Struct scale_info::prelude::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,
}
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>
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,
sourceimpl<T> Decode for PhantomData<T>
impl<T> Decode for PhantomData<T>
sourcefn 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,
Attempt to deserialise the value from input.
sourcefn skip<I>(input: &mut I) -> Result<(), Error> where
I: Input,
fn skip<I>(input: &mut I) -> Result<(), Error> where
I: Input,
Attempt to skip the encoded value from input. Read more
sourcefn encoded_fixed_size() -> Option<usize>
fn encoded_fixed_size() -> Option<usize>
Returns the fixed encoded size of the type. Read more
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<'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>,
Deserialize this value from the given Serde deserializer. Read more
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
The type produced by using this seed.
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>,
Equivalent to the more common Deserialize::deserialize
method, except
with some initial piece of data (the seed) passed in. Read more
sourceimpl<T> Encode for PhantomData<T>
impl<T> Encode for PhantomData<T>
sourcefn encode_to<W>(&self, _dest: &mut W) where
W: Output + ?Sized,
fn encode_to<W>(&self, _dest: &mut W) where
W: Output + ?Sized,
Convert self to a slice and append it to the destination.
sourcefn size_hint(&self) -> usize
fn size_hint(&self) -> usize
If possible give a hint of expected size of the encoding. Read more
sourcefn encode(&self) -> Vec<u8, Global>ⓘNotable traits for Vec<u8, A>impl<A> Write for Vec<u8, A> where
A: Allocator,
fn encode(&self) -> Vec<u8, Global>ⓘNotable traits for Vec<u8, A>impl<A> Write for Vec<u8, A> where
A: Allocator,
A: Allocator,
Convert self to an owned vector.
sourcefn using_encoded<R, F>(&self, f: F) -> R where
F: FnOnce(&[u8]) -> R,
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.
sourcefn encoded_size(&self) -> usize
fn encoded_size(&self) -> usize
Calculates the encoded size. 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
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,
Serialize this value into the given Serde serializer. Read more
sourceimpl<T> TypeInfo for PhantomData<T>
impl<T> TypeInfo for PhantomData<T>
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,
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> DecodeLimit for T where
T: Decode,
impl<T> DecodeLimit for T where
T: Decode,
impl<T> FmtForward for T
impl<T> FmtForward for T
fn fmt_binary(self) -> FmtBinary<Self> where
Self: Binary,
fn fmt_binary(self) -> FmtBinary<Self> where
Self: Binary,
Causes self
to use its Binary
implementation when Debug
-formatted. Read more
fn fmt_display(self) -> FmtDisplay<Self> where
Self: Display,
fn fmt_display(self) -> FmtDisplay<Self> where
Self: Display,
Causes self
to use its Display
implementation when
Debug
-formatted. Read more
fn fmt_lower_exp(self) -> FmtLowerExp<Self> where
Self: LowerExp,
fn fmt_lower_exp(self) -> FmtLowerExp<Self> where
Self: LowerExp,
Causes self
to use its LowerExp
implementation when
Debug
-formatted. Read more
fn fmt_lower_hex(self) -> FmtLowerHex<Self> where
Self: LowerHex,
fn fmt_lower_hex(self) -> FmtLowerHex<Self> where
Self: LowerHex,
Causes self
to use its LowerHex
implementation when
Debug
