Multiple bounds

Combine multiple trait bounds with +.

Multiple bounds

If a type parameter needs several behaviours, the bounds are combined with +:

use std::fmt::Display;

fn buyuk_olan<T: PartialOrd + Display>(a: T, b: T) -> String {
    if a > b {
        format!("{a}")
    } else {
        format!("{b}")
    }
}

PartialOrd is needed for comparing, Display for formatting. The order doesn't matter.

PartialOrd vs Ord

PartialOrd allows some values to be incomparable: f64::NAN < 1.0 and f64::NAN > 1.0 are both false. Ord requires a total order; integers and strings are Ord, floats are not.

let mut v = vec![3, 1, 2];
v.sort();                 // needs Ord: i32 is fine
let mut f = vec![2.5, 1.0];
// f.sort();              // error: f64 isn't Ord
f.sort_by(|a, b| a.total_cmp(b));

Exercise Print the larger

Write the buyuk_olan function above. Print the results for (3, 8) and ("elma", "armut") separated by a space.

Show example solution

use std::fmt::Display;

fn buyuk_olan<T: PartialOrd + Display>(a: T, b: T) -> String {
    if a > b {
        format!("{a}")
    } else {
        format!("{b}")
    }
}

fn main() {
    println!("{} {}", buyuk_olan(3, 8), buyuk_olan("elma", "armut"));
}

Test yourself

What does <code>buyuk_olan("elma", "armut")</code> return?
- "elma"
- "armut"
- Compile error
- "elmaarmut"
Show answer

"elma" — Texts compare in lexicographic order; since "e" > "a", elma is greater.

Combine bounds

fn buyuk_olan<T: PartialOrd ____ Display>(a: T, b: T) -> String

Show answer

fn buyuk_olan<T: PartialOrd + Display>(a: T, b: T) -> String

Type and what it implements

i32

Ord + Copy
f64

PartialOrd (not Ord)
&str

Ord + Display
Vec<i32>

Ord, not Display

Next lesson: where clause

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