Rust by Example

A trait that is generic over its container type has type specification requirements - users of the trait must specify all of its generic types.

In the example below, the Contains trait allows the use of the generic types A and B. The trait is then implemented for the Container type, specifying i32 for A and B so that it can be used with fn difference().

Because Contains is generic, we are forced to explicitly state all of the generic types for fn difference(). In practice, we want a way to express that A and B are determined by the input C. As you will see in the next section, associated types provide exactly that capability.

struct Container(i32, i32);

// A trait which checks if 2 items are stored inside of container.

// Also retrieves first or last value.

trait Contains {

fn contains(&self, _: &A, _: &B) -> bool; // Explicitly requires `A` and `B`.

fn first(&self) -> i32; // Doesn't explicitly require `A` or `B`.

fn last(&self) -> i32;  // Doesn't explicitly require `A` or `B`.

}

impl Contains for Container {

// True if the numbers stored are equal.

fn contains(&self, number_1: &i32, number_2: &i32) -> bool {

(&self.0 == number_1) && (&self.1 == number_2)

}

// Grab the first number.

fn first(&self) -> i32 { self.0 }

// Grab the last number.

fn last(&self) -> i32 { self.1 }

}

// `C` contains `A` and `B`. In light of that, having to express `A` and

// `B` again is a nuisance.

fn difference(container: &C) -> i32 where

C: Contains {

container.last() - container.first()

}

fn main() {

let number_1 = 3;

let number_2 = 10;

let container = Container(number_1, number_2);

println!("Does container contain {} and {}: {}",

&number_1, &number_2,

container.contains(&number_1, &number_2));

println!("First number: {}", container.first());

println!("Last number: {}", container.last());

println!("The difference is: {}", difference(&container));

}

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