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() -> Self {
        let size_of_res = mem::size_of::<R>();
        Self {
            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, 

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

Returns a copy of the value.

pub 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, 

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

Formats the value using the given formatter.

impl<T> Decode for PhantomData<T>

pub 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, 

pub fn default() -> PhantomData<T>

Returns the “default value” for a type.

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

pub 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.

pub 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>

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

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

pub fn size_hint(&self) -> usize

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

pub fn encode(&self) -> Vec<u8, Global>

Convert self to an owned vector.

pub 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, 

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

Feeds this value into the given Hasher.

pub 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>

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

Measure the heap usage of all descendant heap-allocated structures, but not the space taken up by the value itself. If T::size_of is a constant, consider implementing constant_size as well.

pub fn constant_size() -> Option<usize>

Used to optimize MallocSizeOf implementation for collections like Vec and HashMap to avoid iterating over them unnecessarily. The Self: Sized bound is for object safety.

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

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

This method returns an Ordering between self and other.

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

Compares and returns the maximum of two values.

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

Compares and returns the minimum of two values.

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

Restrict a value to a certain interval.

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

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

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

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

This method tests for !=.

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

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

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

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

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

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

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

#[must_use]pub 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, 

pub 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.

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

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