1.20.0[][src]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`.
    }
}Run

Methods

impl<T> ManuallyDrop<T>[src]

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

Wrap a value to be manually dropped.

Examples

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

pub const fn into_inner(slot: ManuallyDrop<T>) -> T[src]

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`.Run

#[must_use = "if you don\'t need the value, you can use `ManuallyDrop::drop` instead"] pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T[src]

🔬 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
[src]

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

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. In particular, this function can only be called called at most once for a given instance of ManuallyDrop<T>.

Trait Implementations

impl<T> Hash for ManuallyDrop<T> where
    T: Hash + ?Sized
[src]

impl<T> DerefMut for ManuallyDrop<T> where
    T: ?Sized
[src]

impl<T> Eq for ManuallyDrop<T> where
    T: Eq + ?Sized
[src]

impl<T> Deref for ManuallyDrop<T> where
    T: ?Sized
[src]

type Target = T

The resulting type after dereferencing.

impl<T> PartialEq<ManuallyDrop<T>> for ManuallyDrop<T> where
    T: PartialEq<T> + ?Sized
[src]

impl<T> PartialOrd<ManuallyDrop<T>> for ManuallyDrop<T> where
    T: PartialOrd<T> + ?Sized
[src]

impl<T> Ord for ManuallyDrop<T> where
    T: Ord + ?Sized
[src]

impl<T> Clone for ManuallyDrop<T> where
    T: Clone + ?Sized
[src]

impl<T> Debug for ManuallyDrop<T> where
    T: Debug + ?Sized
[src]

impl<T> Default for ManuallyDrop<T> where
    T: Default + ?Sized
[src]

impl<T> Copy for ManuallyDrop<T> where
    T: Copy + ?Sized
[src]

Auto Trait Implementations

impl<T: ?Sized> UnwindSafe for ManuallyDrop<T> where
    T: UnwindSafe

impl<T: ?Sized> RefUnwindSafe for ManuallyDrop<T> where
    T: RefUnwindSafe

impl<T: ?Sized> Unpin for ManuallyDrop<T> where
    T: Unpin

impl<T: ?Sized> Send for ManuallyDrop<T> where
    T: Send

impl<T: ?Sized> Sync for ManuallyDrop<T> where
    T: Sync

Blanket Implementations

impl<T> From<T> for T[src]

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 
[src]

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> Into<U> for T where
    U: From<T>, 
[src]

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 
[src]

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.

impl<T> Borrow<T> for T where
    T: ?Sized
[src]

impl<T> BorrowMut<T> for T where
    T: ?Sized
[src]

impl<T> Any for T where
    T: 'static + ?Sized
[src]

impl<T> ToOwned for T where
    T: Clone
[src]

type Owned = T

The resulting type after obtaining ownership.