How do I store different types that implement the same trait in a vector and call common functions on them?

问题

I'm learning Rust and I'm having difficulties in implementing polymorphism. I want to use an array to store either Circle or Test.

trait Poli {
    fn area(&self) -> f64;
}

struct Circle {
    x:      f64,
    y:      f64,
    radius: f64,
}

impl Circle {
    fn new (xx: f64, yy: f64, r: f64) -> Circle{
        Circle{ x: xx, y: yy, radius: r }
    }
}

impl Poli for Circle {
   fn area(&self) -> f64 {
       std::f64::consts::PI * (self.radius * self.radius)
   }
}

struct Test {
    x:      f64,
    y:      f64,
    radius: f64,
    test:   f64,
}

impl Test {
    fn new (xx: f64, yy: f64, r: f64, t: f64) -> Circle{
        Test{ x: xx, y: yy, radius: r, test: t, }
    }
}

impl Poli for Test {
    fn area(&self) -> f64 {
        std::f64::consts::PI * (self.radius * self.radius)
    }
}

I do not know how to make a vector to store types with the same trait:

let cir  = Circle::new(10f64, 10f64, 10f64);
let test = Test::new(10f64, 10f64, 10f64, 10f64);

//let mut vec: Vec<Poli> = Vec::new();   <---

I'd like to iterate the vector and call functions from the trait. Is there any way to do this, or some alternative?

I read the trait object documentation but I think it's not what I want.

回答1:

As mentioned in the page you linked to, you'll need to either store the Poli implementing structs as Vec<&Poli> or Vec<Box<Poli>> depending on whether you want to own the values or just store a reference:

// Owned
let circle = Circle::new(10f64, 10f64, 10f64);
let test = Test::new(10f64, 10f64, 10f64, 10f64);

let polis = vec![Box::new(circle) as Box<Poli>, Box::new(test) as Box<Poli>];

for poli in polis {
    println!("{}", poli.area());
}

// Reference
let circle = Circle::new(10f64, 10f64, 10f64);
let test = Test::new(10f64, 10f64, 10f64, 10f64);

let polis = vec![&circle as &Poli, &test as &Poli];

for poli in polis {
    println!("{}", poli.area());
}

Output

314.1592653589793
314.1592653589793
314.1592653589793
314.1592653589793

Demo

回答2:

After Dogbert's response, I played a bit and found this way works for me. I am new to Rust and I do not know if this is obvious, but I'll leave it here to help others.

let circle = Circle::new(10f64, 10f64, 10f64);
let test   = Test::new(10f64, 10f64, 10f64, 10f64);

let mut vec: Vec<&Poli> = Vec::<&Poli>::new();
    vec.push(&circle);
    vec.push(&test);

for v in vec {
    println!("new {}", v.area());
}

Demo

来源：https://stackoverflow.com/questions/36357995/how-do-i-store-different-types-that-implement-the-same-trait-in-a-vector-and-cal

标签

polymorphism

rust

traits