How would I use trait objects for function callbacks?

Question

I'm trying to wrap my head around trait objects and how I can use them. One scenario is I might want to pass a function for a callback, when some condition is met that callback is called.

fn bind_callback( key: u64, /* pass function */) {
    // when key is matched with the event, call the function
}

How can I do this though? I hear I can use trait objects for something like this, but how would I go about implementing this? Can someone show me an example? Here's what I'm at:

trait Callback {
    fn callback(self);
}

fn pass_callback(f: &Callback) {
    f.callback();
}

fn run_me() {
    println!("Hello World!");
}

fn main() {
    pass_callback(&run_me); // run simple no arg void ret function
    pass_callback(|| println!("Hello World!")); // same thing
}

I know this is terribly wrong, I'm trying to understand how I would accomplish something like this. My error output is:

<anon>:14:19: 14:26 error: the trait `Callback` is not implemented for the type `fn() {run_me}` [E0277]
<anon>:14     pass_callback(&run_me);
                            ^~~~~~~
<anon>:14:19: 14:26 help: see the detailed explanation for E0277
<anon>:14:19: 14:26 note: required for the cast to the object type `Callback`
<anon>:15:19: 15:46 error: mismatched types:
 expected `&Callback`,
    found `[closure@<anon>:15:19: 15:46]`
(expected &-ptr,
    found closure) [E0308]
<anon>:15     pass_callback(|| println!("Hello World!"));
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:15:19: 15:46 help: see the detailed explanation for E0308
error: aborting due to 2 previous errors
playpen: application terminated with error code 101

Answer 1

If you want to use closures and functions as parameters, you cannot use your own traits. Instead, you use one of the Fn* family:

fn pass_callback<F>(f: F)
    where F: Fn()
{
    f();
}

fn run_me() {
    println!("Hello World!");
}

fn main() {
    pass_callback(run_me);
    pass_callback(|| println!("Hello World!"));
}

• Fn
• FnOnce
• FnMut

---

If you really want to use your own trait, you can, but then you need to implement the trait on something and pass that item into your function:

trait Callback {
    fn callback(&self, value: u8) -> bool;
}

struct IsEven;
impl Callback for IsEven {
    fn callback(&self, value: u8) -> bool {
        value % 2 == 0
    }
}

fn pass_callback<C>(f: C)
    where C: Callback
{
    if f.callback(42) {
        println!("Callback passed");
    }
}

fn main() {
    pass_callback(IsEven);
}

---

As an aside, your comment

no arg void ret function

isn't quite true. Rust doesn't have a void "type", it has the empty tuple, often called the unit type.

Answer 2

The reason your code doesn't work is that the function run_me and closure || println!("Hello World!") don't have implementations for your callback trait. You can fix this by adding a Callback implementation for all types implementing the Fn() trait:

trait Callback {
    fn callback(&self);
}

fn pass_callback(f: &Callback) {
    f.callback();
}

fn run_me() {
    println!("Hello World!");
}

// For every type T that implements Fn(),
// this is the Callback implementation.
impl<T: Fn()> Callback for T {
    fn callback(&self) {
        self()
    }
}

fn main() {
    pass_callback(&run_me); // a simple function
    pass_callback(&|| println!("Hello World!")); // a closure
}