Bounds - Rust By Example
Description: Rust by Example (RBE) is a collection of runnable examples that illustrate various Rust concepts and standard libraries.

# Bounds

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: Display>(t: T) {
        println!("{}", t);
    }

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

    struct S<T: Display>(T);

    // Error! `Vec<T>` 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: Debug>(t: &T) {
        println!("{:?}", t);
    }

    // `T` must implement `HasArea`. Any type which meets
    // the bound can access `HasArea`'s function `area`.
    fn area<T: HasArea>(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`.
    }

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

### See also:

`std::fmt`, `struct`s, and `trait`s