Struct frame_support::pallet_prelude::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,
}
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
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>
sourcefn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
source
. Read moresourceimpl<T> Debug for PhantomData<T>where
T: ?Sized,
impl<T> Debug for PhantomData<T>where
T: ?Sized,
impl<T> Decode for PhantomData<T>
impl<T> Decode for PhantomData<T>
fn decode<I>(_input: &mut I) -> Result<PhantomData<T>, Error>where
I: Input,
fn decode<I>(_input: &mut I) -> Result<PhantomData<T>, Error>where
I: Input,
fn skip<I>(input: &mut I) -> Result<(), Error>where
I: Input,
fn skip<I>(input: &mut I) -> Result<(), Error>where
I: Input,
fn encoded_fixed_size() -> Option<usize>
fn encoded_fixed_size() -> Option<usize>
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>
sourceimpl<'de, T> Deserialize<'de> for PhantomData<T>where
T: ?Sized,
impl<'de, T> Deserialize<'de> for PhantomData<T>where
T: ?Sized,
sourcefn deserialize<D>(
deserializer: D
) -> Result<PhantomData<T>, <D as Deserializer<'de>>::Error>where
D: Deserializer<'de>,
fn deserialize<D>(
deserializer: D
) -> Result<PhantomData<T>, <D as Deserializer<'de>>::Error>where
D: Deserializer<'de>,
sourceimpl<'de, T> DeserializeSeed<'de> for PhantomData<T>where
T: Deserialize<'de>,
impl<'de, T> DeserializeSeed<'de> for PhantomData<T>where
T: Deserialize<'de>,
type Value = T
type Value = T
sourcefn deserialize<D>(
self,
deserializer: D
) -> Result<T, <D as Deserializer<'de>>::Error>where
D: Deserializer<'de>,
fn deserialize<D>(
self,
deserializer: D
) -> Result<T, <D as Deserializer<'de>>::Error>where
D: Deserializer<'de>,
Deserialize::deserialize
method, except
with some initial piece of data (the seed) passed in. Read moreimpl<T> Encode for PhantomData<T>
impl<T> Encode for PhantomData<T>
fn encode_to<W>(&self, _dest: &mut W)where
W: Output + ?Sized,
fn encode_to<W>(&self, _dest: &mut W)where
W: Output + ?Sized,
fn using_encoded<R, F>(&self, f: F) -> Rwhere
F: FnOnce(&[u8]) -> R,
fn using_encoded<R, F>(&self, f: F) -> Rwhere
F: FnOnce(&[u8]) -> R,
fn encoded_size(&self) -> usize
fn encoded_size(&self) -> usize
sourceimpl<T> Hash for PhantomData<T>where
T: ?Sized,
impl<T> Hash for PhantomData<T>where
T: ?Sized,
impl<T> MaxEncodedLen for PhantomData<T>
impl<T> MaxEncodedLen for PhantomData<T>
fn max_encoded_len() -> usize
fn max_encoded_len() -> usize
sourceimpl<T> Ord for PhantomData<T>where
T: ?Sized,
impl<T> Ord for PhantomData<T>where
T: ?Sized,
sourcefn cmp(&self, _other: &PhantomData<T>) -> Ordering
fn cmp(&self, _other: &PhantomData<T>) -> Ordering
1.21.0 · sourcefn max(self, other: Self) -> Selfwhere
Self: Sized,
fn max(self, other: Self) -> Selfwhere
Self: Sized,
1.21.0 · sourcefn min(self, other: Self) -> Selfwhere
Self: Sized,
fn min(self, other: Self) -> Selfwhere
Self: Sized,
1.50.0 · sourcefn clamp(self, min: Self, max: Self) -> Selfwhere
Self: Sized + PartialOrd<Self>,
fn clamp(self, min: Self, max: Self) -> Selfwhere
Self: Sized + PartialOrd<Self>,
sourceimpl<T> PalletError for PhantomData<T>
impl<T> PalletError for PhantomData<T>
sourceconst MAX_ENCODED_SIZE: usize = 0usize
const MAX_ENCODED_SIZE: usize = 0usize
sourceimpl<T> PartialEq<PhantomData<T>> for PhantomData<T>where
T: ?Sized,
impl<T> PartialEq<PhantomData<T>> for PhantomData<T>where
T: ?Sized,
sourcefn eq(&self, _other: &PhantomData<T>) -> bool
fn eq(&self, _other: &PhantomData<T>) -> bool
sourceimpl<T> PartialOrd<PhantomData<T>> for PhantomData<T>where
T: ?Sized,
impl<T> PartialOrd<PhantomData<T>> for PhantomData<T>where
T: ?Sized,
sourceimpl<T> Serialize for PhantomData<T>where
T: ?Sized,
impl<T> Serialize for PhantomData<T>where
T: ?Sized,
sourcefn serialize<S>(
&self,
serializer: S
) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>where
S: Serializer,
fn serialize<S>(
&self,
serializer: S
) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>where
S: Serializer,
impl<T> TypeInfo for PhantomData<T>
impl<T> TypeInfo for PhantomData<T>
type Identity = PhantomData<()>
type Identity = PhantomData<()>
fn type_info() -> Type<MetaForm>
fn type_info() -> Type<MetaForm>
Self
.impl<Z> Zeroize for PhantomData<Z>
impl<Z> Zeroize for PhantomData<Z>
PhantomData
is always zero sized so provide a [Zeroize
] implementation.
impl<T> Copy for PhantomData<T>where
T: ?Sized,
impl<T> EncodeLike<PhantomData<T>> for PhantomData<T>
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,
impl<Z> ZeroizeOnDrop for PhantomData<Z>
[PhantomData
is always zero sized so provide a ZeroizeOnDrop implementation.
