# 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`.

```rust,ignore
// 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:

```rust,ignore
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:

```rust,editable
// 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`][where] clauses can also be used to apply bounds in
some cases to be more expressive.

### See also:

[`std::fmt`][fmt], [`struct`s][structs], and [`trait`s][traits]

[fmt]: ../hello/print.md
[methods]: ../fn/methods.md
[structs]: ../custom_types/structs.md
[traits]: ../trait.md
[where]: ../generics/where.md
