Struct TypeId

pub struct TypeId { /* private fields */ }

A TypeId represents a globally unique identifier for a type.

Each TypeId is an opaque object which does not allow inspection of what’s inside but does allow basic operations such as cloning, comparison, printing, and showing.

A TypeId is currently only available for types which ascribe to 'static, but this limitation may be removed in the future.

While TypeId implements Hash, PartialOrd, and Ord, it is worth noting that the hashes and ordering will vary between Rust releases. Beware of relying on them inside of your code!

Danger of Improper Variance

You might think that subtyping is impossible between two static types, but this is false; there exists a static type with a static subtype. To wit, fn(&str), which is short for for<'any> fn(&'any str), and fn(&'static str), are two distinct, static types, and yet, fn(&str) is a subtype of fn(&'static str), since any value of type fn(&str) can be used where a value of type fn(&'static str) is needed.

This means that abstractions around TypeId, despite its 'static bound on arguments, still need to worry about unnecessary and improper variance: it is advisable to strive for invariance first. The usability impact will be negligible, while the reduction in the risk of unsoundness will be most welcome.

Examples

Suppose SubType is a subtype of SuperType, that is, a value of type SubType can be used wherever a value of type SuperType is expected. Suppose also that CoVar<T> is a generic type, which is covariant over T (like many other types, including PhantomData<T> and Vec<T>).

Then, by covariance, CoVar<SubType> is a subtype of CoVar<SuperType>, that is, a value of type CoVar<SubType> can be used wherever a value of type CoVar<SuperType> is expected.

Then if CoVar<SuperType> relies on TypeId::of::<SuperType>() to uphold any invariants, those invariants may be broken because a value of type CoVar<SuperType> can be created without going through any of its methods, like so:

type SubType = fn(&());
type SuperType = fn(&'static ());
type CoVar<T> = Vec<T>; // imagine something more complicated

let sub: CoVar<SubType> = CoVar::new();
// we have a `CoVar<SuperType>` instance without
// *ever* having called `CoVar::<SuperType>::new()`!
let fake_super: CoVar<SuperType> = sub;

The following is an example program that tries to use TypeId::of to implement a generic type Unique<T> that guarantees unique instances for each Unique<T>, that is, and for each type T there can be at most one value of type Unique<T> at any time.

mod unique {
    use std::any::TypeId;
    use std::collections::BTreeSet;
    use std::marker::PhantomData;
    use std::sync::Mutex;

    static ID_SET: Mutex<BTreeSet<TypeId>> = Mutex::new(BTreeSet::new());

    // TypeId has only covariant uses, which makes Unique covariant over TypeAsId 🚨
    #[derive(Debug, PartialEq)]
    pub struct Unique<TypeAsId: 'static>(
        // private field prevents creation without `new` outside this module
        PhantomData<TypeAsId>,
    );

    impl<TypeAsId: 'static> Unique<TypeAsId> {
        pub fn new() -> Option<Self> {
            let mut set = ID_SET.lock().unwrap();
            (set.insert(TypeId::of::<TypeAsId>())).then(|| Self(PhantomData))
        }
    }

    impl<TypeAsId: 'static> Drop for Unique<TypeAsId> {
        fn drop(&mut self) {
            let mut set = ID_SET.lock().unwrap();
            (!set.remove(&TypeId::of::<TypeAsId>())).then(|| panic!("duplicity detected"));
        }
    }
}

use unique::Unique;

// `OtherRing` is a subtype of `TheOneRing`. Both are 'static, and thus have a TypeId.
type TheOneRing = fn(&'static ());
type OtherRing = fn(&());

fn main() {
    let the_one_ring: Unique<TheOneRing> = Unique::new().unwrap();
    assert_eq!(Unique::<TheOneRing>::new(), None);

    let other_ring: Unique<OtherRing> = Unique::new().unwrap();
    // Use that `Unique<OtherRing>` is a subtype of `Unique<TheOneRing>` 🚨
    let fake_one_ring: Unique<TheOneRing> = other_ring;
    assert_eq!(fake_one_ring, the_one_ring);

    std::mem::forget(fake_one_ring);
}

Implementations

impl TypeId

pub fn of<T>() -> TypeId

where T: 'static + ?Sized,

Returns the TypeId of the generic type parameter.

Examples

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

fn is_string<T: ?Sized + Any>(_s: &T) -> bool {
    TypeId::of::<String>() == TypeId::of::<T>()
}

assert_eq!(is_string(&0), false);
assert_eq!(is_string(&"cookie monster".to_string()), true);

Trait Implementations

impl Clone for TypeId

fn clone(&self) -> TypeId

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for TypeId

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl Hash for TypeId

fn hash<H>(&self, state: &mut H)

where H: Hasher,

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Ord for TypeId

fn cmp(&self, other: &TypeId) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl PartialEq for TypeId

fn eq(&self, other: &TypeId) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl PartialOrd for TypeId

fn partial_cmp(&self, other: &TypeId) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl Copy for TypeId

impl Eq for TypeId