-formatted. Read more
fn fmt_octal(self) -> FmtOctal<Self> where
Self: Octal,
fn fmt_octal(self) -> FmtOctal<Self> where
Self: Octal,
Causes self
to use its Octal
implementation when Debug
-formatted. Read more
fn fmt_pointer(self) -> FmtPointer<Self> where
Self: Pointer,
fn fmt_pointer(self) -> FmtPointer<Self> where
Self: Pointer,
Causes self
to use its Pointer
implementation when
Debug
-formatted. Read more
fn fmt_upper_exp(self) -> FmtUpperExp<Self> where
Self: UpperExp,
fn fmt_upper_exp(self) -> FmtUpperExp<Self> where
Self: UpperExp,
Causes self
to use its UpperExp
implementation when
Debug
-formatted. Read more
fn fmt_upper_hex(self) -> FmtUpperHex<Self> where
Self: UpperHex,
fn fmt_upper_hex(self) -> FmtUpperHex<Self> where
Self: UpperHex,
Causes self
to use its UpperHex
implementation when
Debug
-formatted. Read more
fn fmt_list(self) -> FmtList<Self> where
&'a Self: for<'a> IntoIterator,
fn fmt_list(self) -> FmtList<Self> where
&'a Self: for<'a> IntoIterator,
Formats each item in a sequence. Read more
impl<T> Pipe for T where
T: ?Sized,
impl<T> Pipe for T where
T: ?Sized,
fn pipe<R>(self, func: impl FnOnce(Self) -> R) -> R
fn pipe<R>(self, func: impl FnOnce(Self) -> R) -> R
Pipes by value. This is generally the method you want to use. Read more
fn pipe_ref<'a, R>(&'a self, func: impl FnOnce(&'a Self) -> R) -> R where
R: 'a,
fn pipe_ref<'a, R>(&'a self, func: impl FnOnce(&'a Self) -> R) -> R where
R: 'a,
Borrows self
and passes that borrow into the pipe function. Read more
fn pipe_ref_mut<'a, R>(&'a mut self, func: impl FnOnce(&'a mut Self) -> R) -> R where
R: 'a,
fn pipe_ref_mut<'a, R>(&'a mut self, func: impl FnOnce(&'a mut Self) -> R) -> R where
R: 'a,
Mutably borrows self
and passes that borrow into the pipe function. Read more
fn pipe_borrow<'a, B, R>(&'a self, func: impl FnOnce(&'a B) -> R) -> R where
Self: Borrow<B>,
B: 'a + ?Sized,
R: 'a,
fn pipe_borrow<'a, B, R>(&'a self, func: impl FnOnce(&'a B) -> R) -> R where
Self: Borrow<B>,
B: 'a + ?Sized,
R: 'a,
Borrows self
, then passes self.borrow()
into the pipe function. Read more
fn pipe_borrow_mut<'a, B, R>(
&'a mut self,
func: impl FnOnce(&'a mut B) -> R
) -> R where
Self: BorrowMut<B>,
B: 'a + ?Sized,
R: 'a,
fn pipe_borrow_mut<'a, B, R>(
&'a mut self,
func: impl FnOnce(&'a mut B) -> R
) -> R where
Self: BorrowMut<B>,
B: 'a + ?Sized,
R: 'a,
Mutably borrows self
, then passes self.borrow_mut()
into the pipe
function. Read more
fn pipe_as_ref<'a, U, R>(&'a self, func: impl FnOnce(&'a U) -> R) -> R where
Self: AsRef<U>,
U: 'a + ?Sized,
R: 'a,
fn pipe_as_ref<'a, U, R>(&'a self, func: impl FnOnce(&'a U) -> R) -> R where
Self: AsRef<U>,
U: 'a + ?Sized,
R: 'a,
Borrows self
, then passes self.as_ref()
into the pipe function.
fn pipe_as_mut<'a, U, R>(&'a mut self, func: impl FnOnce(&'a mut U) -> R) -> R where
Self: AsMut<U>,
U: 'a + ?Sized,
R: 'a,
fn pipe_as_mut<'a, U, R>(&'a mut self, func: impl FnOnce(&'a mut U) -> R) -> R where
Self: AsMut<U>,
U: 'a + ?Sized,
R: 'a,
Mutably borrows self
, then passes self.as_mut()
into the pipe
function. Read more
fn pipe_deref<'a, T, R>(&'a self, func: impl FnOnce(&'a T) -> R) -> R where
Self: Deref<Target = T>,
T: 'a + ?Sized,
R: 'a,
fn pipe_deref<'a, T, R>(&'a self, func: impl FnOnce(&'a T) -> R) -> R where
Self: Deref<Target = T>,
T: 'a + ?Sized,
R: 'a,
Borrows self
, then passes self.deref()
into the pipe function.
