Module 12: Traits

Practice Tasks

Complete these tasks to reinforce what you learned in this module.

Task 1: Define and implement a trait

Define a trait Describe with one method fn describe(&self) -> String. Implement it for a struct Dog with field name: String so that describe() returns format!("Dog named {}", self.name). In main, create a Dog, call describe(), and print the result.

Expected Output:

Hint: Define the trait with one method; implement it for a struct that has a name field. The theory shows trait definition and impl Trait for Type.

Starter Code:

task.rs

// Task 1: Describe trait and Dog. See tasks/TASKS.md.

fn main() {
    todo!("Define Describe, Dog, impl Describe for Dog, call describe()");
}

Task 2: Trait bound

Write a generic function print_describe<T: Describe>(x: &T) that prints the result of x.describe(). Use the same Describe and Dog from Task 1 (or redefine them). Call print_describe with a Dog in main.

Expected Output:

Hint: The generic function needs a trait bound so it can call .describe() on x. The theory explains trait bounds with T: Trait.

Starter Code:

task.rs

// Task 2: print_describe with trait bound. See tasks/TASKS.md.

fn main() {
    todo!("Define Describe, Dog, print_describe<T: Describe>, call with Dog");
}

Task 3: Conversions with `From`, `Into`, and `TryFrom`

Define two newtype structs Celsius(i32) and Fahrenheit(i32). Implement From<Celsius> for Fahrenheit using the formula c * 9 / 5 + 32. Implement TryFrom<i32> for Celsius whose error is a String: return Err(format!("{value} is below absolute zero")) when value < -273, otherwise Ok(Celsius(value)). Write a function fn convert(raw: i32) -> Result<Fahrenheit, String> that builds a Celsius with Celsius::try_from(raw)? and then converts it with .into(). In main, call convert(100) and convert(-300), printing "{raw}C = {}F" (with the inner i32) on success and "error: {e}" on failure.

Expected Output:

Hint: Implement From and you get Into for free, so c.into() works once the target type is known. TryFrom returns a Result, so ? can early-return its error. The theory file theory/02_conversions.md covers From/Into and TryFrom.

Starter Code:

task.rs

// Task 3: From / Into / TryFrom conversions. See tasks/TASKS.md.

fn main() {
    todo!("Define Celsius/Fahrenheit, impl From + TryFrom, write convert(), print results");
}

Task 4: A heterogeneous collection with trait objects

The Shape trait and two types (Square, Rectangle) with area() are provided in the editor. In main, put a Square { side: 3 } and a Rectangle { w: 2, h: 4 } into a single Vec<Box<dyn Shape>>, then sum their areas and print total area = <sum>.

Expected Output:

Hint: A Vec<Box<dyn Shape>> can hold different concrete types behind the Shape trait — wrap each value in Box::new(...). Then shapes.iter().map(|s| s.area()).sum::<i32>(). (This is C#'s List<IShape>.) See theory/03_trait_objects.md.

Starter Code:

task.rs

// Task 4: store different shapes in one Vec<Box<dyn Shape>> and sum their areas.
// See tasks/TASKS.md and theory/03_trait_objects.md.

trait Shape {
    fn area(&self) -> i32;
}

struct Square {
    side: i32,
}
struct Rectangle {
    w: i32,
    h: i32,
}

impl Shape for Square {
    fn area(&self) -> i32 {
        self.side * self.side
    }
}
impl Shape for Rectangle {
    fn area(&self) -> i32 {
        self.w * self.h
    }
}

fn main() {
    // YOUR CODE HERE: build a Vec<Box<dyn Shape>> containing a Square { side: 3 }
    // and a Rectangle { w: 2, h: 4 }, sum their areas, and print "total area = <sum>".
    todo!("build a Vec<Box<dyn Shape>> and print the total area");
}

// Task 4: store different shapes in one Vec<Box<dyn Shape>> and sum their areas. // See tasks/TASKS.md and theory/03_trait_objects.md. trait Shape { fn area(&self) -> i32; } struct Square { side: i32, } struct Rectangle { w: i32, h: i32, } impl Shape for Square { fn area(&self) -> i32 { self.side * self.side } } impl Shape for Rectangle { fn area(&self) -> i32 { self.w * self.h } } fn main() { // YOUR CODE HERE: build a Vec<Box<dyn Shape>> containing a Square { side: 3 } // and a Rectangle { w: 2, h: 4 }, sum their areas, and print "total area = <sum>". todo!("build a Vec<Box<dyn Shape>> and print the total area"); }