9.0 KiB
% Traits
Do you remember the impl keyword, used to call a function with method
syntax?
struct Circle {
x: f64,
y: f64,
radius: f64,
}
impl Circle {
fn area(&self) -> f64 {
std::f64::consts::PI * (self.radius * self.radius)
}
}
Traits are similar, except that we define a trait with just the method signature, then implement the trait for that struct. Like this:
struct Circle {
x: f64,
y: f64,
radius: f64,
}
trait HasArea {
fn area(&self) -> f64;
}
impl HasArea for Circle {
fn area(&self) -> f64 {
std::f64::consts::PI * (self.radius * self.radius)
}
}
As you can see, the trait block looks very similar to the impl block,
but we don’t define a body, just a type signature. When we impl a trait,
we use impl Trait for Item, rather than just impl Item.
We can use traits to constrain our generics. Consider this function, which does not compile, and gives us a similar error:
fn print_area<T>(shape: T) {
println!("This shape has an area of {}", shape.area());
}
Rust complains:
error: type `T` does not implement any method in scope named `area`
Because T can be any type, we can’t be sure that it implements the area
method. But we can add a ‘trait constraint’ to our generic T, ensuring
that it does:
# trait HasArea {
# fn area(&self) -> f64;
# }
fn print_area<T: HasArea>(shape: T) {
println!("This shape has an area of {}", shape.area());
}
The syntax <T: HasArea> means any type that implements the HasArea trait.
Because traits define function type signatures, we can be sure that any type
which implements HasArea will have an .area() method.
Here’s an extended example of how this works:
trait HasArea {
fn area(&self) -> f64;
}
struct Circle {
x: f64,
y: f64,
radius: f64,
}
impl HasArea for Circle {
fn area(&self) -> f64 {
std::f64::consts::PI * (self.radius * self.radius)
}
}
struct Square {
x: f64,
y: f64,
side: f64,
}
impl HasArea for Square {
fn area(&self) -> f64 {
self.side * self.side
}
}
fn print_area<T: HasArea>(shape: T) {
println!("This shape has an area of {}", shape.area());
}
fn main() {
let c = Circle {
x: 0.0f64,
y: 0.0f64,
radius: 1.0f64,
};
let s = Square {
x: 0.0f64,
y: 0.0f64,
side: 1.0f64,
};
print_area(c);
print_area(s);
}
This program outputs:
This shape has an area of 3.141593
This shape has an area of 1
As you can see, print_area is now generic, but also ensures that we have
passed in the correct types. If we pass in an incorrect type:
print_area(5);
We get a compile-time error:
error: failed to find an implementation of trait main::HasArea for int
So far, we’ve only added trait implementations to structs, but you can
implement a trait for any type. So technically, we could implement HasArea
for i32:
trait HasArea {
fn area(&self) -> f64;
}
impl HasArea for i32 {
fn area(&self) -> f64 {
println!("this is silly");
*self as f64
}
}
5.area();
It is considered poor style to implement methods on such primitive types, even though it is possible.
This may seem like the Wild West, but there are two other restrictions around
implementing traits that prevent this from getting out of hand. The first is
that if the trait isn’t defined in your scope, it doesn’t apply. Here’s an
example: the standard library provides a Write trait which adds
extra functionality to Files, for doing file I/O. By default, a File
won’t have its methods:
let mut f = std::fs::File::open("foo.txt").ok().expect("Couldn’t open foo.txt");
let result = f.write("whatever".as_bytes());
# result.unwrap(); // ignore the error
Here’s the error:
error: type `std::fs::File` does not implement any method in scope named `write`
let result = f.write(b"whatever");
^~~~~~~~~~~~~~~~~~
We need to use the Write trait first:
use std::io::Write;
let mut f = std::fs::File::open("foo.txt").ok().expect("Couldn’t open foo.txt");
let result = f.write("whatever".as_bytes());
# result.unwrap(); // ignore the error
This will compile without error.
This means that even if someone does something bad like add methods to int,
it won’t affect you, unless you use that trait.
There’s one more restriction on implementing traits. Either the trait or the
type you’re writing the impl for must be defined by you. So, we could
implement the HasArea type for i32, because HasArea is in our code. But
if we tried to implement Float, a trait provided by Rust, for i32, we could
not, because neither the trait nor the type are in our code.