impl<T> Tap for T
impl<T> Tap for T
fn tap_borrow<B>(self, func: impl FnOnce(&B)) -> Self where
Self: Borrow<B>,
B: ?Sized,
fn tap_borrow<B>(self, func: impl FnOnce(&B)) -> Self where
Self: Borrow<B>,
B: ?Sized,
Immutable access to the Borrow<B>
of a value. Read more
fn tap_borrow_mut<B>(self, func: impl FnOnce(&mut B)) -> Self where
Self: BorrowMut<B>,
B: ?Sized,
fn tap_borrow_mut<B>(self, func: impl FnOnce(&mut B)) -> Self where
Self: BorrowMut<B>,
B: ?Sized,
Mutable access to the BorrowMut<B>
of a value. Read more
fn tap_ref<R>(self, func: impl FnOnce(&R)) -> Self where
Self: AsRef<R>,
R: ?Sized,
fn tap_ref<R>(self, func: impl FnOnce(&R)) -> Self where
Self: AsRef<R>,
R: ?Sized,
Immutable access to the AsRef<R>
view of a value. Read more
fn tap_ref_mut<R>(self, func: impl FnOnce(&mut R)) -> Self where
Self: AsMut<R>,
R: ?Sized,
fn tap_ref_mut<R>(self, func: impl FnOnce(&mut R)) -> Self where
Self: AsMut<R>,
R: ?Sized,
Mutable access to the AsMut<R>
view of a value. Read more
fn tap_deref<T>(self, func: impl FnOnce(&T)) -> Self where
Self: Deref<Target = T>,
T: ?Sized,
fn tap_deref<T>(self, func: impl FnOnce(&T)) -> Self where
Self: Deref<Target = T>,
T: ?Sized,
Immutable access to the Deref::Target
of a value. Read more
fn tap_deref_mut<T>(self, func: impl FnOnce(&mut T)) -> Self where
Self: DerefMut<Target = T> + Deref,
T: ?Sized,
fn tap_deref_mut<T>(self, func: impl FnOnce(&mut T)) -> Self where
Self: DerefMut<Target = T> + Deref,
T: ?Sized,
Mutable access to the Deref::Target
of a value. Read more
fn tap_dbg(self, func: impl FnOnce(&Self)) -> Self
fn tap_dbg(self, func: impl FnOnce(&Self)) -> Self
Calls .tap()
only in debug builds, and is erased in release builds.
fn tap_mut_dbg(self, func: impl FnOnce(&mut Self)) -> Self
fn tap_mut_dbg(self, func: impl FnOnce(&mut Self)) -> Self
Calls .tap_mut()
only in debug builds, and is erased in release
builds. Read more
fn tap_borrow_dbg<B>(self, func: impl FnOnce(&B)) -> Self where
Self: Borrow<B>,
B: ?Sized,
fn tap_borrow_dbg<B>(self, func: impl FnOnce(&B)) -> Self where
Self: Borrow<B>,
B: ?Sized,
Calls .tap_borrow()
only in debug builds, and is erased in release
builds. Read more
fn tap_borrow_mut_dbg<B>(self, func: impl FnOnce(&mut B)) -> Self where
Self: BorrowMut<B>,
B: ?Sized,
fn tap_borrow_mut_dbg<B>(self, func: impl FnOnce(&mut B)) -> Self where
Self: BorrowMut<B>,
B: ?Sized,
Calls .tap_borrow_mut()
only in debug builds, and is erased in release
builds. Read more
fn tap_ref_dbg<R>(self, func: impl FnOnce(&R)) -> Self where
Self: AsRef<R>,
R: ?Sized,
fn tap_ref_dbg<R>(self, func: impl FnOnce(&R)) -> Self where
Self: AsRef<R>,
R: ?Sized,
Calls .tap_ref()
only in debug builds, and is erased in release
builds. Read more
fn tap_ref_mut_dbg<R>(self, func: impl FnOnce(&mut R)) -> Self where
Self: AsMut<R>,
R: ?Sized,
fn tap_ref_mut_dbg<R>(self, func: impl FnOnce(&mut R)) -> Self where
Self: AsMut<R>,
R: ?Sized,
Calls .tap_ref_mut()
only in debug builds, and is erased in release
builds. 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)
toowned_clone_into
)Uses borrowed data to replace owned data, usually by cloning. Read more