Juan Rodriguez
Senior Software Engineer - M.Sc. in Computer Science.
Generic Types, Traits, and Lifetimes
by: sjdonado
#backend-chapter - Rust. 2021
Duplicating code is tedious and error prone
- Operate with abstract types
- Make sure that a type has the behavior we want
- Ensure that references are valid as long as we need them to be
Generic Data Types == Generics
- In Function Definitions
- In Struct Definitions
- In Enum Definitions
- In Method Definitions
- Monomorphization
In Function Definitions
fn largest_i32(list: &[i32]) -> &i32 {
let mut largest = &list[0];
for item in list {
if item > largest {
largest = item;
}
}
largest
}
fn largest_char(list: &[char]) -> &char {
let mut largest = &list[0];
for item in list {
if item > largest {
largest = item;
}
}
largest
}
fn main() {
let number_list = vec![34, 50, 25, 100, 65];
let result = largest_i32(&number_list);
println!("The largest number is {}", result);
assert_eq!(result, &100);
let char_list = vec!['y', 'm', 'a', 'q'];
let result = largest_char(&char_list);
println!("The largest char is {}", result);
assert_eq!(result, &'y');
}
In Function Definitions
fn largest<T>(list: &[T]) -> &T {
let mut largest = &list[0];
for item in list {
if item > largest {
largest = item;
}
}
largest
}
fn main() {
let number_list = vec![34, 50, 25, 100, 65];
let result = largest(&number_list);
println!("The largest number is {}", result);
let char_list = vec!['y', 'm', 'a', 'q'];
let result = largest(&char_list);
println!("The largest char is {}", result);
}🤔
In Struct Definitions
struct Point<T> {
x: T,
y: T,
}
fn main() {
let integer = Point { x: 5, y: 10 };
let float = Point { x: 1.0, y: 4.0 };
}
!=
struct Point<T, U> {
x: T,
y: U,
}
fn main() {
let both_integer = Point { x: 5, y: 10 };
let both_float = Point { x: 1.0, y: 4.0 };
let integer_and_float = Point { x: 5, y: 4.0 };
}
In Enum Definitions
enum Option<T> {
Some(T),
None,
}enum Result<T, E> {
Ok(T),
Err(E),
}In Method Definitions
struct Point<T, U> {
x: T,
y: U,
}
impl<T, U> Point<T, U> {
fn x(&self) -> &T {
&self.x
}
fn mixup<V, W>(self, other: Point<V, W>) -> Point<T, W> {
Point {
x: self.x,
y: other.y,
}
}
}
fn main() {
let p1 = Point { x: 5, y: 10.4 };
let p2 = Point { x: "Hello", y: 'c' };
let p3 = p1.mixup(p2);
}
impl Point<f32> {
fn distance_from_origin(&self) -> f32 {
(self.x.powi(2) + self.y.powi(2)).sqrt()
}
}
Monomorphization
enum Option<T> {
Some(T),
None,
}
fn main() {
let integer = Some(5);
let float = Some(5.0);
}
enum Option_i32 {
Some(i32),
None,
}
enum Option_f64 {
Some(f64),
None,
}
fn main() {
let integer = Option_i32::Some(5);
let float = Option_f64::Some(5.0);
}
Traits ≅ interfaces
- Implementing a Trait on a Type
- Orphan rule
- Default Implementations
- Traits as Parameters
- Returning Types that Implement Traits
- Using Trait Bounds to Conditionally Implement Methods
Implementing a Trait on a Type
pub trait Summary {
fn summarize(&self) -> String;
}
pub struct NewsArticle {
pub headline: String,
pub location: String,
pub author: String,
pub content: String,
}
impl Summary for NewsArticle {
fn summarize(&self) -> String {
format!("{}, by {} ({})", self.headline, self.author, self.location)
}
}
pub struct Tweet {
pub username: String,
pub content: String,
pub reply: bool,
pub retweet: bool,
}
impl Summary for Tweet {
fn summarize(&self) -> String {
format!("{}: {}", self.username, self.content)
}
}
fn main() {
let tweet = Tweet {
username: String::from("horse_ebooks"),
content: String::from(
"of course, as you probably already know, people",
),
reply: false,
retweet: false,
};
println!("1 new tweet: {}", tweet.summarize());
}
Orphan rule
We can’t implement external traits on external types
- Implement std::fmt::Display on a custom type like Tweet ✅
- Implement Summary on Vec<T> ✅
- Implement the Display trait on Vec<T> ❌
Examples in our same crate:
Default Implementations
pub trait Summary {
fn summarize_author(&self) -> String;
fn summarize(&self) -> String {
format!("(Read more from {}...)", self.summarize_author())
}
