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Advanced Features.md
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@ -10,7 +10,7 @@ The features covered here are useful in very specific situations.
In this chapter we will cover In this chapter we will cover
- [Unsafe Rust](./Unsafe%20Rust.md): How to opt out of some of Rust's guarantees and take responsibility for manually upholding those guarantees - [Unsafe Rust](./Unsafe%20Rust.md): How to opt out of some of Rust's guarantees and take responsibility for manually upholding those guarantees
- [Advanced traits](./Advanced%20Traits.md): associated types, default type parameters, fully qualified syntax, supertraits, and the new type pattern in relation to traits - [Advanced traits](./Advanced%20Traits.md): associated types, default type parameters, fully qualified syntax, supertraits, and the new type pattern in relation to traits
- [Advanced types](): more about the newtype pattern, type aliases, the never type, and dynamically sized types - [Advanced types](./Advanced%20Types.md): more about the newtype pattern, type aliases, the never type, and dynamically sized types
- [Advanced functions and closures](): function pointers and returning closures - [Advanced functions and closures](): function pointers and returning closures
- [Macros](): ways to define code that defines more code at compile time - [Macros](): ways to define code that defines more code at compile time
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# Advanced Traits # Advanced Traits
Here we will go into the nitty-gritty of traits..
## Specifying Placeholder Types in Trait Definitions with Associated Types
*Associated types* connect 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 instead of the placeholder type for the particular implementation.
This way we can define a trait that uses some types without needing to know exactly what those types are until the trait is implemented.
We have described most of the advanced features in this chapter as being rarely needed.
Associated types are somewhere in the middle: they are used more rarely than features explained in the rest of the book, but more commonly than many of the other features discussed in this chapter.
One example of trait with an associated type is the `Iterator` trait that the std library provides.
The associated type is named `Item` and stands in for the type of the values the type implementing the `Iterator` trait is iterating over.
Here is the definition of the `Iterator` trait.
```rust
pub trait Iterator {
type Item;
fn next(&mut self) -> Option<Self::Item>;
}
```
The type `Item` is a placeholder, and the `next` method's definition shows that it will return values of type `Option<Self::Item>`.
Implementors of the `Iterator` trait will specify the concrete type for `Item` and the `next` method will return an `Option` containing a value of that concrete type.
Associated types may seem like a similar concept to generics, in that the latter allow us to define a function without specifying what types it can handle.
To examine the difference between the two, we will look at the implementation of the `Iterator` trait on a type named `Counter` that specifies the `Item` type is `u32`.
```rust
impl Iterator for Counter {
type Item = u32;
fn next(&mut self) -> Option<Self::Item> {
// --snip--
```
This syntax seems similar to that of generics.
So why not just define the `Iterator` trait with generics
```rust
pub trait Iterator<T> {
fn next(&mut self) -> Option<T>;
}
```
The differences is that when using generics, we must annotate the types in each implementation.
Because we can also implement `Iterator<String> for Counter` or any other type, we could have multiple implementations of `Iterator` for `Counter`.
This could also be said, when a trait has a generic parameter, it can be implemented for a type multiple times, changing the concrete types of the generic type parameters each time.
When we use the `next` method on `Counter`, we would have to provide type annotations to indicate which implementation of `Iterator` we want to use.
With associated types, we don't need to annotate types because we can't implement a trait on a type multiple times.
In the first definition that uses associated types, we can only choose what the type of `Item` will be once because 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`.
Associated types also become part of the trait's contract.
Implementors of the trait must also provide a type to stand in for the associated type placeholder.
Associated types often have a name that describes how the type will be used, and documenting the associated type in the API documentation is good practice.
## Default Generic Type Parameters and Operator Overloading
When we use generic type parameters, we can specify a default concrete type for the generic type.
This eliminates the need for implementors of the trait to specify a concrete type if the default type works.
You can specify a default type when declaring a generic type with the `<PlaceholderType=ConcreteType>` syntax.
A good example of a situation where this technique is useful is with *operator overloading*, where you customize the behavior of an operator (such as `+`) in particular situations.
Rust doesn't allow you to create your own operators or overload arbitrary operators.
You can overload the operation and corresponding traits listed in `std::ops` by implementing the traits associated with the operator.
For example here we overload the `+` operator to add two `Point` instances together.
