Idiomatic callbacks in Rust

In C/C++ I'd normally do callbacks with a plain function pointer, maybe passing a void* userdata parameter too. Something like this:

typedef void (*Callback)();

class Processor
{
public:
    void setCallback(Callback c)
    {
        mCallback = c;
    }

    void processEvents()
    {
        for (...)
        {
            ...
            mCallback();
        }
    }
private:
    Callback mCallback;
};

What is the idiomatic way of doing this in Rust? Specifically, what types should my setCallback() function take, and what type should mCallback be? Should it take an Fn? Maybe FnMut? Do I save it Boxed? An example would be amazing.

Short answer: For maximum flexibility, you can store the callback as a boxed FnMut object, with the callback setter generic on callback type. The code for this is shown in the last example in the answer. For a more detailed explanation, read on.

"Function pointers": callbacks as `fn`

The closest equivalent of the C++ code in the question would be declaring callback as a fn type. fn encapsulates functions defined by the fn keyword, much like C++'s function pointers:

type Callback = fn();

struct Processor {
    callback: Callback,
}

impl Processor {
    fn set_callback(&mut self, c: Callback) {
        self.callback = c;
    }

    fn process_events(&self) {
        (self.callback)();
    }
}

fn simple_callback() {
    println!("hello world!");
}

fn main() {
    let mut p = Processor { callback: simple_callback };
    p.process_events();         // hello world!
}

This code could be extended to include an Option<Box<Any>> to hold the "user data" associated with the function. Even so, it would not be idiomatic Rust. The Rust way to associate data with a function is to capture it in an anonymous closure, just like in modern C++. Since closures are not fn, set_callback will need to accept other kinds of function objects.

Callbacks as generic function objects

In both Rust and C++ closures with the same call signature come in different sizes to accommodate different sizes of the captured values they store in the closure object. Additionally, each closure site generates a distinct anonymous type which is the type of the closure object at compile time. Due to these constraints, it the struct cannot refer to the callback type by name or a type alias.

One way to own a closure in the struct without referring to a concrete type is by making the struct generic. The struct will automatically adapt its size and the type of callback for the concrete function or closure you pass to it:

struct Processor<CB> where CB: FnMut() {
    callback: CB,
}

impl<CB> Processor<CB> where CB: FnMut() {
    fn set_callback(&mut self, c: CB) {
        self.callback = c;
    }

    fn process_events(&mut self) {
        (self.callback)();
    }
}

fn main() {
    let s = "world!".to_string();
    let callback = || println!("hello {}", s);
    let mut p = Processor { callback: callback };
    p.process_events();
}

As before, the new definition of callback will be able to accept top-level functions defined with fn, but this one will also accept closures as || println!("hello world!"), as well as closures that capture values, such as || println!("{}", somevar). Because of this the closure doesn't need a separate userdata argument; it can simply capture the data from its environment and it will be available when it is called.

But what's the deal with the FnMut, why not just Fn? Since closures hold captured values, Rust enforces the same rules on them that it enforces on other container objects. Depending on what the closures do with the values they hold, they are grouped in three families, each marked with a trait:

Fn are closures that only read data, and may be safely called multiple times, possibly from multiple threads. Both above closures are Fn.
FnMut are closures that modify data, e.g. by writing to a captured mut variable. They may also be called multiple times, but not in parallel. (Calling a FnMut closure from multiple threads would lead to a data race, so it can only be done with the protection of a mutex.) The closure object must be declared mutable by the caller.
FnOnce are closures that consume the data they capture, e.g. by moving it to a function that owns them. As the name implies, these may be called only once, and the caller must own them.

Somewhat counter-intuitively, when specifying a trait bound for the type of an object that accepts a closure, FnOnce is actually the most permissive one. Declaring that a generic callback type must satisfy the FnOnce trait means that it will accept literally any closure. But that comes with a price: it means the holder is only allowed to call it once. Since process_events() may opt to invoke the callback multiple times, and as the method itself may be called more than once, the next most permissive bound is FnMut. Note that we had to mark process_events as mutating self.

Non-generic callbacks: function trait objects

Even though the generic implementation of the callback is extremely efficient, it has serious interface limitations. It requires each Processor instance to be parameterized with a concrete callback type, which means that a single Processor can only deal with a single callback type. Given that each closure has a distinct type, the generic Processor cannot handle proc.set_callback(|| println!("hello")) followed by proc.set_callback(|| println!("world")). Extending the struct to support two callbacks fields would require the whole struct to be parameterized to two types, which would quickly become unwieldy as the number of callbacks grows. Adding more type parameters wouldn't work if the number of callbacks needed to be dynamic, e.g. to implement an add_callback function that maintains a vector of different callbacks.

To remove the type parameter, we can take advantage of trait objects, the feature of Rust that allows automatic creation of dynamic interfaces based on traits. This is sometimes referred to as type erasure and is a popular technique in C++[1][2], not to be confused with Java and FP languages' somewhat different use of the term. Readers familiar with C++ will recognize the distinction between a closure that implements Fn and an Fn trait object as equivalent to the distinction between general function objects and std::function values in C++.

A trait object is created by borrowing an object with the & operator and casting or coercing it to a reference to the specific trait. In this case, since Processor needs to own the callback object, we cannot use borrowing, but must store the callback in a heap-allocated Box<Trait> (the Rust equivalent of std::unique_ptr), which is functionally equivalent to a trait object.

If Processor stores Box<FnMut()>, it no longer needs to be generic, but the set_callback method is now generic, so it can properly box whatever callable you give it before storing the box in the Processor. The callback can be of any kind as long as it doesn't consume the captured values. set_callback being generic doesn't incur limitations discussed above, as it doesn't affect the interface of the data stored in the struct.

struct Processor {
    callback: Box<FnMut()>,
}

impl Processor {
    fn set_callback<CB: 'static + FnMut()>(&mut self, c: CB) {
        self.callback = Box::new(c);
    }

    fn process_events(&mut self) {
        (self.callback)();
    }
}

fn simple_callback() {
    println!("hello");
}

fn main() {
    let mut p = Processor { callback: Box::new(simple_callback) };
    p.process_events();
    let s = "world!".to_string();
    let callback2 = move || println!("hello {}", s);
    p.set_callback(callback2);
    p.process_events();
}