wiki/content/20200917161757-generics.md

date	id	title
20200917	9062c9c0-b2d5-4719-b15b-6fe5122b6a9e	Rust Generics

Function definitions

We read the following definition as: the function largest is generic over some type T. This function has one parameter named list, which is a slice of values of type T. The largest function will return a reference to a value of the same type T.

fn largest<T>(list: &[T]) -> &T {
    let mut largest = list[0];

    for item in list {
        if item > largest {
            largest = item;
        }
    }

    largest
}

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 };
}

A struct can also have multiple generic types:

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 };
}

Enum Definitions

#![allow(unused)]
fn main() {
    enum Option<T> {
        Some(T),
        None,
    }
}

And again with multiple generics:

#![allow(unused)]
fn main() {
    enum Result<T, E> {
        Ok(T),
        Err(E),
    }
}

Method Definitions

struct Point<T> {
    x: T,
    y: T,
}

impl<T> Point<T> {
    fn x(&self) -> &T {
        &self.x
    }
    fn y(&self) -> &T {
        &self.y
    }
}

fn main() {
    let p = Point { x: 5, y: 10 };

    println!("p.x = {} and p.y={}", p.x(), p.y());
}

And again with multiple generics:

struct Point<T, U> {
    x: T,
    y: U,
}

impl<T, U> Point<T, U> {
    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);

    println!("p3.x = {}, p3.y = {}", p3.x, p3.y);
}

Performance of Code Using Generics

The Rust compiler is very very clever and using generics has no performance penalty