We do this by implementing the `Add` trait on a `Point` struct.
```rust
use std::ops::Add;
#[derive(Debug, Copy, Clone, 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 }
);
}
```
The `add` method adds the `x` values of two `Point` instances and the `y` values of two `Point` instances to create a new `Point`.
The `Add` trait has an associated type named `Output` that determines the type returned from the `add` method.
The default generic type in this code is within the `Add` trait.
Here is its definition
```rust
trait Add<Rhs=Self> {
type Output;
fn add(self, rhs: Rhs) -> Self::Output;
}
```
This should look generally familiar: a trait with one method and an associated type.
The new part is `Rhs=Self`: this syntax is called *default type parameters*.
The `Rhs` generic type parameter (short for "right hand side") defines the type of the `rhs` parameter in the `add` method.
If we didn't specify a concrete type for `Rhs` when we implement the `Add` trait, the type of `Rhs` will default to `Self`, which will be the type we are implementing `Add` on.
When we implemented `Add` for `Point`, we used the default for `Rhs` because we wanted to add two `Point` instances.
Now lets look at an example of implementing the `Add` trait where we want to customize the `Rhs` type rather than using the default.
Here we want two structs `Millimeters` and `MEters`, which hold values in different units.
This thin wrapping of an existing type in another struct is known as the *newtype pattern*, which will be described in more detail in the ["Using the Newtype Pattern to Implement External Traits on External Types"]() section.
We want to add values in millimeters to values in meters and have the implementation of `Add` do the conversion correctly.
We can implement `Add` for `Millimeters` with `Meters` as the `Rhs`.
```rust
use std::ops::Add;
struct Millimeters(u32);
struct Meters(u32);
impl Add<Meters> for Millimeters {
type Output = Millimeters;
fn add(self, other: Meters) -> Millimeters {
Millimeters(self.0 + (other.0 * 1000))
}
}
```
To add `Millimeters` and `Meters`, we specify `impl Add<Meters>` to set the value of the `Rhs` type parameter instead of using the default of `Self`.
You will use default type parameters in two main ways:
- To extend a type without breaking existing code
- To allow customization in specific cases most users won't need
The std library's `Add` trait is an example of the second purpose.
Usually, you will add two like types, but the `Add` trait provides the ability to customize beyond that.
Using a default type parameter in the `Add` trait definition means you don't have to specify the extra parameter most of the time.
A bit of implementation boilerplate isn't needed, making it easier to use the trait.
The first purpose is similar to the second but in reverse.
If you want to add a type parameter to an existing trait, you can give it a default to allow extension of the functionality of the trait without breaking the existing implementation code.
## Fully Qualified Syntax for Disambiguation: Calling Methods with the Same Name
Nothing in Rust prevents a trait from having a method with the same name as another trait's methods.
Nor does Rust prevent you from implementing both traits on one type.
It is also possible to implement a method directly on the type with the same name as methods form traits.
When calling methods with the same name, you need to specify to Rust which one you want to use.
Consider this code where we have defined two traits, `Pilot` and `Wizard`, that both have a method called `fly`.
Then implement both traits on a type `Human` that already has a method named `fly` implemented on it.
Each `fly` method does something different.
```rust
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*");
}
}
```
When we call `fly` on an instance of `Human`, the compiler defaults to calling the method that is directly implemented on the type.
```rust
fn main() {
let person = Human;
person.fly();
}
```
The output of this code will print `*waving arms furiously*`, showing that Rust called the `fly` method implemented on `Human` directly.
To call the `fly` methods from either the `Pilot` trait or the `Wizard` trait, we need to use more specific syntax to specify which `fly` method we mean.
Here is a demonstration of this syntax,
```rust
fn main() {
let person = Human;
Pilot::fly(&person);
Wizard::fly(&person);
person.fly();
}
```
Specifying the trait name before the method name clarifies to Rust and Us which implementation of `fly` we want to call.
We could also write `Human::fly(&person)` but that us the same as `person.fly()`.
This is also a bit longer to write if we don't need to disambiguate.
Output
```
$ cargo run
Compiling traits-example v0.1.0 (file:///projects/traits-example)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.46s
Running `target/debug/traits-example`
This is your captain speaking.
Up!
