Summary - Traits

Combine trait and impl to write a Summary trait.

Traits summary

A trait is a contract of behaviour: trait T { fn m(&self); }, implemented with impl T for Type. Methods can have default bodies that can be overridden. Parameters ask for behaviour with &impl T or <X: T>; + combines several traits. The orphan rule: either the trait or the type must be yours. Common traits are implemented automatically with #[derive(..)]: Debug, Clone, PartialEq, Default, Hash.

Ready-made traits with derive

Many standard traits don't need to be written by hand:

#[derive(Debug, Clone, PartialEq, Default)]
struct Nokta {
    x: i32,
    y: i32,
}

let a = Nokta { x: 1, y: 2 };
let b = a.clone();
println!("{:?} {}", b, a == b);           // Nokta { x: 1, y: 2 } true
println!("{:?}", Nokta::default());       // Nokta { x: 0, y: 0 }

Copy can only be derived if all fields are Copy, and it's written together with Clone.

Exercise Derived traits

Define Nokta { x: i32, y: i32 } with #[derive(Debug, Clone, PartialEq)]. Let a = Nokta { x: 1, y: 2 } and b = a.clone(); print b with {:?} and the result of a == b, separated by a space.

Show example solution

#[derive(Debug, Clone, PartialEq)]
struct Nokta {
    x: i32,
    y: i32,
}

fn main() {
    let a = Nokta { x: 1, y: 2 };
    let b = a.clone();
    println!("{:?} {}", b, a == b);
}

Test yourself

What does <code>#[derive(Clone)]</code> give a struct?
- == comparison
- {:?} printing
- A .clone() method copying all fields
- Automatic Copy behaviour
Show answer

A .clone() method copying all fields — derive(Clone) generates an implementation calling clone on every field; for that, all fields must be Clone.

Add comparison

#[derive(Debug, ____)]
struct Nokta { x: i32, y: i32 }
// a == b artık yazılabilir

Show answer

#[derive(Debug, PartialEq)]
struct Nokta { x: i32, y: i32 }
// a == b artık yazılabilir

derive and what it gives

Debug

{:?}
Clone

.clone()
PartialEq

== and !=
Default

Type::default()

Next lesson: Generic function

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