Struct std::marker::PhantomData
1.0.0 · source · [−]pub struct PhantomData<T>
where
T: ?Sized;
Expand description
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,
}
RunThe 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>,
}
RunThis 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,
}
}
RunUnused 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);
}
}
RunOwnership 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
sourceimpl<T> Clone for PhantomData<T> where
T: ?Sized,
impl<T> Clone for PhantomData<T> where
T: ?Sized,
sourcefn clone(&self) -> PhantomData<T>
fn clone(&self) -> PhantomData<T>
Returns a copy of the value. Read more
sourcefn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
Performs copy-assignment from source
. Read more
sourceimpl<T> Debug for PhantomData<T> where
T: ?Sized,
impl<T> Debug for PhantomData<T> where
T: ?Sized,
const: unstable · sourceimpl<T> Default for PhantomData<T> where
T: ?Sized,
impl<T> Default for PhantomData<T> where
T: ?Sized,
const: unstable · sourcefn default() -> PhantomData<T>
fn default() -> PhantomData<T>
Returns the “default value” for a type. Read more
sourceimpl<T> Hash for PhantomData<T> where
T: ?Sized,
impl<T> Hash for PhantomData<T> where
T: ?Sized,
sourceimpl<T> Ord for PhantomData<T> where
T: ?Sized,
impl<T> Ord for PhantomData<T> where
T: ?Sized,
sourceimpl<T> PartialEq<PhantomData<T>> for PhantomData<T> where
T: ?Sized,
impl<T> PartialEq<PhantomData<T>> for PhantomData<T> where
T: ?Sized,
sourceimpl<T> PartialOrd<PhantomData<T>> for PhantomData<T> where
T: ?Sized,
impl<T> PartialOrd<PhantomData<T>> for PhantomData<T> where
T: ?Sized,
sourcefn partial_cmp(&self, _other: &PhantomData<T>) -> Option<Ordering>
fn partial_cmp(&self, _other: &PhantomData<T>) -> Option<Ordering>
This method returns an ordering between self
and other
values if one exists. Read more
sourcefn lt(&self, other: &Rhs) -> bool
fn lt(&self, other: &Rhs) -> bool
This method tests less than (for self
and other
) and is used by the <
operator. Read more
sourcefn le(&self, other: &Rhs) -> bool
fn le(&self, other: &Rhs) -> bool
This method tests less than or equal to (for self
and other
) and is used by the <=
operator. Read more
impl<T> Copy for PhantomData<T> where
T: ?Sized,
impl<T> Eq for PhantomData<T> where
T: ?Sized,
impl<T> StructuralEq for PhantomData<T> where
T: ?Sized,
impl<T> StructuralPartialEq for PhantomData<T> where
T: ?Sized,
Auto Trait Implementations
impl<T: ?Sized> RefUnwindSafe for PhantomData<T> where
T: RefUnwindSafe,
impl<T: ?Sized> Send for PhantomData<T> where
T: Send,
impl<T: ?Sized> Sync for PhantomData<T> where
T: Sync,
impl<T: ?Sized> Unpin for PhantomData<T> where
T: Unpin,
impl<T: ?Sized> UnwindSafe for PhantomData<T> where
T: UnwindSafe,
Blanket Implementations
sourceimpl<T> BorrowMut<T> for T where
T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
const: unstable · sourcefn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
Mutably borrows from an owned value. Read more
sourceimpl<T> ToOwned for T where
T: Clone,
impl<T> ToOwned for T where
T: Clone,
type Owned = T
type Owned = T
The resulting type after obtaining ownership.
sourcefn clone_into(&self, target: &mut T)
fn clone_into(&self, target: &mut T)
Uses borrowed data to replace owned data, usually by cloning. Read more