Rust by Example

When working with generics, the type parameters often must use traits as bounds to stipulate what functionality a type implements. For example, the following example uses the trait Display to print and so it requires T to be bound by Display; that is, T must implement Display.

// Define a function `printer` that takes a generic type `T` which

// must implement trait `Display`.

fn printer(t: T) {

println!("{}", t);

}

Bounding restricts the generic to types that conform to the bounds. That is:

struct S(T);

// Error! `Vec` does not implement `Display`. This

// specialization will fail.

let s = S(vec![1]);

Another effect of bounding is that generic instances are allowed to access the methods of traits specified in the bounds. For example:

// A trait which implements the print marker: `{:?}`.

use std::fmt::Debug;

trait HasArea {

fn area(&self) -> f64;

}

impl HasArea for Rectangle {

fn area(&self) -> f64 { self.length * self.height }

}

#[derive(Debug)]

struct Rectangle { length: f64, height: f64 }

#[allow(dead_code)]

struct Triangle  { length: f64, height: f64 }

// The generic `T` must implement `Debug`. Regardless

// of the type, this will work properly.

fn print_debug(t: &T) {

println!("{:?}", t);

}

// `T` must implement `HasArea`. Any type which meets

// the bound can access `HasArea`'s function `area`.

fn area(t: &T) -> f64 { t.area() }

fn main() {

let rectangle = Rectangle { length: 3.0, height: 4.0 };

let _triangle = Triangle  { length: 3.0, height: 4.0 };

print_debug(&rectangle);

println!("Area: {}", area(&rectangle));

//print_debug(&_triangle);

//println!("Area: {}", area(&_triangle));

// ^ TODO: Try uncommenting these.

// | Error: Does not implement either `Debug` or `HasArea`.

}

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As an additional note, where clauses can also be used to apply bounds in some cases to be more expressive.