Utilities for dynamic typing or type reflection.

Any and TypeId

Any itself can be used to get a TypeId, and has more features when used as a trait object. As &dyn Any (a borrowed trait object), it has the is and downcast_ref methods, to test if the contained value is of a given type, and to get a reference to the inner value as a type. As &mut dyn Any, there is also the downcast_mut method, for getting a mutable reference to the inner value. Box<dyn Any> adds the downcast method, which attempts to convert to a Box<T>. See the Box documentation for the full details.

Note that &dyn Any is limited to testing whether a value is of a specified concrete type, and cannot be used to test whether a type implements a trait.

Smart pointers and dyn Any

One piece of behavior to keep in mind when using Any as a trait object, especially with types like Box<dyn Any> or Arc<dyn Any>, is that simply calling .type_id() on the value will produce the TypeId of the container, not the underlying trait object. This can be avoided by converting the smart pointer into a &dyn Any instead, which will return the object's TypeId. For example:

use std::any::{Any, TypeId};

let boxed: Box<dyn Any> = Box::new(3_i32);

// You're more likely to want this:
let actual_id = (&*boxed).type_id();
// ... than this:
let boxed_id = boxed.type_id();

assert_eq!(actual_id, TypeId::of::<i32>());
assert_eq!(boxed_id, TypeId::of::<Box<dyn Any>>());

Examples

Consider a situation where we want to log a value passed to a function. We know the value we're working on implements Debug, but we don't know its concrete type. We want to give special treatment to certain types: in this case printing out the length of String values prior to their value. We don't know the concrete type of our value at compile time, so we need to use runtime reflection instead.

use std::fmt::Debug;
use std::any::Any;

// Logger function for any type that implements `Debug`.
fn log<T: Any + Debug>(value: &T) {
    let value_any = value as &dyn Any;

    // Try to convert our value to a `String`. If successful, we want to
    // output the `String`'s length as well as its value. If not, it's a
    // different type: just print it out unadorned.
    match value_any.downcast_ref::<String>() {
        Some(as_string) => {
            println!("String ({}): {}", as_string.len(), as_string);
        }
        None => {
            println!("{value:?}");
        }
    }
}

// This function wants to log its parameter out prior to doing work with it.
fn do_work<T: Any + Debug>(value: &T) {
    log(value);
    // ...do some other work
}

fn main() {
    let my_string = "Hello World".to_string();
    do_work(&my_string);

    let my_i8: i8 = 100;
    do_work(&my_i8);
}