How do I create a heterogeneous collection of obje

Question

I want to use trait objects in a Vec. In C++ I could make a base class Thing from which is derived Monster1 and Monster2. I could then create a std::vector<Thing*>. Thing objects must store some data e.g. x : int, y : int, but derived classes need to add more data.

Currently I have something like

struct Level {
    // some stuff here
    pub things: Vec<Box<ThingTrait + 'static>>,
}

struct ThingRecord {
    x: i32,
    y: i32,
}

struct Monster1 {
    thing_record: ThingRecord,
    num_arrows: i32,
}

struct Monster2 {
    thing_record: ThingRecord,
    num_fireballs: i32,
}

I define a ThingTrait with methods for get_thing_record(), attack(), make_noise() etc. and implement them for Monster1 and Monster2.

Answer 1

Trait objects

The most extensible way to implement a heterogeneous collection (in this case a vector) of objects is exactly what you have:

Vec<Box<ThingTrait + 'static>>

Although there are times where you might want a lifetime that's not 'static, so you'd need something like:

Vec<Box<ThingTrait + 'a>>

You could also have a collection of references to traits, instead of boxed traits:

Vec<&ThingTrait>

An example:

trait ThingTrait {
    fn attack(&self);
}

impl ThingTrait for Monster1 {
    fn attack(&self) {
        println!("monster 1 attacks")
    }
}

impl ThingTrait for Monster2 {
    fn attack(&self) {
        println!("monster 2 attacks")
    }
}

fn main() {
    let m1 = Monster1 {
        thing_record: ThingRecord { x: 42, y: 32 },
        num_arrows: 2,
    };

    let m2 = Monster2 {
        thing_record: ThingRecord { x: 42, y: 32 },
        num_fireballs: 65,
    };

    let things: Vec<Box<ThingTrait>> = vec![Box::new(m1), Box::new(m2)];
}

Box<SomeTrait>, Rc<SomeTrait>, &SomeTrait, etc. are all trait objects. These allow implementation of the trait on an infinite number of types, but the tradeoff is that it requires some amount of indirection and dynamic dispatch.

See also:

• What makes something a "trait object"?

Enums

As mentioned in the comments, if you have a fixed number of known alternatives, a less open-ended solution is to use an enum. This doesn't require that the values be Boxed, but it will still have a small amount of dynamic dispatch to decide which concrete enum variant is present at runtime:

enum Monster {
    One(Monster1),
    Two(Monster2),
}

impl Monster {
    fn attack(&self) {
        match *self {
            Monster::One(_) => println!("monster 1 attacks"),
            Monster::Two(_) => println!("monster 2 attacks"),
        }
    }
}

fn main() {
    let m1 = Monster1 {
        thing_record: ThingRecord { x: 42, y: 32 },
        num_arrows: 2,
    };

    let m2 = Monster2 {
        thing_record: ThingRecord { x: 42, y: 32 },
        num_fireballs: 65,
    };

    let things = vec![Monster::One(m1), Monster::Two(m2)];
}