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Structs, Enums and Traits

Rust Generics

Write a function or struct once and use it with many types, with no cost at run time.

  • Intermediate
  • 3 min read
  • 1 runnable example
What you'll learn
  • Type parameters
  • Trait bounds
  • Generic structs
Run code →
largest<T>
largest(&[3, 9, 4])9
largest(&[2.5, 1.5])2.5
largest(&['x', 'b'])'x'

One definition, many types

Generics let you write code once and use it with different types. A type parameter such as T stands for a type that is chosen where the code is used.

T: PartialOrd + Copy is a trait bound: largest accepts any type that can be compared and copied, so it works with integers, floats and characters. Pair<T> is a generic struct, and its impl block is generic too. The compiler creates a separate version for each type, so generics cost nothing at run time.

Key points

  1. T stands for a type chosen at the call.
  2. T: PartialOrd limits T to comparable types.
  3. The compiler makes a version per type.

Runnable Rust example

Rust
Run code →
main.rs
fn largest<T: PartialOrd + Copy>(items: &[T]) -> T {
    let mut largest = items[0];
    for &item in items {
        if item > largest {
            largest = item;
        }
    }
    largest
}

#[derive(Debug)]
struct Pair<T> {
    left: T,
    right: T,
}

impl<T: Copy> Pair<T> {
    fn swap(&self) -> Pair<T> {
        Pair { left: self.right, right: self.left }
    }
}

fn main() {
    println!("{}", largest(&[3, 9, 4]));
    println!("{}", largest(&[2.5, 1.5]));
    println!("{}", largest(&['x', 'b']));

    let pair = Pair { left: "a", right: "b" };
    println!("{:?}", pair.swap());
}
Get a Run button for this example Result of running the code
9
2.5
x
Pair { left: "b", right: "a" }

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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