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 Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
const: unstable · sourcefn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
sourceimpl<T> CheckedConversion for T
impl<T> CheckedConversion for T
sourcefn checked_from<T>(t: T) -> Option<Self>where
Self: TryFrom<T>,
fn checked_from<T>(t: T) -> Option<Self>where
Self: TryFrom<T>,
sourcefn checked_into<T>(self) -> Option<T>where
Self: TryInto<T>,
fn checked_into<T>(self) -> Option<T>where
Self: TryInto<T>,
impl<T> DecodeAll for Twhere
T: Decode,
impl<T> DecodeAll for Twhere
T: Decode,
fn decode_all(input: &mut &[u8]) -> Result<T, Error>
fn decode_all(input: &mut &[u8]) -> Result<T, Error>
Self
and consume all of the given input data. Read moreimpl<T> DecodeLimit for Twhere
T: Decode,
impl<T> DecodeLimit for Twhere
T: Decode,
fn decode_all_with_depth_limit(limit: u32, input: &mut &[u8]) -> Result<T, Error>
fn decode_all_with_depth_limit(limit: u32, input: &mut &[u8]) -> Result<T, Error>
Self
and consume all of the given input data. Read moreimpl<T> Downcast for Twhere
T: Any,
impl<T> Downcast for Twhere
T: Any,
fn into_any(self: Box<T, Global>) -> Box<dyn Any + 'static, Global>
fn into_any(self: Box<T, Global>) -> Box<dyn Any + 'static, Global>
Box<dyn Trait>
(where Trait: Downcast
) to Box<dyn Any>
. Box<dyn Any>
can
then be further downcast
into Box<ConcreteType>
where ConcreteType
implements Trait
. Read morefn into_any_rc(self: Rc<T>) -> Rc<dyn Any + 'static>
fn into_any_rc(self: Rc<T>) -> Rc<dyn Any + 'static>
Rc<Trait>
(where Trait: Downcast
) to Rc<Any>
. Rc<Any>
can then be
further downcast
into Rc<ConcreteType>
where ConcreteType
implements Trait
. Read morefn as_any(&self) -> &(dyn Any + 'static)
fn as_any(&self) -> &(dyn Any + 'static)
&Trait
(where Trait: Downcast
) to &Any
. This is needed since Rust cannot
generate &Any
’s vtable from &Trait
’s. Read morefn as_any_mut(&mut self) -> &mut (dyn Any + 'static)
fn as_any_mut(&mut self) -> &mut (dyn Any + 'static)
&mut Trait
(where Trait: Downcast
) to &Any
. This is needed since Rust cannot
generate &mut Any
’s vtable from &mut Trait
’s. Read moresourceimpl<Q, K> Equivalent<K> for Qwhere
Q: Eq + ?Sized,
K: Borrow<Q> + ?Sized,
impl<Q, K> Equivalent<K> for Qwhere
Q: Eq + ?Sized,
K: Borrow<Q> + ?Sized,
sourcefn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
key
and return true
if they are equal.impl<Q, K> Equivalent<K> for Qwhere
Q: Eq + ?Sized,
K: Borrow<Q> + ?Sized,
impl<Q, K> Equivalent<K> for Qwhere
Q: Eq + ?Sized,
K: Borrow<Q> + ?Sized,
fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
sourceimpl<T> Instrument for T
impl<T> Instrument for T
sourcefn instrument(self, span: Span) -> Instrumented<Self>
fn instrument(self, span: Span) -> Instrumented<Self>
sourcefn in_current_span(self) -> Instrumented<Self>
fn in_current_span(self) -> Instrumented<Self>
sourceimpl<T, Outer> IsWrappedBy<Outer> for Twhere
Outer: AsRef<T> + AsMut<T> + From<T>,
T: From<Outer>,
impl<T, Outer> IsWrappedBy<Outer> for Twhere
Outer: AsRef<T> + AsMut<T> + From<T>,
T: From<Outer>,
impl<T> KeyedVec for Twhere
T: Codec,
impl<T> KeyedVec for Twhere
T: Codec,
impl<T> Pointable for T
impl<T> Pointable for T
sourceimpl<T> SaturatedConversion for T
impl<T> SaturatedConversion for T
sourcefn saturated_from<T>(t: T) -> Selfwhere
Self: UniqueSaturatedFrom<T>,
fn saturated_from<T>(t: T) -> Selfwhere
Self: UniqueSaturatedFrom<T>,
sourcefn saturated_into<T>(self) -> Twhere
Self: UniqueSaturatedInto<T>,
fn saturated_into<T>(self) -> Twhere
Self: UniqueSaturatedInto<T>,
T
. Read moresourceimpl<S, T> UncheckedInto<T> for Swhere
T: UncheckedFrom<S>,
impl<S, T> UncheckedInto<T> for Swhere
T: UncheckedFrom<S>,
sourcefn unchecked_into(self) -> T
fn unchecked_into(self) -> T
unchecked_from
.sourceimpl<T, S> UniqueSaturatedInto<T> for Swhere
T: Bounded,
S: TryInto<T>,
impl<T, S> UniqueSaturatedInto<T> for Swhere
T: Bounded,
S: TryInto<T>,
sourcefn unique_saturated_into(self) -> T
fn unique_saturated_into(self) -> T
T
.