}
pub struct Tweet {
pub username: String,
pub content: String,
pub reply: bool,
pub retweet: bool,
}
impl Summary for Tweet {
fn summarize_author(&self) -> String {
format!("@{}", self.username)
}
}
fn main() {
let tweet = Tweet {
username: String::from("horse_ebooks"),
content: String::from(
"of course, as you probably already know, people",
),
reply: false,
retweet: false,
};
println!("1 new tweet: {}", tweet.summarize());
}
Traits as Parameters
pub fn notify(item: &impl Summary) {
println!("Breaking news! {}", item.summarize());
}
pub fn notify(item1: &impl Summary, item2: &impl Summary) {
pub fn notify<T: Summary>(item1: &T, item2: &T) {
Trait Bound Syntax
Specifying Multiple Trait Bounds with the + Syntax
pub fn notify(item: &(impl Summary + Display)) {pub fn notify<T: Summary + Display>(item: &T) {
==
Traits as Parameters
Clearer Trait Bounds with where Clauses
fn some_function<T: Display + Clone, U: Clone + Debug>(t: &T, u: &U) -> i32 {
fn some_function<T, U>(t: &T, u: &U) -> i32
where T: Display + Clone,
U: Clone + Debug
{
Returning Types that Implement Traits
fn returns_summarizable() -> impl Summary {
Tweet {
username: String::from("horse_ebooks"),
content: String::from(
"of course, as you probably already know, people",
),
reply: false,
retweet: false,
}
}
Using Trait Bounds to Conditionally Implement Methods
impl<T: Display + PartialOrd> Pair<T> {
fn cmp_display(&self) {
if self.x >= self.y {
println!("The largest member is x = {}", self.x);
} else {
println!("The largest member is y = {}", self.y);
}
}
}impl<T: Display> ToString for T {
// --snip--
}
let s = 3.to_string();
Similar to extend:
Lifetime -> references
- The Borrow Checker
- Lifetime Annotations in Function Signatures
- Lifetime Annotations in Struct Definitions
- Lifetime Elision
- The Static Lifetime
We must annotate types when multiple types are possible. In a similar way, we must annotate lifetimes when the lifetimes of references could be related in a few different ways
The Borrow Checker
{
let r; // ---------+-- 'a
// |
{ // |
let x = 5; // -+-- 'b |
r = &x; // | |
} // -+ |
// |
println!("r: {}", r); // |
} Preventing Dangling References with Lifetimes
{
let x = 5; // ----------+-- 'b
// |
let r = &x; // --+-- 'a |
// | |
println!("r: {}", r); // | |
// --+ |
} // ----------+
Lifetime Annotations in Function Signatures
fn longest(x: &str, y: &str) -> &str {
if x.len() > y.len() {
x
} else {
y
}
}fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
if x.len() > y.len() {
x
} else {
y
}
}fn main() {
let string1 = String::from("long string is long");
let result;
{
let string2 = String::from("xyz");
result = longest(string1.as_str(), string2.as_str());
}
println!("The longest string is {}", result);
}🤔
Lifetime Annotations in Struct Definitions
struct ImportantExcerpt<'a> {
part: &'a str,
}
fn main() {
let novel = String::from("Call me Ishmael. Some years ago...");
let first_sentence = novel.split('.').next().expect("Could not find a '.'");
let i = ImportantExcerpt {
part: first_sentence,
};
}
Lifetime Elision
fn first_word(s: &str) -> &str {
First rule
fn first_word<'a>(s: &'a str) -> &str {
fn first_word<'a>(s: &'a str) -> &str {
fn first_word(s: &str) -> &str {
let bytes = s.as_bytes();
...
}Second rule
fn first_word<'a>(s: &'a str) -> &str {
fn first_word<'a>(s: &'a str) -> &'a str {
Lifetime Elision
struct ImportantExcerpt<'a> {
part: &'a str,
}
impl<'a> ImportantExcerpt<'a> {
fn announce_and_return_part(&self, announcement: &str) -> &str {
println!("Attention please: {}", announcement);
self.part
}
}Third rule
The Static Lifetime
fn main() {
let s: &'static str = "I have a static lifetime.";
}
Generic Type Parameters, Trait Bounds, and Lifetimes Together
use std::fmt::Display;
fn longest_with_an_announcement<'a, T>(
x: &'a str,
y: &'a str,
ann: T,
) -> &'a str
where
T: Display,
{
println!("Announcement! {}", ann);
if x.len() > y.len() {
x
} else {
y
}
}
Generic Types, Traits, and Lifetimes
By Juan Rodriguez
Generic Types, Traits, and Lifetimes
- 1,059
