Add trait on
Millimeters to be able to add Millimeters to Millimeters and
Millimeters to Meters
If we’re adding Millimeters to other Millimeters, we don’t need to
parameterize the RHS type for Add since the default Self type is what we
want. If we want to implement adding Millimeters and Meters, then we need
to say impl Add<Meters> to set the value of the RHS type parameter.
Default type parameters are used in two main ways:
- To extend a type without breaking existing code.
- To allow customization in a way most users don’t want.
The Add trait is an example of the second purpose: most of the time, you’re
adding two like types together. Using a default type parameter in the Add
trait definition makes it easier to implement the trait since you don’t have to
specify the extra parameter most of the time. In other words, we’ve removed a
little bit of implementation boilerplate.
The first purpose is similar, but in reverse: since existing implementations of a trait won’t have specified a type parameter, if we want to add a type parameter to an existing trait, giving it a default will let us extend the functionality of the trait without breaking the existing implementation code.
Fully Qualified Syntax for Disambiguation
Rust cannot prevent a trait from having a method with the same name as another trait’s method, nor can it prevent us from implementing both of these traits on one type. We can also have a method implemented directly on the type with the same name as well! In order to be able to call each of the methods with the same name, then, we need to tell Rust which one we want to use.
Consider the code in Listing 19-27 where we've defined two traits, Pilot and
Wizard, that both have a method called fly. We then implement both traits
on a type Human that itself already has a method named fly implemented on
it. Each fly method does something different:
Filename: src/main.rs
# #![allow(unused_variables)] #fn main() { trait Pilot { fn fly(&self); } trait Wizard { fn fly(&self); } struct Human; impl Pilot for Human { fn fly(&self) { println!("This is your captain speaking."); } } impl Wizard for Human { fn fly(&self) { println!("Up!"); } } impl Human { fn fly(&self) { println!("*waving arms furiously*"); } } #}
Listing 19-27: Two traits defined to have a fly method,
and implementations of those traits on the Human type in addition to a fly
method on Human directly
When we call fly on an instance of Human, the compiler defaults to calling
the method that is directly implemented on the type, as shown in Listing 19-28:
Filename: src/main.rs
# trait Pilot { # fn fly(&self); # } # # trait Wizard { # fn fly(&self); # } # # struct Human; # # impl Pilot for Human { # fn fly(&self) { # println!("This is your captain speaking."); # } # } # # impl Wizard for Human { # fn fly(&self) { # println!("Up!"); # } # } # # impl Human { # fn fly(&self) { # println!("*waving arms furiously*"); # } # } # fn main() { let person = Human; person.fly(); }
Listing 19-28: Calling fly on an instance of
Human
Running this will print out *waving arms furiously*, which shows that Rust
called the fly method implemented on Human directly.
In order to call the fly methods from either the Pilot trait or the
Wizard trait, we need to use more explicit syntax in order to specify which
fly method we mean. This syntax is demonstrated in Listing 19-29:
Filename: src/main.rs
# trait Pilot { # fn fly(&self); # } # # trait Wizard { # fn fly(&self); # } # # struct Human; # # impl Pilot for Human { # fn fly(&self) { # println!("This is your captain speaking."); # } # } # # impl Wizard for Human { # fn fly(&self) { # println!("Up!"); # } # } # # impl Human { # fn fly(&self) { # println!("*waving arms furiously*"); # } # } # fn main() { let person = Human; Pilot::fly(&person); Wizard::fly(&person); person.fly(); }
Listing 19-29: Specifying which trait's fly method we
want to call
Specifying the trait name before the method name clarifies to Rust which
implementation of fly we want to call. We could also choose to write
Human::fly(&person), which is equivalent to person.fly() that we had in
Listing 19-28, but is a bit longer to write if we don't need to disambiguate.
Running this code will print:
This is your captain speaking.
Up!
*waving arms furiously*
Because the fly method takes a self parameter, if we had two types that
both implement one trait, Rust can figure out which implementation of a trait
to use based on the type of self.