*waving arms furiously*
```
Because `fly` takes a `self` parameter, if we had two *types* that both implement one *trait*, Rust would be able to figure out which implementation of a `trait` to use based on the type of `self`.
Associated function that are not methods do not have a `self` parameter.
When there are multiple types of traits that define non-method functions with the same function name, Rust doesn't always know which type you mean unless you use *fully qualified syntax*.
For example, we here we create a trait for an animal shelter that wants to name all baby dogs *Spot*.
We make an `Animal` trait with an associated non-method function `baby_name`.
The `Animal` trait is implemented for the struct `Dog`, which we also provide an associated non-method function `baby_name` directly.
```rust
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());
}
```
We implement the code for naming all puppies `Spot` 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 call the `Dog::baby_name` function, which calls the associated function defined on `Dog` directly.
This outputs
```
$ cargo run
Compiling traits-example v0.1.0 (file:///projects/traits-example)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.54s
Running `target/debug/traits-example`
A baby dog is called a Spot
```
This isn't thew output we wanted.
We want to call the `baby_name` function that is part of the `Animal` trait that we implemented on `Dog` so that it prints `A baby dog is called a puppy`.
The technique of specifying the trait name that we used here doesn't help here.
If we changed `main` to the code below, we get a compiler error.
```rust
fn main() {
println!("A baby dog is called a {}", Animal::baby_name());
}
```
Because `Animal::baby_name` doesn't have a `self` parameter and there could be other types implements the `Animal` trait, Rust can't figure out which implementation of `Animal::baby_name` we want.
We get this compiler error
```
$ cargo run
Compiling traits-example v0.1.0 (file:///projects/traits-example)
error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
--> src/main.rs:20:43
|
2 | fn baby_name() -> String;
| ------------------------- `Animal::baby_name` defined here
...
20 | println!("A baby dog is called a {}", Animal::baby_name());
| ^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
|
help: use the fully-qualified path to the only available implementation
|
20 | println!("A baby dog is called a {}", <Dog as Animal>::baby_name());
| +++++++ +
For more information about this error, try `rustc --explain E0790`.
error: could not compile `traits-example` (bin "traits-example") due to 1 previous error
```
To disambiguate and tell Rust that we want to use the implementation of `Animal` for `Dog` as opposed to the implementation of `Animal` for some other type.
We need to use fully qualified syntax.
Here demonstrates how to use fully qualified syntax
```rust
fn main() {
println!("A baby dog is called a {}", <Dog as Animal>::baby_name());
}
```
Here we provide Rust with a type annotation within the angle brackets.
This indicates we want to call the `baby_name` method from the `Animal` trait as implementation on `Dog` by saying that we want to treat the `Dog` type as an `Animal` for this function call.
Here is the new output
```
$ cargo run
Compiling traits-example v0.1.0 (file:///projects/traits-example)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.48s
Running `target/debug/traits-example`
A baby dog is called a puppy
```
Here us a fully qualified syntax is defined as follows
```
<Type as Trait>::function(receiver_if_method, next_arg, ...);
```
For associated function that aren't methods, there would not be a `receiver`: there would only be the list of other arguments.
You could use fully qualified syntax everywhere that you call functions or methods.
You are allowed to omit any part of this syntax that Rust can figure out from other information in the program.
You only need to use this this more verbose syntax in cases where there are multiple implementations that use the same name and Rust needs help to identify which implementation you want to call.
## Using Supertraits to Require One Trait's Functionality Within Another Trait
Sometimes you might write a trait definition that depends on another trait.
For a type to implement the first trait, you want to require that type to also implement the second trait.
You would do this so that your trait definition can make use of the associated items of the second trait.
The trait your trait definition is relying on is called a *supertrait* of your trait.
Lets say we want to make an `OutlinePrint` trait with an `outline_print` method that will print a given value formatted so that it is framed in asterisks.
Given that a `Point` struct that implements the std library trait `Display` to result in `(x, y)`.
When we call `outline_print` on a `Point` instance that has `1` for `x` and `3` for `y`, it should print the following
```
**********
* *
* (1, 3) *
* *
**********
```
The implementation of the `outline_print` method we want to use the `Display` trait's functionality.
Therefore we need to specify that the `OutlinePrint` trait will work only for types that also implement `Display` and provide the functionality that `OutlinePrint` needs.
