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 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`.
    }
}
Run

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>[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>) -> T1.42.0[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
[src]

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

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
[src]

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

type Target = T

The resulting type after dereferencing.

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

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

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

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

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

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

Auto Trait Implementations

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

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

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

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

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

Blanket Implementations

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

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

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

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

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

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

type Owned = T

The resulting type after obtaining ownership.

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> 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.