However, associated functions that are part of traits don't have a self
parameter. When two types in the same scope implement that trait, Rust can't
figure out which type we mean unless we use fully qualified syntax. For
example, take the Animal trait in Listing 19-30 that has the associated
function baby_name, the implementation of Animal for the struct Dog, and
the associated function baby_name defined on Dog directly:
Filename: src/main.rs
trait Animal { fn baby_name() -> String; } struct Dog; impl Dog { fn baby_name() -> String { String::from("Spot") } } impl Animal for Dog { fn baby_name() -> String { String::from("puppy") } } fn main() { println!("A baby dog is called a {}", Dog::baby_name()); }
Listing 19-30: A trait with an associated function and a type that has an associated function with the same name that also implements the trait
This code is for an animal shelter where they want to give all puppies the name
Spot, which is implemented in the baby_name associated function that is
defined on Dog. The Dog type also implements the trait Animal, which
describes characteristics that all animals have. Baby dogs are called puppies,
and that is expressed in the implementation of the Animal trait on Dog in
the baby_name function associated with the Animal trait.
In main, we're calling the Dog::baby_name function, which calls the
associated function defined on Dog directly. This code prints:
A baby dog is called a Spot
This isn't really what we wanted, in this case we want to call the baby_name
function that's part of the Animal trait that we implemented on Dog, so
that we can print A baby dog is called a puppy. The technique we used in
Listing 19-29 doesn't help here; if we change main to be the code in Listing
19-31:
Filename: src/main.rs
fn main() {
println!("A baby dog is called a {}", Animal::baby_name());
}
Listing 19-31: Attempting to call the baby_name
function from the Animal trait, but Rust doesn't know which implementation to
use
Because Animal::baby_name is an associated function rather than a method, and
thus doesn't have a self parameter, Rust has no way to figure out which
implementation of Animal::baby_name we want. We'll get this compiler error:
error[E0283]: type annotations required: cannot resolve `_: Animal`
--> src/main.rs
|
20 | println!("A baby dog is called a {}", Animal::baby_name());
| ^^^^^^^^^^^^^^^^^
|
= note: required by `Animal::baby_name`
In order to tell Rust that we want to use the implementation of Animal for
Dog, we need to use fully qualified syntax, which is the most specific we
can be when calling a function. Listing 19-32 demonstrates how to use fully
qualified syntax in this case:
Filename: src/main.rs
# trait Animal { # fn baby_name() -> String; # } # # struct Dog; # # impl Dog { # fn baby_name() -> String { # String::from("Spot") # } # } # # impl Animal for Dog { # fn baby_name() -> String { # String::from("puppy") # } # } # fn main() { println!("A baby dog is called a {}", <Dog as Animal>::baby_name()); }
Listing 19-32: Using fully qualified syntax to specify
that we want to call the baby_name function from the Animal trait as
implemented on Dog
We're providing Rust with a type annotation within the angle brackets, and
we're specifying that we want to call the baby_name method from the Animal
trait as implemented on Dog by saying that we want to treat the Dog type as
an Animal for this function call. This code will now print what we want:
A baby dog is called a puppy
In general, fully qualified syntax is defined as:
<Type as Trait>::function(receiver_if_method, next_arg, ...);
For associated functions, there would not be a receiver, there would only be
the list of other arguments. We could choose to use fully qualified syntax
everywhere that we call functions or methods. However, we're allowed to leave
out any part of this syntax that Rust is able to figure out from other
information in the program. We only need to use this more verbose syntax in
cases where there are multiple implementations that use the same name and Rust
needs help in order to know which implementation we want to call.
Supertraits to Use One Trait’s Functionality Within Another Trait
Sometimes, we may want a trait to be able to rely on another trait also being implemented wherever our trait is implemented, so that our trait can use the other trait’s functionality. The required trait is a supertrait of the trait we’re implementing.
