1.20.0[−][src]Struct sp_std::mem::ManuallyDrop
A wrapper to inhibit compiler from automatically calling T
’s destructor.
This wrapper is 0-cost.
ManuallyDrop<T>
is subject to the same layout optimizations as T
.
As a consequence, it has no effect on the assumptions that the compiler makes
about its contents. For example, initializing a ManuallyDrop<&mut T>
with mem::zeroed
is undefined behavior.
If you need to handle uninitialized data, use MaybeUninit<T>
instead.
Note that accessing the value inside a ManuallyDrop<T>
is safe.
This means that a ManuallyDrop<T>
whose content has been dropped must not
be exposed through a public safe API.
Correspondingly, ManuallyDrop::drop
is unsafe.
Examples
This wrapper can be used to enforce a particular drop order on fields, regardless of how they are defined in the struct:
use std::mem::ManuallyDrop; struct Peach; struct Banana; struct Melon; struct FruitBox { // Immediately clear there’s something non-trivial going on with these fields. peach: ManuallyDrop<Peach>, melon: Melon, // Field that’s independent of the other two. banana: ManuallyDrop<Banana>, } impl Drop for FruitBox { fn drop(&mut self) { unsafe { // Explicit ordering in which field destructors are run specified in the intuitive // location – the destructor of the structure containing the fields. // Moreover, one can now reorder fields within the struct however much they want. ManuallyDrop::drop(&mut self.peach); ManuallyDrop::drop(&mut self.banana); } // After destructor for `FruitBox` runs (this function), the destructor for Melon gets // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`. } }
However, care should be taken when using this pattern as it can lead to leak amplification.
In this example, if the Drop
implementation for Peach
were to panic, the banana
field
would also be leaked.
In contrast, the automatically-generated compiler drop implementation would have ensured that all fields are dropped even in the presence of panics. This is especially important when working with pinned data, where reusing the memory without calling the destructor could lead to Undefined Behaviour.
Implementations
impl<T> ManuallyDrop<T>
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#[must_use =
"if you don't need the wrapper, you can use `mem::forget` instead"]pub const fn new(value: T) -> ManuallyDrop<T>
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Wrap a value to be manually dropped.
Examples
use std::mem::ManuallyDrop; let mut x = ManuallyDrop::new(String::from("Hello World!")); x.truncate(5); // You can still safely operate on the value assert_eq!(*x, "Hello"); // But `Drop` will not be run here
pub const fn into_inner(slot: ManuallyDrop<T>) -> T
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Extracts the value from the ManuallyDrop
container.
This allows the value to be dropped again.
Examples
use std::mem::ManuallyDrop; let x = ManuallyDrop::new(Box::new(())); let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`.
#[must_use =
"if you don't need the value, you can use `ManuallyDrop::drop` instead"]pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T
1.42.0[src]
Takes the value from the ManuallyDrop<T>
container out.
This method is primarily intended for moving out values in drop.
Instead of using ManuallyDrop::drop
to manually drop the value,
you can use this method to take the value and use it however desired.
Whenever possible, it is preferable to use into_inner
instead, which prevents duplicating the content of the ManuallyDrop<T>
.
Safety
This function semantically moves out the contained value without preventing further usage,
leaving the state of this container unchanged.
It is your responsibility to ensure that this ManuallyDrop
is not used again.
impl<T> ManuallyDrop<T> where
T: ?Sized,
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T: ?Sized,
pub unsafe fn drop(slot: &mut ManuallyDrop<T>)
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Manually drops the contained value. This is exactly equivalent to calling
ptr::drop_in_place
with a pointer to the contained value. As such, unless
the contained value is a packed struct, the destructor will be called in-place
without moving the value, and thus can be used to safely drop pinned data.
If you have ownership of the value, you can use ManuallyDrop::into_inner
instead.
Safety
This function runs the destructor of the contained value. Other than changes made by
the destructor itself, the memory is left unchanged, and so as far as the compiler is
concerned still holds a bit-pattern which is valid for the type T
.
However, this "zombie" value should not be exposed to safe code, and this function
should not be called more than once. To use a value after it's been dropped, or drop
a value multiple times, can cause Undefined Behavior (depending on what drop
does).
This is normally prevented by the type system, but users of ManuallyDrop
must
uphold those guarantees without assistance from the compiler.
Trait Implementations
impl<T> Clone for ManuallyDrop<T> where
T: Clone + ?Sized,
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T: Clone + ?Sized,
fn clone(&self) -> ManuallyDrop<T>
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fn clone_from(&mut self, source: &Self)
1.0.0[src]
impl<T> Copy for ManuallyDrop<T> where
T: Copy + ?Sized,
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T: Copy + ?Sized,
impl<T> Debug for ManuallyDrop<T> where
T: Debug + ?Sized,
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T: Debug + ?Sized,
impl<T> Default for ManuallyDrop<T> where
T: Default + ?Sized,
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T: Default + ?Sized,
fn default() -> ManuallyDrop<T>
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impl<T> Deref for ManuallyDrop<T> where
T: ?Sized,
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T: ?Sized,
impl<T> DerefMut for ManuallyDrop<T> where
T: ?Sized,
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T: ?Sized,
impl<T> Eq for ManuallyDrop<T> where
T: Eq + ?Sized,
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T: Eq + ?Sized,
impl<T> Hash for ManuallyDrop<T> where
T: Hash + ?Sized,
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T: Hash + ?Sized,
fn hash<__H>(&self, state: &mut __H) where
__H: Hasher,
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__H: Hasher,
fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,
1.3.0[src]
H: Hasher,
impl<T> Ord for ManuallyDrop<T> where
T: Ord + ?Sized,
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T: Ord + ?Sized,
fn cmp(&self, other: &ManuallyDrop<T>) -> Ordering
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#[must_use]fn max(self, other: Self) -> Self
1.21.0[src]
#[must_use]fn min(self, other: Self) -> Self
1.21.0[src]
#[must_use]fn clamp(self, min: Self, max: Self) -> Self
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impl<T> PartialEq<ManuallyDrop<T>> for ManuallyDrop<T> where
T: PartialEq<T> + ?Sized,
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T: PartialEq<T> + ?Sized,
fn eq(&self, other: &ManuallyDrop<T>) -> bool
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fn ne(&self, other: &ManuallyDrop<T>) -> bool
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impl<T> PartialOrd<ManuallyDrop<T>> for ManuallyDrop<T> where
T: PartialOrd<T> + ?Sized,
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T: PartialOrd<T> + ?Sized,
fn partial_cmp(&self, other: &ManuallyDrop<T>) -> Option<Ordering>
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fn lt(&self, other: &ManuallyDrop<T>) -> bool
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fn le(&self, other: &ManuallyDrop<T>) -> bool
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fn gt(&self, other: &ManuallyDrop<T>) -> bool
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fn ge(&self, other: &ManuallyDrop<T>) -> bool
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impl<T> StructuralEq for ManuallyDrop<T> where
T: ?Sized,
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T: ?Sized,
impl<T> StructuralPartialEq for ManuallyDrop<T> where
T: ?Sized,
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T: ?Sized,
Auto Trait Implementations
impl<T: ?Sized> RefUnwindSafe for ManuallyDrop<T> where
T: RefUnwindSafe,
T: RefUnwindSafe,
impl<T: ?Sized> Send for ManuallyDrop<T> where
T: Send,
T: Send,
impl<T: ?Sized> Sync for ManuallyDrop<T> where
T: Sync,
T: Sync,
impl<T: ?Sized> Unpin for ManuallyDrop<T> where
T: Unpin,
T: Unpin,
impl<T: ?Sized> UnwindSafe for ManuallyDrop<T> where
T: UnwindSafe,
T: UnwindSafe,
Blanket Implementations
impl<T> Any for T where
T: 'static + ?Sized,
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T: 'static + ?Sized,
impl<T> Borrow<T> for T where
T: ?Sized,
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T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
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T: ?Sized,
fn borrow_mut(&mut self) -> &mut Tⓘ
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impl<T> From<T> for T
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impl<T, U> Into<U> for T where
U: From<T>,
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U: From<T>,
impl<T> ToOwned for T where
T: Clone,
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T: Clone,
type Owned = T
The resulting type after obtaining ownership.
fn to_owned(&self) -> T
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fn clone_into(&self, target: &mut T)
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impl<T, U> TryFrom<U> for T where
U: Into<T>,
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U: Into<T>,
type Error = Infallible
The type returned in the event of a conversion error.
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
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impl<T, U> TryInto<U> for T where
U: TryFrom<T>,
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U: TryFrom<T>,