We can do this in the trait definition by specifying `OutlinePrint: Display`.
This technique is similar to adding a trait bound to the trait.
Here shows an implementation of the `OutlinePrint` trait
```rust
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));
}
}
```
Because we specified that `OutlinePrint` requires the `Display` trait.
We can use the `to_string` function that is automatically implemented for any type that implements `Display`.
If we attempted to use `to_string` without adding a color and specifying the `Display` trait after the trait name, we would get an error saying that no method named `to_string` was found for the type `&Self` in the current scope.
Lets see what happens when we try to implement `OutlinePrint` on a type that doesn't implement `Display`, such as the `Point` struct
```rust
struct Point {
x: i32,
y: i32,
}
impl OutlinePrint for Point {}
```
We still get an error saying that `Display` is required but not implemented
```
$ cargo run
Compiling traits-example v0.1.0 (file:///projects/traits-example)
error[E0277]: `Point` doesn't implement `std::fmt::Display`
--> src/main.rs:20:23
|
20 | impl OutlinePrint for Point {}
| ^^^^^ `Point` cannot be formatted with the default formatter
|
= help: the trait `std::fmt::Display` is not implemented for `Point`
= note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
note: required by a bound in `OutlinePrint`
--> src/main.rs:3:21
|
3 | trait OutlinePrint: fmt::Display {
| ^^^^^^^^^^^^ required by this bound in `OutlinePrint`
error[E0277]: `Point` doesn't implement `std::fmt::Display`
--> src/main.rs:24:7
|
24 | p.outline_print();
| ^^^^^^^^^^^^^ `Point` cannot be formatted with the default formatter
|
= help: the trait `std::fmt::Display` is not implemented for `Point`
= note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
note: required by a bound in `OutlinePrint::outline_print`
--> src/main.rs:3:21
|
3 | trait OutlinePrint: fmt::Display {
| ^^^^^^^^^^^^ required by this bound in `OutlinePrint::outline_print`
4 | fn outline_print(&self) {
| ------------- required by a bound in this associated function
For more information about this error, try `rustc --explain E0277`.
error: could not compile `traits-example` (bin "traits-example") due to 2 previous errors
```
In order to fix this, we implement `Display` on `Point` and satisfy the constraint that `OutlinePrint` requires.
Like this
```rust
use std::fmt;
impl fmt::Display for Point {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "({}, {})", self.x, self.y)
}
}
```
Now implementing the `OutlinePrint` trait on `Point` will compile successfully.
We can call `outline_print` on a `Point` instance to display it within an outline of asterisks.
## Using the Newtype Pattern to Implement Traits on External Types
Previously we mentioned the orphan rule that states we are only allowed to implement a trait on a type if either the trait or the type are local to our crate.
It is possible to get around this restriction using the *newtype pattern*.
This involves creating a new type in a tuple struct.
The tuple struct will have one field and be 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 that originates from the Haskell programming language.
There is no runtime performance penalty for using this pattern, and wrapper type is elided at compile time.
For example let's say we want to implement `Display` on `Vec<T>`, which the orphan rule prevents us from doing directly because the `Display` trait and the `Vec<T>` type are defined outside our crate.
We can make a `Wrapper` struct that holds an instance of `Vec<T>`.
Next we can implement `Display` on `Wrapper` and use the `Vec<T>` value.
```rust
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}");
}
```
The implementation of `Display` uses `self.0` to access the inner `Vec<T>`.
Because `Wrapper` is a tuple is a tuple struct and `Vec<T>` is the item at index 0 in the tuple.
Then we can use the functionality of the `Display` trait on `Wrapper`.
The downside of this technique is that `Wrapper` is a new type, so it doesn't have the methods of the value it is holding.
We would need to implement all the methods of `Vec<T>` directly on `Wrapper` such that the methods delegate to `self.0`, which would allows us to treat `Wrapper` exactly like a `Vec<T>`.
If we wanted the new type to have every method the inner type has, implementing the `Deref` trait on the `Wrapper` to return the inner type would be a solution.
If we didn't want the `Wrapper` type to have all the methods of the inner type.
For example, to restrict the `Wrapper` type's behavior we would have to implement just the methods we do want manually.
This newtype pattern is useful even when traits are not involved.

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# Advanced Types