One last thing about traits: generic functions with a trait bound use ‘monomorphization’ (mono: one, morph: form), so they are statically dispatched. What’s that mean? Check out the chapter on trait objects for more details.
Multiple trait bounds
You’ve seen that you can bound a generic type parameter with a trait:
fn foo<T: Clone>(x: T) {
x.clone();
}
If you need more than one bound, you can use +:
use std::fmt::Debug;
fn foo<T: Clone + Debug>(x: T) {
x.clone();
println!("{:?}", x);
}
T now needs to be both Clone as well as Debug.
Where clause
Writing functions with only a few generic types and a small number of trait bounds isn’t too bad, but as the number increases, the syntax gets increasingly awkward:
use std::fmt::Debug;
fn foo<T: Clone, K: Clone + Debug>(x: T, y: K) {
x.clone();
y.clone();
println!("{:?}", y);
}
The name of the function is on the far left, and the parameter list is on the far right. The bounds are getting in the way.
Rust has a solution, and it’s called a ‘where clause’:
use std::fmt::Debug;
fn foo<T: Clone, K: Clone + Debug>(x: T, y: K) {
x.clone();
y.clone();
println!("{:?}", y);
}
fn bar<T, K>(x: T, y: K) where T: Clone, K: Clone + Debug {
x.clone();
y.clone();
println!("{:?}", y);
}
fn main() {
foo("Hello", "world");
bar("Hello", "workd");
}
foo() uses the syntax we showed earlier, and bar() uses a where clause.
All you need to do is leave off the bounds when defining your type parameters,
and then add where after the parameter list. For longer lists, whitespace can
be added:
use std::fmt::Debug;
fn bar<T, K>(x: T, y: K)
where T: Clone,
K: Clone + Debug {
x.clone();
y.clone();
println!("{:?}", y);
}
This flexibility can add clarity in complex situations.
where is also more powerful than the simpler syntax. For example:
trait ConvertTo<Output> {
fn convert(&self) -> Output;
}
impl ConvertTo<i64> for i32 {
fn convert(&self) -> i64 { *self as i64 }
}
// can be called with T == i32
fn normal<T: ConvertTo<i64>>(x: &T) -> i64 {
x.convert()
}
// can be called with T == i64
fn inverse<T>() -> T
// this is using ConvertTo as if it were "ConvertFrom<i32>"
where i32: ConvertTo<T> {
1i32.convert()
}
This shows off the additional feature of where clauses: they allow bounds
where the left-hand side is an arbitrary type (i32 in this case), not just a
plain type parameter (like T).
Default methods
There’s one last feature of traits we should cover: default methods. It’s easiest just to show an example:
trait Foo {
fn bar(&self);
fn baz(&self) { println!("We called baz."); }
}
Implementors of the Foo trait need to implement bar(), but they don’t
need to implement baz(). They’ll get this default behavior. They can
override the default if they so choose:
# trait Foo {
# fn bar(&self);
# fn baz(&self) { println!("We called baz."); }
# }
struct UseDefault;
impl Foo for UseDefault {
fn bar(&self) { println!("We called bar."); }
}
struct OverrideDefault;
impl Foo for OverrideDefault {
fn bar(&self) { println!("We called bar."); }
fn baz(&self) { println!("Override baz!"); }
}
let default = UseDefault;
default.baz(); // prints "We called baz."
let over = OverrideDefault;
over.baz(); // prints "Override baz!"
Inheritance
Sometimes, implementing a trait requires implementing another trait:
trait Foo {
fn foo(&self);
}
trait FooBar : Foo {
fn foobar(&self);
}
Implementors of FooBar must also implement Foo, like this:
# trait Foo {
# fn foo(&self);
# }
# trait FooBar : Foo {
# fn foobar(&self);
# }
struct Baz;
impl Foo for Baz {
fn foo(&self) { println!("foo"); }
}
impl FooBar for Baz {
fn foobar(&self) { println!("foobar"); }
}
If we forget to implement Foo, Rust will tell us:
error: the trait `main::Foo` is not implemented for the type `main::Baz` [E0277]