Using a trait as field’s type

It is possible to use a trait as a field’s type inside a structure. This allows to store in this field any object whose type implements the trait. To do that we need:

To use the dyn keyword, in order to state that we use a Dynamically Sized Type (DST). Only the trait is specified, not the real type of the object that will be stored. The real type, and thus its size, will only be known at compile time.
To use a reference (&) on the dyn Trait. The reference is a fat pointer, that contains a pointer to the real object, the object’s size, a pointer to the vtable (virtual table) that contains the available methods of the object and its traits.
Use a lifetime specifier to link the lifetime of the pointed object to the lifetime of the structure.

As an example, we will use a structure for a drawing in which we store a single shape as a trait. The trait provides a draw() method that we will use to simulate the drawing on screen.

Let us start with the trait definition:

trait Shape {
  fn draw(&self);
}

We define a Rectangle structure:

struct Rectangle {
  x: i32,
  y: i32,
  width: u32,
  height: u32,
}

With its implementation of the Shape trait that just prints a message:

impl Shape for Rectangle {
  fn draw(&self) {
    println!("Drawing rectangle {}x{} at ({}, {}).",
             self.width, self.height, self.x, self.y);
  }
}

We now define a structure Drawing that stores a single shape as a trait:

struct Drawing<'a> {
  shape: &'a (dyn Shape + 'a),
}

We implement a draw() method for it that calls the draw() method of the Shape object:

impl<'a> Drawing<'a> {
  fn draw(&self) {
    self.shape.draw();
  }
}

Let us instantiate a Rectangle object:

let rect = Rectangle { x: 4, y: 6, width: 20, height: 30 };

We also instantiate a Drawing object using our Rectangle object for the field’s value. The draw() method of Drawing calls the draw() method of the Shape object without knowing anything about its concrete type:

{
  let drawing = Drawing { shape: &rect };
  drawing.draw();
}

Drawing rectangle 20x30 at (4, 6).

()

see solution

32.1 Using a trait as field’s type