Boyang Yan

rust Generics

2 min read

Generics

Rust support generics, which lets you abstract algorithms or data structures (such as sorting or a binary tree) over the types used or stored.

Generic Data Types

You can use generics to abstract over the concrete field type:

#[derive(Debug)]
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 };
    println!("{integer:?} and {float:?}");
}
Point { x: 5, y: 10 } and Point { x: 1.0, y: 4.0 }

Notes

  • Try declaring a new variable let p = Point { x: 5, y: 10.0 };.
  • Fix the code to allow points that have elements of different types.

Generic Methods

You can declare a generic type on your impl block:

#[derive(Debug)]
struct Point<T>(T, T);
 
impl<T> Point<T> {
    fn x(&self) -> &T {
        &self.0  // + 10
    }
 
    // fn set_x(&mut self, x: T)
}
 
fn main() {
    let p = Point(5, 10);
    println!("p.x = {}", p.x());
}
p.x = 5

Notes Q: Why T is specified twice in impl<T> Point<T> {}? Isn’t that redundant?

  • This is because it is a generic implementation section for generic type. They are independently generic.
  • It means these methods are defined for any T.
  • It is possible to write impl Point<u32> { .. }.
    • Point is still generic and you can use Point<f64>, but methods in this block will only be available for Point<u32>.

Monomorphization

Generic code is turned into non-generic code based on the call sites:

fn main() {
    let integer = Some(5);
    let float = Some(5.0);
}

behaves as if you wrote

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);
}

This is a zero-cost abstraction: you get exactly the same result as if you had hand-coded the data structures without the abstraction.

Reference List

  1. https://google.github.io/comprehensive-rust/generics.html

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