What makes something a "trait object"?

Question

Recent Rust changes have made "trait objects" more prominent to me, but I only have a nebulous grasp of what actually makes something into a trait object. One change in particular is the upcoming change to allow trait objects to forward trait implementations to the inner type.

Given a trait Foo, I'm pretty sure that Box<Foo> / Box<dyn Foo> is a trait object. Is &Foo / &dyn Foo also a trait object? What about other smart-pointer things like Rc or Arc? How could I make my own type that would count as a trait object?

The reference only mentions trait objects once, but nothing like a definition.

Answer 1

You have trait objects when you have a pointer to a trait. Box, Arc, Rc and the reference & are all, at their core, pointers. In terms of defining a "trait object" they work in the same way.

"Trait objects" are Rust's take on dynamic dispatch. Here's an example that I hope helps show what trait objects are:

// define an example struct, make it printable
#[derive(Debug)]
struct Foo;

// an example trait
trait Bar {
    fn baz(&self);
}

// implement the trait for Foo
impl Bar for Foo {
    fn baz(&self) {
        println!("{:?}", self)
    }
}

// This is a generic function that takes any T that implements trait Bar.
// It must resolve to a specific concrete T at compile time.
// The compiler creates a different version of this function
// for each concrete type used to call it so &T here is NOT
// a trait object (as T will represent a known, sized type
// after compilation)
fn static_dispatch<T>(t: &T)
where
    T: Bar,
{
    t.baz(); // we can do this because t implements Bar
}

// This function takes a pointer to a something that implements trait Bar
// (it'll know what it is only at runtime). &dyn Bar is a trait object.
// There's only one version of this function at runtime, so this
// reduces the size of the compiled program if the function
// is called with several different types vs using static_dispatch.
// However performance is slightly lower, as the &dyn Bar that
// dynamic_dispatch receives is a pointer to the object +
// a vtable with all the Bar methods that the object implements.
// Calling baz() on t means having to look it up in this vtable.
fn dynamic_dispatch(t: &dyn Bar) {
    // ----------------^
    // this is the trait object! It would also work with Box<dyn Bar> or
    // Rc<dyn Bar> or Arc<dyn Bar>
    //
    t.baz(); // we can do this because t implements Bar
}

fn main() {
    let foo = Foo;
    static_dispatch(&foo);
    dynamic_dispatch(&foo);
}

For further reference, there is a good Trait Objects chapter of the Rust book

Answer 2

Short Answer: You can only make object-safe traits into trait objects.

Object-Safe Traits: Traits that do not resolve to concrete type of implementation. In practice two rules govern if a trait is object-safe.

1. The return type isn’t Self.
2. There are no generic type parameters.

Any trait satisfying these two rules can be used as trait objects.

Example of trait that is object-safe can be used as trait object:

trait Draw {
    fn draw(&self);
}

Example of trait that cannot be used as trait object:

trait Draw {
    fn draw(&self) -> Self;
}

For detailed explanation: https://doc.rust-lang.org/book/second-edition/ch17-02-trait-objects.html

Answer 3

Trait objects are the Rust implementation of dynamic dispatch. Dynamic dispatch allows one particular implementation of a polymorphic operation (trait methods) to be chosen at run time. Dynamic dispatch allows a very flexible architecture because we can swap function implementations out at runtime. However, there is a small runtime cost associated with dynamic dispatch.

The variables/parameters which hold the trait objects are fat pointers which consists of the following components:

• pointer to the object in memory
• pointer to that object’s vtable, a vtable is a table with pointers which point to the actual method(s) implementation(s).

Example

struct Point {
    x: i64,
    y: i64,
    z: i64,
}

trait Print {
    fn print(&self);
}

// dyn Print is actually a type and we can implement methods on it
impl dyn Print + 'static {
    fn print_traitobject(&self) {
        println!("from trait object");
    }
}

impl Print for Point {
    fn print(&self) {
        println!("x: {}, y: {}, z: {}", self.x, self.y, self.z);
    }
}

// static dispatch (compile time): compiler must know specific versions
// at compile time generates a version for each type

// compiler will use monomorphization to create different versions of the function
// for each type. However, because they can be inlined, it generally has a faster runtime
// compared to dynamic dispatch
fn static_dispatch<T: Print>(point: &T) {
    point.print();
}

// dynamic dispatch (run time): compiler doesn't need to know specific versions
// at compile time because it will use a pointer to the data and the vtable.
// The vtable contains pointers to all the different different function implementations.
// Because it has to do lookups at runtime it is generally slower compared to static dispatch

// point_trait_obj is a trait object
fn dynamic_dispatch(point_trait_obj: &(dyn Print + 'static)) {
    point_trait_obj.print();
    point_trait_obj.print_traitobject();
}

fn main() {
    let point = Point { x: 1, y: 2, z: 3 };

    // On the next line the compiler knows that the generic type T is Point
    static_dispatch(&point);

    // This function takes any obj which implements Print trait
    // We could, at runtime, change the specfic type as long as it implements the Print trait
    dynamic_dispatch(&point);
}