Struct frame_support::dispatch::marker::PhantomData

#[lang = "phantom_data"]pub struct PhantomData<T>
 where
    T: ?Sized;

Zero-sized type used to mark things that "act like" they own a T.

Adding a PhantomData<T> field to your type tells the compiler that your type acts as though it stores a value of type T, even though it doesn't really. This information is used when computing certain safety properties.

For a more in-depth explanation of how to use PhantomData<T>, please see the Nomicon.

A ghastly note 👻👻👻

Though they both have scary names, PhantomData and 'phantom types' are related, but not identical. A phantom type parameter is simply a type parameter which is never used. In Rust, this often causes the compiler to complain, and the solution is to add a "dummy" use by way of PhantomData.

Examples

Unused lifetime parameters

Perhaps the most common use case for PhantomData is a struct that has an unused lifetime parameter, typically as part of some unsafe code. For example, here is a struct Slice that has two pointers of type *const T, presumably pointing into an array somewhere:

struct Slice<'a, T> {
    start: *const T,
    end: *const T,
}

The intention is that the underlying data is only valid for the lifetime 'a, so Slice should not outlive 'a. However, this intent is not expressed in the code, since there are no uses of the lifetime 'a and hence it is not clear what data it applies to. We can correct this by telling the compiler to act as if the Slice struct contained a reference &'a T:

use std::marker::PhantomData;

struct Slice<'a, T: 'a> {
    start: *const T,
    end: *const T,
    phantom: PhantomData<&'a T>,
}

This also in turn requires the annotation T: 'a, indicating that any references in T are valid over the lifetime 'a.

When initializing a Slice you simply provide the value PhantomData for the field phantom:

fn borrow_vec<T>(vec: &Vec<T>) -> Slice<'_, T> {
    let ptr = vec.as_ptr();
    Slice {
        start: ptr,
        end: unsafe { ptr.add(vec.len()) },
        phantom: PhantomData,
    }
}

Unused type parameters

It sometimes happens that you have unused type parameters which indicate what type of data a struct is "tied" to, even though that data is not actually found in the struct itself. Here is an example where this arises with FFI. The foreign interface uses handles of type *mut () to refer to Rust values of different types. We track the Rust type using a phantom type parameter on the struct ExternalResource which wraps a handle.

use std::marker::PhantomData;
use std::mem;

struct ExternalResource<R> {
   resource_handle: *mut (),
   resource_type: PhantomData<R>,
}

impl<R: ResType> ExternalResource<R> {
    fn new() -> ExternalResource<R> {
        let size_of_res = mem::size_of::<R>();
        ExternalResource {
            resource_handle: foreign_lib::new(size_of_res),
            resource_type: PhantomData,
        }
    }

    fn do_stuff(&self, param: ParamType) {
        let foreign_params = convert_params(param);
        foreign_lib::do_stuff(self.resource_handle, foreign_params);
    }
}

Ownership and the drop check

Adding a field of type PhantomData<T> indicates that your type owns data of type T. This in turn implies that when your type is dropped, it may drop one or more instances of the type T. This has bearing on the Rust compiler's drop check analysis.

If your struct does not in fact own the data of type T, it is better to use a reference type, like PhantomData<&'a T> (ideally) or PhantomData<*const T> (if no lifetime applies), so as not to indicate ownership.

Trait Implementations

impl<T> Clone for PhantomData<T> where
    T: ?Sized, 

fn clone(&self) -> PhantomData<T>

Returns a copy of the value.

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

Performs copy-assignment from source.

impl<T> Copy for PhantomData<T> where
    T: ?Sized, 

impl<T> Debug for PhantomData<T> where
    T: ?Sized, 

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

Formats the value using the given formatter.

impl<T> Decode for PhantomData<T>

fn decode<I>(_input: &mut I) -> Result<PhantomData<T>, Error> where
    I: Input, 

Attempt to deserialise the value from input.

impl<T> Default for PhantomData<T> where
    T: ?Sized, 

fn default() -> PhantomData<T>

Returns the "default value" for a type.

impl<'de, T> Deserialize<'de> for PhantomData<T> where
    T: ?Sized, 

fn deserialize<D>(
    deserializer: D
) -> Result<PhantomData<T>, <D as Deserializer<'de>>::Error> where
    D: Deserializer<'de>, 

Deserialize this value from the given Serde deserializer.

impl<'de, T> DeserializeSeed<'de> for PhantomData<T> where
    T: Deserialize<'de>, 

type Value = T

The type produced by using this seed.

fn deserialize<D>(
    self,
    deserializer: D
) -> Result<T, <D as Deserializer<'de>>::Error> where
    D: Deserializer<'de>, 

Equivalent to the more common Deserialize::deserialize method, except with some initial piece of data (the seed) passed in.

impl<T> Encode for PhantomData<T>

fn encode_to<W>(&self, _dest: &mut W) where
    W: Output, 

Convert self to a slice and append it to the destination.

fn size_hint(&self) -> usize

If possible give a hint of expected size of the encoding.

fn encode(&self) -> Vec<u8>

Convert self to an owned vector.

fn using_encoded<R, F>(&self, f: F) -> R where
    F: FnOnce(&[u8]) -> R, 

Convert self to a slice and then invoke the given closure with it.

impl<T> EncodeLike<PhantomData<T>> for PhantomData<T>

impl<T> Eq for PhantomData<T> where
    T: ?Sized, 

impl<T> Hash for PhantomData<T> where
    T: ?Sized, 

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

Feeds this value into the given Hasher.

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

Feeds a slice of this type into the given Hasher.

impl<T> MallocSizeOf for PhantomData<T>

fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize

Measure the heap usage of all descendant heap-allocated structures, but not the space taken up by the value itself.

impl<T> Ord for PhantomData<T> where
    T: ?Sized, 

fn cmp(&self, _other: &PhantomData<T>) -> Ordering

This method returns an Ordering between self and other.

#[must_use]fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

#[must_use]fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

#[must_use]fn clamp(self, min: Self, max: Self) -> Self

🔬 This is a nightly-only experimental API. (clamp)

Restrict a value to a certain interval.

impl<T> PartialEq<PhantomData<T>> for PhantomData<T> where
    T: ?Sized, 

fn eq(&self, _other: &PhantomData<T>) -> bool

This method tests for self and other values to be equal, and is used by ==.

#[must_use]fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl<T> PartialOrd<PhantomData<T>> for PhantomData<T> where
    T: ?Sized, 

fn partial_cmp(&self, _other: &PhantomData<T>) -> Option<Ordering>

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

#[must_use]fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

#[must_use]fn le(&self, other: &Rhs) -> bool

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

#[must_use]fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

#[must_use]fn ge(&self, other: &Rhs) -> bool

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

impl<T> Serialize for PhantomData<T> where
    T: ?Sized, 

fn serialize<S>(
    &self,
    serializer: S
) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error> where
    S: Serializer, 

Serialize this value into the given Serde serializer.