For example, let’s say we want to make an OutlinePrint trait with an
outline_print method that will print out a value outlined in asterisks. That
is, if our Point struct implements Display to result in (x, y), calling
outline_print on a Point instance that has 1 for x and 3 for y would
look like:
**********
* *
* (1, 3) *
* *
**********
In the implementation of outline_print, since we want to be able to use the
Display trait’s functionality, we need to be able to say that the
OutlinePrint trait will only work for types that also implement Display and
provide the functionality that OutlinePrint needs. We can do that in the
trait definition by specifying OutlinePrint: Display. It’s like adding a
trait bound to the trait. Listing 19-33 shows an implementation of the
OutlinePrint trait:
# #![allow(unused_variables)] #fn main() { use std::fmt; trait OutlinePrint: fmt::Display { fn outline_print(&self) { let output = self.to_string(); let len = output.len(); println!("{}", "*".repeat(len + 4)); println!("*{}*", " ".repeat(len + 2)); println!("* {} *", output); println!("*{}*", " ".repeat(len + 2)); println!("{}", "*".repeat(len + 4)); } } #}
Listing 19-33: Implementing the OutlinePrint trait that
requires the functionality from Display
Because we’ve specified that OutlinePrint requires the Display trait, we
can use to_string in outline_print (to_string is automatically
implemented for any type that implements Display). If we hadn’t added the : Display after the trait name and we tried to use to_string in
outline_print, we’d get an error that no method named to_string was found
for the type &Self in the current scope.
If we try to implement OutlinePrint on a type that doesn’t implement
Display, such as the Point struct:
# #![allow(unused_variables)] #fn main() { # trait OutlinePrint {} struct Point { x: i32, y: i32, } impl OutlinePrint for Point {} #}
We’ll get an error that Display isn’t implemented and that Display is
required by OutlinePrint:
error[E0277]: the trait bound `Point: std::fmt::Display` is not satisfied
--> src/main.rs:20:6
|
20 | impl OutlinePrint for Point {}
| ^^^^^^^^^^^^ the trait `std::fmt::Display` is not implemented for
`Point`
|
= note: `Point` cannot be formatted with the default formatter; try using
`:?` instead if you are using a format string
= note: required by `OutlinePrint`
Once we implement Display on Point and satisfy the constraint that
OutlinePrint requires, like so:
# #![allow(unused_variables)] #fn main() { # struct Point { # x: i32, # y: i32, # } # use std::fmt; impl fmt::Display for Point { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "({}, {})", self.x, self.y) } } #}
then implementing the OutlinePrint trait on Point will compile successfully
and we can call outline_print on a Point instance to display it within an
outline of asterisks.
The Newtype Pattern to Implement External Traits on External Types
In Chapter 10, we mentioned the orphan rule, which says we’re allowed to implement a trait on a type as long as either the trait or the type are local to our crate. One way to get around this restriction is to use the newtype pattern, which involves creating a new type using a tuple struct with one field as a thin wrapper around the type we want to implement a trait for. Then the wrapper type is local to our crate, and we can implement the trait on the wrapper. “Newtype” is a term originating from the Haskell programming language. There’s no runtime performance penalty for using this pattern. The wrapper type is elided at compile time.
For example, if we wanted to implement Display on Vec, we can make a
Wrapper struct that holds an instance of Vec. Then we can implement
Display on Wrapper and use the Vec value as shown in Listing 19-34:
Filename: src/main.rs
use std::fmt; struct Wrapper(Vec<String>); impl fmt::Display for Wrapper { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "[{}]", self.0.join(", ")) } } fn main() { let w = Wrapper(vec![String::from("hello"), String::from("world")]); println!("w = {}", w); }
Listing 19-34: Creating a Wrapper type around
Vec<String> to be able to implement Display
The implementation of Display uses self.0 to access the inner Vec, and
then we can use the functionality of the Display type on Wrapper.
The downside is that since Wrapper is a new type, it doesn’t have the methods
of the value it’s holding; we’d have to implement all the methods of Vec like
push, pop, and all the rest directly on Wrapper to delegate to self.0
in order to be able to treat Wrapper exactly like a Vec. If we wanted the
new type to have every single method that the inner type has, implementing the
Deref trait that we discussed in Chapter 15 on the wrapper to return the
inner type can be a solution. If we don’t want the wrapper type to have all the
methods of the inner type, in order to restrict the wrapper type’s behavior for
example, we’d have to implement just the methods we do want ourselves.
That’s how the newtype pattern is used in relation to traits; it’s also a useful pattern without having traits involved. Let’s switch focus now to talk about some advanced ways to interact with Rust’s type system.