Sized and using traits as sized objects - rust

Question

struct Point { x: f64, y: f64 }
struct Circle { center: Point, radius: f64 }
struct Square { lowerLeftCorner: Point, side: f64 }
trait ShapeVisitor {
fn visit_circle(&mut self, c: &Circle);
fn visit_square(&mut self, c: &Square);
}
trait Shape {
fn accept<V: ShapeVisitor>(&self, sv: &mut V);
}
impl Shape for Circle {
fn accept<V: ShapeVisitor>(&self, sv: &mut V) {
sv.visit_circle(self);
}
}
impl Shape for Square {
fn accept<V: ShapeVisitor>(&self, sv: &mut V) {
sv.visit_square(self);
}
}
So here's a method that computes the total area:
fn area(shapes: Vec<Box<dyn Shape>>) -> f64 {
struct AreaCalculator {
area: f64,
}
impl ShapeVisitor for AreaCalculator {
fn visit_circle(&mut self, c: &Circle) {
self.area += std::f64::consts::PI * c.radius * c.radius;
}
fn visit_square(&mut self, r: &Square) {
self.area += r.side * r.side;
}
}
let mut calculator = AreaCalculator { area: 0.0 };
for shape in shapes {
(*shape).accept(calculator);
}
calculator.area
}
And here's an example program
fn main() {
let mut shapes: Vec<Box<dyn Shape>> = Vec::new();
let p = Point { x: 0.0, y: 0.0 };
let circle = Circle { center: p, radius: 12.0 };
shapes.push(Box::new(circle));
let area = compute_area(shapes);
println!("Area {:?}", area);
}
Here's the error report:
error[E0038]: the trait `Shape` cannot be made into an object
--> visitor.rs:26:25
|
26 | fn compute_area(shapes: Vec<Box<dyn Shape>>) -> f64 {
| ^^^^^^^^^^^^^^^^^^^ `Shape` cannot be made into an object
|
= help: consider moving `accept` to another trait
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> visitor.rs:11:8
|
10 | trait Shape {
| ----- this trait cannot be made into an object...
11 | fn accept<V: ShapeVisitor>(&self, sv: &mut V);
| ^^^^^^ ...because method `accept` has generic type parameters
error: aborting due to previous error
Is there a simple way to rectify it?

Answer 1

In order to be able to create a trait object, the trait must be object safe. But your Shape is not object safe - it has generic type parameters. Removing the type parameters solves the issue:
#[derive(Copy, Clone)]
struct Point {
x: f64,
y: f64,
}
struct Circle {
center: Point,
radius: f64,
}
struct Square {
lowerLeftCorner: Point,
side: f64,
}
trait ShapeVisitor {
fn visit_circle(&mut self, c: &Circle);
fn visit_square(&mut self, c: &Square);
}
trait Shape {
fn accept(&self, sv: &mut dyn ShapeVisitor);
}
impl Shape for Circle {
fn accept(&self, sv: &mut dyn ShapeVisitor) {
sv.visit_circle(self);
}
}
impl Shape for Square {
fn accept(&self, sv: &mut dyn ShapeVisitor) {
sv.visit_square(self);
}
}
fn area(shapes: Vec<Box<dyn Shape>>) -> f64 {
struct AreaCalculator {
area: f64,
}
impl ShapeVisitor for AreaCalculator {
fn visit_circle(&mut self, c: &Circle) {
self.area += std::f64::consts::PI * c.radius * c.radius;
}
fn visit_square(&mut self, r: &Square) {
self.area += r.side * r.side;
}
}
let mut calculator = AreaCalculator { area: 0.0 };
for shape in shapes {
shape.accept(&mut calculator);
}
calculator.area
}
fn main() {
let mut shapes: Vec<Box<dyn Shape>> = Vec::new();
let p = Point { x: 0.0, y: 0.0 };
let circle = Circle {
center: p,
radius: 1.0,
};
shapes.push(Box::new(circle));
shapes.push(Box::new(Square {
lowerLeftCorner: p,
side: 10.0,
}));
let area = area(shapes);
println!("Area {:?}", area);
}