shapes.md

Shapes

In this exercise we're going to define methods for a struct, define and implement a trait, and look into how to make structs and traits generic.

Learning Goals

You will learn how to:

• implement methods for a struct
• when to use Self, self, &self and &mut self in methods
• define a trait with required methods
• make a type generic over T
• how to constrain T

Tasks

Part 1: Defining Methods for Types

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1. Make a new library project called shapes

2. Make two structs, Circle with field radius and Square with field side to use as types. Decide on appropriate types for radius and side.

3. Make an impl block and implement the following methods for each type. Consider when to use self, &self, &mut self and Self.

   • fn new(...) -> ...

     • creates an instance of the shape with a certain size (radius or side length).

   • fn area(...) -> ...

     • calculates the area of the shape.

   • fn scale(...)

     • changes the size of an instance of the shape.

   • fn destroy(...) -> ...

     • destroys the instance of a shape and returns the value of its field.

Part 2: Defining and Implementing a Trait

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1. Define a Trait HasArea with a mandatory method: fn area(&self) -> f32.
2. Implement HasArea for Square and Circle. You can defer to the existing method but may need to cast the return type.
3. Abstract over Circle and Square by defining an enum Shape that contains both as variants.
4. Implement HasArea for Shape.

Part 3: Making Square generic over T

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We want to make Square and Circle generic over T, so we can use other numeric types and not just u32 and f32.

1. Add the generic type parameter <T> to Square. You can temporarily remove enum Shape to make this easier.

2. Import the num crate, version 0.4.0, to use the num::Num trait as bound for the generic type <T>. This assures that T is a numeric type, and also makes some guarantees about operations that can be performed on <T>.

3. Add a where clause on the methods of Square, as required, e.g.:

   where T: num::Num

4. Depending on the operations performed in that function, you may need to add further trait bounds, such as Copy and std::ops::MulAssign. You can add them to the where clause with a + sign, like T: num::Num + Copy.

5. Add the generic type parameter <T> to Circle and then appropriate where clauses.

6. Re-introduce Shape but with the generic type parameter <T>, and then add appropriate where clauses.

Help

This section gives partial solutions to look at or refer to.

We also recommend using the official Rust documentation to figure out unfamiliar concepts. If you ever feel completely stuck, or if you haven’t understood something specific, please hail the trainers quickly.

Getting Started

Create a new library Cargo project, check the build and see if it runs:

$ cargo new --lib shapes
$ cd shapes
$ cargo run

Creating a Type

Each of your shape types (Square, Circle, etc.) will need some fields (or properties) to identify its geometry. Use /// to add documentation to each field.

/// Describes a human individual
struct Person {
    /// How old this person is
    age: u8
}

Functions that take arguments: self, &self, &mut self

Does your function need to take ownership of the shape in order to calculate its area? Or is it sufficient to merely take a read-only look at the shape for a short period of time?

You can pass arguments by reference in Rust by making your function take x: &MyShape, and passing them with &my_shape.

You can also associate your function with a specific type by placing it inside a block like impl MyShape { ... }

impl Pentagon {
    fn area(&self) -> u32 {
        // calculate the area of the pentagon here...
    }
}

A Shape of many geometries

You can use an enum to provide a single type that can be any of your supported shapes. If we were working with fruit, we might say:

struct Banana { ... }
struct Apple { ... }

enum Fruit {
    Banana(Banana),
    Apple(Apple),
}

If you wanted to count the pips in a piece of Fruit, you might just call to the num_pips() method on the appropriate constituent fruit. This might look like:

impl Fruit {
    fn num_pips(&self) -> u8 {
        match self {
            Fruit::Apple(apple) => apple.num_pips(),
            Fruit::Banana(banana) => banana.num_pips(),
        }
    }
}

I need a π

The f32 type also has its own module in the standard library called std::f32. If you look at the docs, you will find a defined constant for π: std::f32::consts::PI.

I need a π, of type T

If you want to convert a Pi constant to some type T, you need a where bound like:

where T: num::Num + From<f32>

This restricts T to values that can be converted from an f32 (or, types you can convert an f32 into). You can then call let my_pi: T = my_f32_pi.into(); to convert your f32 value into a T value.

Defining a Trait

A trait has a name, and lists function definitions that make guarantees about the name of a method, its arguments and return types.

pub trait Color {
    fn red() -> u8;
}

Adding generic Type parameters

pub struct Square<T> {
    /// The length of one side of the square
    side: T,
}

impl<T> Square<T> {
    // ...
}