Struct wasmer_types::lib::std::marker::PhantomData 1.0.0[−][src]
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,
}
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
type Archived = PhantomData<T>
type Archived = PhantomData<T>
The archived version of this type.
The resolver for this type. It must contain all the information needed to make the archived type from the normal type. Read more
pub fn resolve(
&self,
usize,
<PhantomData<T> as Archive>::Resolver,
&mut MaybeUninit<<PhantomData<T> as Archive>::Archived>
)
pub fn resolve(
&self,
usize,
<PhantomData<T> as Archive>::Resolver,
&mut MaybeUninit<<PhantomData<T> as Archive>::Archived>
)
Creates the archived version of this value at the given position and writes it to the given output. Read more
Returns the “default value” for a type. Read more
pub fn deserialize<D>(
deserializer: D
) -> Result<PhantomData<T>, <D as Deserializer<'de>>::Error> where
D: Deserializer<'de>,
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. Read more
impl<T, D> Deserialize<PhantomData<T>, D> for PhantomData<T> where
D: Fallible + ?Sized,
T: ?Sized,
impl<T, D> Deserialize<PhantomData<T>, D> for PhantomData<T> where
D: Fallible + ?Sized,
T: ?Sized,
Deserializes using the given deserializer
type Value = T
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>,
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. Read more
Returns the size of the referenced value in bytes. Read more
This method returns an ordering between self
and other
values if one exists. Read more
This method tests less than (for self
and other
) and is used by the <
operator. Read more
This method tests less than or equal to (for self
and other
) and is used by the <=
operator. Read more
This method tests greater than (for self
and other
) and is used by the >
operator. Read more
pub fn serialize<S>(
&self,
serializer: S
) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error> where
S: Serializer,
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. Read more
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
type ArchivedMetadata = ()
type ArchivedMetadata = ()
The archived version of the pointer metadata for this type.
Converts some archived metadata to the pointer metadata for itself.
The archived counterpart of this type. Unlike Archive
, it may be unsized.
type MetadataResolver = ()
type MetadataResolver = ()
The resolver for the metadata of this type.
pub fn resolve_metadata(
&self,
usize,
<T as ArchiveUnsized>::MetadataResolver,
&mut MaybeUninit<<<T as ArchiveUnsized>::Archived as ArchivePointee>::ArchivedMetadata>
)
pub fn resolve_metadata(
&self,
usize,
<T as ArchiveUnsized>::MetadataResolver,
&mut MaybeUninit<<<T as ArchiveUnsized>::Archived as ArchivePointee>::ArchivedMetadata>
)
Creates the archived version of the metadata for this value at the given position and writes it to the given output. Read more
fn resolve_unsized(
&self,
from: usize,
to: usize,
resolver: Self::MetadataResolver,
out: &mut MaybeUninit<RelPtr<Self::Archived>>
)
fn resolve_unsized(
&self,
from: usize,
to: usize,
resolver: Self::MetadataResolver,
out: &mut MaybeUninit<RelPtr<Self::Archived>>
)
Resolves a relative pointer to this value with the given from
and to
and writes it to
the given output. Read more
Mutably borrows from an owned value. Read more
Compare self to key
and return true
if they are equal.
Writes the object and returns the position of the archived type.
pub fn serialize_metadata(
&self,
&mut S
) -> Result<<<T as ArchiveUnsized>::Archived as ArchivePointee>::ArchivedMetadata, <S as Fallible>::Error>
pub fn serialize_metadata(
&self,
&mut S
) -> Result<<<T as ArchiveUnsized>::Archived as ArchivePointee>::ArchivedMetadata, <S as Fallible>::Error>
Serializes the metadata for the given type.