TypeId in core::any - Rust
Description: A `TypeId` represents a globally unique identifier for a type.

# Struct TypeId

1.6.0 · Source

    pub struct TypeId { /* private fields */ }

Expand description

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§

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### impl TypeId

1.0.0 (const: unstable) · Source

#### pub fn of<T: ?Sized + 'static>() -> TypeId

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§

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### impl Clone for TypeId

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#### fn clone(&self) -> TypeId

Returns a copy of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl Debug for TypeId

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

Formats the value using the given formatter. Read more
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### impl Hash for TypeId

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

Feeds this value into the given `Hasher`. Read more
1.3.0 · Source§

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

Feeds a slice of this type into the given `Hasher`. Read more
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### impl Ord for TypeId

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

This method returns an `Ordering` between `self` and `other`. Read more
1.21.0 · Source§

#### fn max(self, other: Self) -> Self
where Self: Sized,

Compares and returns the maximum of two values. Read more
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#### fn min(self, other: Self) -> Self
where Self: Sized,

Compares and returns the minimum of two values. Read more
1.50.0 · Source§

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

Restrict a value to a certain interval. Read more
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### impl PartialEq for TypeId

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

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Rhs) -> bool

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

### impl PartialOrd for TypeId

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

This method returns an ordering between `self` and `other` values if one exists. Read more
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#### fn lt(&self, other: &Rhs) -> bool

Tests less than (for `self` and `other`) and is used by the `<` operator. Read more
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#### fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
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#### fn gt(&self, other: &Rhs) -> bool

Tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
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#### fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
1.0.0 · Source§

### impl Copy for TypeId

1.0.0 · Source§

### impl Eq for TypeId

## Auto Trait Implementations§

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### impl Freeze for TypeId

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### impl RefUnwindSafe for TypeId

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### impl Send for TypeId

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### impl Sync for TypeId

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### impl Unpin for TypeId

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### impl UnwindSafe for TypeId

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.