A trait is a language feature that tells the Rust compiler about functionality a type must provide.
Recall 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 first define a trait with a method
signature, then implement the trait for a type. In this example, we implement the trait HasArea for Circle:
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, only a type signature. When we impl a trait,
we use impl Trait for Item, rather than only impl Item.
Traits are useful because they allow a type to make certain promises about its behavior. Generic functions can exploit this to constrain, or bound, the types they accept. Consider this function, which does not compile:
fn main() { fn print_area<T>(shape: T) { println!("This shape has an area of {}", shape.area()); } }fn print_area<T>(shape: T) { println!("This shape has an area of {}", shape.area()); }
Rust complains:
error: no method named `area` found for type `T` in the current scope
Because T can be any type, we can’t be sure that it implements the area
method. But we can add a trait bound to our generic T, ensuring
that it does:
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); }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: the trait bound `_ : HasArea` is not satisfied [E0277]
Your generic structs can also benefit from trait bounds. All you need to
do is append the bound when you declare type parameters. Here is a new
type Rectangle<T> and its operation is_square():
struct Rectangle<T> { x: T, y: T, width: T, height: T, } impl<T: PartialEq> Rectangle<T> { fn is_square(&self) -> bool { self.width == self.height } } fn main() { let mut r = Rectangle { x: 0, y: 0, width: 47, height: 47, }; assert!(r.is_square()); r.height = 42; assert!(!r.is_square()); }
is_square() needs to check that the sides are equal, so the sides must be of
a type that implements the core::cmp::PartialEq trait:
impl<T: PartialEq> Rectangle<T> { ... }
Now, a rectangle can be defined in terms of any type that can be compared for equality.
Here we defined a new struct Rectangle that accepts numbers of any
precision—really, objects of pretty much any type—as long as they can be
compared for equality. Could we do the same for our HasArea structs, Square
and Circle? Yes, but they need multiplication, and to work with that we need
to know more about operator traits.
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 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").expect("Couldn’t open foo.txt"); let buf = b"whatever"; // byte string literal. buf: &[u8; 8] let result = f.write(buf);
Here’s the error:
error: type `std::fs::File` does not implement any method in scope named `write`
let result = f.write(buf);
^~~~~~~~~~
We need to use the Write trait first:
use std::io::Write; let mut f = std::fs::File::open("foo.txt").expect("Couldn’t open foo.txt"); let buf = b"whatever"; let result = f.write(buf);
This will compile without error.
This means that even if someone does something bad like add methods to i32,
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 implementing it for must be defined by you. Or more
precisely, one of them must be defined in the same crate as the impl
you're writing. For more on Rust's module and package system, see the
chapter on crates and modules.
So, we could implement the HasArea type for i32, because we defined
HasArea in our code. But if we tried to implement ToString, a trait
provided by Rust, for i32, we could not, because neither the trait nor
the type are defined in our crate.
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.
You’ve seen that you can bound a generic type parameter with a trait:
fn main() { fn foo<T: Clone>(x: T) { x.clone(); } }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.
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:
fn main() { use std::fmt::Debug; fn foo<T: Clone, K: Clone + Debug>(x: T, y: K) { x.clone(); y.clone(); println!("{:?}", y); } }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", "world"); }
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 "ConvertTo<i64>" where i32: ConvertTo<T> { 42.convert() }
This shows off the additional feature of where clauses: they allow bounds
on the left-hand side not only of type parameters T, but also of types (i32 in this case). In this example, i32 must implement
ConvertTo<T>. Rather than defining what i32 is (since that's obvious), the
where clause here constrains T.
A default method can be added to a trait definition if it is already known how a typical implementor will define a method. For example, is_invalid() is defined as the opposite of is_valid():
trait Foo { fn is_valid(&self) -> bool; fn is_invalid(&self) -> bool { !self.is_valid() } }
Implementors of the Foo trait need to implement is_valid() but not is_invalid() due to the added default behavior. This default behavior can still be overridden as in:
struct UseDefault; impl Foo for UseDefault { fn is_valid(&self) -> bool { println!("Called UseDefault.is_valid."); true } } struct OverrideDefault; impl Foo for OverrideDefault { fn is_valid(&self) -> bool { println!("Called OverrideDefault.is_valid."); true } fn is_invalid(&self) -> bool { println!("Called OverrideDefault.is_invalid!"); true // overrides the expected value of is_invalid() } } let default = UseDefault; assert!(!default.is_invalid()); // prints "Called UseDefault.is_valid." let over = OverrideDefault; assert!(over.is_invalid()); // prints "Called OverrideDefault.is_invalid!"
Sometimes, implementing a trait requires implementing another trait:
fn main() { trait Foo { fn foo(&self); } trait FooBar : Foo { fn foobar(&self); } }trait Foo { fn foo(&self); } trait FooBar : Foo { fn foobar(&self); }
Implementors of FooBar must also implement Foo, like this:
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 bound `main::Baz : main::Foo` is not satisfied [E0277]
Implementing traits like Debug and Default repeatedly can become
quite tedious. For that reason, Rust provides an attribute that
allows you to let Rust automatically implement traits for you:
#[derive(Debug)] struct Foo; fn main() { println!("{:?}", Foo); }
However, deriving is limited to a certain set of traits: