Module std::any

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 out 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);
}

Provider and Demand

Provider and the associated APIs support generic, type-driven access to data, and a mechanism for implementers to provide such data. The key parts of the interface are the Provider trait for objects which can provide data, and the request_value and request_ref functions for requesting data from an object which implements Provider. Generally, end users should not call request_* directly, they are helper functions for intermediate implementers to use to implement a user-facing interface. This is purely for the sake of ergonomics, there is no safety concern here; intermediate implementers can typically support methods rather than free functions and use more specific names.

Typically, a data provider is a trait object of a trait which extends Provider. A user will request data from a trait object by specifying the type of the data.

Data flow

• A user requests an object of a specific type, which is delegated to request_value or request_ref
• request_* creates a Demand object and passes it to Provider::provide
• The data provider’s implementation of Provider::provide tries providing values of different types using Demand::provide_*. If the type matches the type requested by the user, the value will be stored in the Demand object.
• request_* unpacks the Demand object and returns any stored value to the user.

Examples

use std::any::{Provider, Demand, request_ref};

// Definition of MyTrait, a data provider.
trait MyTrait: Provider {
    // ...
}

// Methods on `MyTrait` trait objects.
impl dyn MyTrait + '_ {
    /// Get a reference to a field of the implementing struct.
    pub fn get_context_by_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
        request_ref::<T>(self)
    }
}

// Downstream implementation of `MyTrait` and `Provider`.
impl MyTrait for SomeConcreteType {
    // ...
}

impl Provider for SomeConcreteType {
    fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
        // Provide a string reference. We could provide multiple values with
        // different types here.
        demand.provide_ref::<String>(&self.some_string);
    }
}

// Downstream usage of `MyTrait`.
fn use_my_trait(obj: &dyn MyTrait) {
    // Request a &String from obj.
    let _ = obj.get_context_by_ref::<String>().unwrap();
}

In this example, if the concrete type of obj in use_my_trait is SomeConcreteType, then the get_context_by_ref call will return a reference to obj.some_string with type &String.

Structs

DemandExperimental

A helper object for providing data by type.

TypeId

A TypeId represents a globally unique identifier for a type.

Traits

ProviderExperimental

Trait implemented by a type which can dynamically provide values based on type.

Any

A trait to emulate dynamic typing.

Functions

request_refExperimental

Request a reference from the Provider.

request_valueExperimental

Request a value from the Provider.

type_name_of_valExperimental

Returns the name of the type of the pointed-to value as a string slice. This is the same as type_name::<T>(), but can be used where the type of a variable is not easily available.

type_name

Returns the name of a type as a string slice.