Struct std::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<'a, T>(vec: &'a Vec<T>) -> Slice<'a, T> {
    let ptr = vec.as_ptr();
    Slice {
        start: ptr,
        end: unsafe { ptr.offset(vec.len() as isize) },
        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> PartialEq<PhantomData<T>> for PhantomData<T> where T: ?Sized [src]

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

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

impl<T> Default for PhantomData<T> where T: ?Sized [src]

fn default() -> PhantomData<T>

impl<T> Clone for PhantomData<T> where T: ?Sized [src]

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

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

impl<T> Eq for PhantomData<T> where T: ?Sized [src]

impl<T> Debug for PhantomData<T> where T: ?Sized [src]

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

impl<T> Ord for PhantomData<T> where T: ?Sized [src]

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

impl<T> Copy for PhantomData<T> where T: ?Sized [src]

impl<T> Hash for PhantomData<T> where T: ?Sized [src]

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

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

1.3.0

impl<T> PartialOrd<PhantomData<T>> for PhantomData<T> where T: ?Sized [src]

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

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

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

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

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