Struct std::mem::ManuallyDrop

#[lang = "manually_drop"]
#[repr(transparent)]
pub struct ManuallyDrop<T> where
    T: ?Sized,  { /* fields omitted */ }

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>

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

Wrap a value to be manually dropped.

Examples

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

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

Extract 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`.

impl<T> ManuallyDrop<T> where     T: ?Sized,

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

Manually drops the contained value.

If you have ownership of the value, you can use ManuallyDrop::into_inner instead.

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> where     T: ?Sized,

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

Mutably dereferences the value.

impl<T> Default for ManuallyDrop<T> where     T: Default + ?Sized,

fn default() -> ManuallyDrop<T>

Returns the "default value" for a type.

impl<T> Hash for ManuallyDrop<T> where     T: Hash + ?Sized,

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

Feeds this value into the given [Hasher].

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

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

impl<T> Deref for ManuallyDrop<T> where     T: ?Sized,

type Target = T

The resulting type after dereferencing.

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

Dereferences the value.

impl<T> Debug for ManuallyDrop<T> where     T: Debug + ?Sized,

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

Formats the value using the given formatter.

impl<T> Clone for ManuallyDrop<T> where     T: Clone + ?Sized,

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

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> PartialOrd<ManuallyDrop<T>> for ManuallyDrop<T> where     T: PartialOrd<T> + ?Sized,

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

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

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

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

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

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

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

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

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 + ?Sized,

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

This method returns an Ordering between self and other.

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

Compares and returns the maximum of two values.

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

Compares and returns the minimum of two values.

impl<T> Eq for ManuallyDrop<T> where     T: Eq + ?Sized,

impl<T> PartialEq<ManuallyDrop<T>> for ManuallyDrop<T> where     T: PartialEq<T> + ?Sized,

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

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

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

This method tests for !=.

impl<T> Copy for ManuallyDrop<T> where     T: Copy + ?Sized,