In this example, we will create a trait that will contain the signature for two functions: calc_perimeter and calc_area. To start with, we construct a struct. In this case, we will have two structs:

struct Perimeter 
{
  side_one: i32,
  side_two: i32, 
}

struct Oval 
{
  radius: f32,
  height: f32,
}

We need to create a trait for each. The general format for a trait looks like this:

trait TraitName 
{ 
  fn function_name(&self) -> return_type; 
}

In our case, we would have the following:

trait CalcPerimeter
{
  fn calc_perimeter(&self) -> i32; 
}

trait CalcArea 
{
  fn calc_area(&self) -> f32; 
}

We now need to create an implementation for both of these traits. The impl, though, will not look quite the same.

Before, we had the following:

impl SomeImplement
{ 
  ...
}

This time, we have to give the name of the struct it relates to:

impl SomeImplement for MyStruct 
{
  ...
}

If the impl defines the trait and the trait is just a stub, why do we need to say which struct it is for?

This is a fair question.

Without the trait, an impl operates in a similar way to a function. We supply the parameters to the impl via &self. When we have a trait, the impl has to say what &self refers to.

Our impl for the first trait will be as follows:

impl CalcPerimeter for Perimeter 
{ 
  fn calc_perimeter(&self) -> i32 
  {
    self.side_one * 2 + self.side_two * 2 
  }
}

Note that the function can access side_one and side_two from the Perimeter struct.

The second impl will look like this:

impl CalcArea for Oval
{ 
  fn calc_area(&self) -> f32 
  { 
    3.1415927f32 * self.radius * self.height 
  }
}

Finally, the calls to the implementations. Unlike previous examples, both of the structures have to be initialized and then the implementation call given:

fn main() 
{ 
  let peri = Perimeter 
  {
    side_one: 5, side_two: 12 };
    println!("Side 1 = 5, Side 2 = 12, Perimeter = {}", 
              peri.calc_perimeter());
    let area = Oval 
    {
      radius: 5.1f32,
      height: 12.3f32
    };
    println!("Radius = 5.1, Height = 12.3, Area = {}", 
              area.calc_area()); }

Once the code has been compiled, the expected answer is as shown in the following screenshot: