Union std::mem::ManuallyDrop

[+] Show declaration

pub union ManuallyDrop<T> {
    // some fields omitted
}

[−] Expand description

A wrapper to inhibit compiler from automatically calling T’s destructor.

This wrapper is 0-cost.

Examples

This wrapper helps with explicitly documenting the drop order dependencies between fields of the type:

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`.
    }
}

Methods

impl<T> ManuallyDrop<T>

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pub const fn new(value: T) -> ManuallyDrop<T>

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Wrap a value to be manually dropped.

Examples

use std::mem::ManuallyDrop;
ManuallyDrop::new(Box::new(()));

pub fn into_inner(slot: ManuallyDrop<T>) -> T

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Extract the value from the ManuallyDrop container.

Examples

use std::mem::ManuallyDrop;
let x = ManuallyDrop::new(Box::new(()));
let _: Box<()> = ManuallyDrop::into_inner(x);

pub unsafe fn drop(slot: &mut ManuallyDrop<T>)

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Manually drops the contained value.

Safety

This function runs the destructor of the contained value and thus the wrapped value now represents uninitialized data. It is up to the user of this method to ensure the uninitialized data is not actually used.

Trait Implementations

impl<T> DerefMut for ManuallyDrop<T>

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fn deref_mut(&mut self) -> &mut <ManuallyDrop<T> as Deref>::Target

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Mutably dereferences the value.

impl<T> Deref for ManuallyDrop<T>

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type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &<ManuallyDrop<T> as Deref>::Target

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Dereferences the value.

impl<T> Debug for ManuallyDrop<T> where     T: Debug,

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fn fmt(&self, fmt: &mut Formatter) -> Result<(), Error>

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Formats the value using the given formatter.

impl<T> Default for ManuallyDrop<T> where     T: Default,

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fn default() -> ManuallyDrop<T>

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Returns the "default value" for a type.

impl<T> Copy for ManuallyDrop<T> where     T: Copy,

impl<T> Clone for ManuallyDrop<T> where     T: Clone,

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fn clone(&self) -> ManuallyDrop<T>

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Returns a copy of the value.

fn clone_from(&mut self, source: &ManuallyDrop<T>)

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Performs copy-assignment from source.

impl<T> PartialOrd<ManuallyDrop<T>> for ManuallyDrop<T> where     T: PartialOrd<T>,

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fn partial_cmp(&self, other: &ManuallyDrop<T>) -> Option<Ordering>

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This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &ManuallyDrop<T>) -> bool

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This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &ManuallyDrop<T>) -> bool

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This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &ManuallyDrop<T>) -> bool

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This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &ManuallyDrop<T>) -> bool

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This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<T> Ord for ManuallyDrop<T> where     T: Ord,

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fn cmp(&self, other: &ManuallyDrop<T>) -> Ordering

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This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

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Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

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Compares and returns the minimum of two values.

impl<T> Eq for ManuallyDrop<T> where     T: Eq,

impl<T> PartialEq<ManuallyDrop<T>> for ManuallyDrop<T> where     T: PartialEq<T>,

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fn eq(&self, other: &ManuallyDrop<T>) -> bool

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This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &ManuallyDrop<T>) -> bool

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This method tests for !=.

impl<T> Hash for ManuallyDrop<T> where     T: Hash,

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fn hash<H>(&self, state: &mut H) where     H: Hasher,

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Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where     H: Hasher,

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Feeds a slice of this type into the given [Hasher].