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.

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 all values being initialized at their type. In particular, initializing a ManuallyDrop<&mut T> with mem::zeroed is undefined behavior. If you need to handle uninitialized data, use MaybeUninit<T> instead.

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

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

🔬 This is a nightly-only experimental API. (manually_drop_take #55422)

Takes the contained value 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. Drop will be invoked on the returned value following normal end-of-scope rules.

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

Safety

This function semantically moves out the contained value without preventing further usage. It is up to the user of this method to ensure that this container is not used again.

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, 

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W

fn deref_mut(&mut self) -> &mut T

Mutably dereferences the value.

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.

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W

fn deref(&self) -> &T

Dereferences the value.

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

fn default() -> ManuallyDrop<T>

Returns the "default value" for a type.

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

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

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.

fn clamp(self, min: Self, max: Self) -> Self

🔬 This is a nightly-only experimental API. (clamp #44095)

Restrict a value to a certain interval.

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 !=.