Rust
Summary - Trait bounds
Combine trait bounds and where to write a generic max function.
Trait bounds summary
A trait bound defines a generic type's behaviour: <T: Display>, several bounds with +, long bounds with where. impl Trait in a parameter is shorthand for a generic; in return position it means a single hidden type. dyn Trait is runtime dispatch; it holds different types in the same collection and is used behind a pointer. The default choice is generics; use dyn when you need a mixed collection or a plugin structure.
Finding the largest area
Iterator methods can be used on trait objects too. Since f64 isn't Ord, the largest is found with fold instead of max:
fn en_buyuk_alan(v: &[Box<dyn Sekil>]) -> f64 {
v.iter().map(|s| s.alan()).fold(0.0, f64::max)
}
f64::max, a function taking two arguments, can be passed directly to fold.
Exercise Largest area
With the Sekil, Kare and Daire definitions from the previous lesson, write fn en_buyuk_alan(v: &[Box<dyn Sekil>]) -> f64. Print the result with {:.2} for a list containing Kare(2.0), Kare(3.0) and Daire(1.0).
Show example solution
trait Sekil {
fn alan(&self) -> f64;
fn ad(&self) -> String;
}
struct Kare(f64);
struct Daire(f64);
impl Sekil for Kare {
fn alan(&self) -> f64 {
self.0 * self.0
}
fn ad(&self) -> String {
String::from("kare")
}
}
impl Sekil for Daire {
fn alan(&self) -> f64 {
std::f64::consts::PI * self.0 * self.0
}
fn ad(&self) -> String {
String::from("daire")
}
}
fn en_buyuk_alan(v: &[Box<dyn Sekil>]) -> f64 {
v.iter().map(|s| s.alan()).fold(0.0, f64::max)
}
fn main() {
let v: Vec<Box<dyn Sekil>> = vec![Box::new(Kare(2.0)), Box::new(Kare(3.0)), Box::new(Daire(1.0))];
println!("{:.2}", en_buyuk_alan(&v));
}
Test yourself
-
How is a <code>Vec<Box<dyn Sekil>></code> passed to a function taking <code>&[Box<dyn Sekil>]</code>?
- With &v
- With v
- v.into()
- It can't be
Show answer
With &v — &Vec<T> converts to &[T] automatically (deref coercion).
-
Largest by folding
v.iter().map(|s| s.alan()).____(0.0, f64::max)
Show answer
v.iter().map(|s| s.alan()).fold(0.0, f64::max)
-
Group summary
- <T: A + B>
- Multiple bounds
- where
- Readable bounds
- -> impl Trait
- Single hidden type
- Box<dyn Trait>
- Dynamic dispatch
Next lesson: Crate skeleton
