core::mem::ManuallyDrop - Rust
Description: API documentation for the Rust `ManuallyDrop` struct in crate `core`.
Keywords: rust, rustlang, rust-lang, ManuallyDrop

# 1.20.0[−][src]Struct core::mem::ManuallyDrop

    #[lang = "manually_drop"]#[repr(transparent)]pub struct ManuallyDrop<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>) -> T`1.42.0[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: ?Sized> ManuallyDrop<T>`[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 + ?Sized> Clone for ManuallyDrop<T>`[src]

#### `fn clone(&self) -> ManuallyDrop<T>`[src]

Returns a copy of the value. Read more

#### `fn clone_from(&mut self, source: &Self)`1.0.0[src]

Performs copy-assignment from `source`. Read more

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

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

#### `fn fmt(&self, f: &mut Formatter) -> Result`[src]

Formats the value using the given formatter. Read more

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

#### `fn default() -> ManuallyDrop<T>`[src]

Returns the "default value" for a type. Read more

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

#### `type Target = T`

The resulting type after dereferencing.

#### `fn deref(&self) -> &T`[src]

Dereferences the value.

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

#### `fn deref_mut(&mut self) -> &mut T`[src]

Mutably dereferences the value.

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

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

#### `fn hash<__H: Hasher>(&self, state: &mut __H)`[src]

Feeds this value into the given [`Hasher`]. Read more

#### `fn hash_slice<H: Hasher>(data: &[Self], state: &mut H) where
Self: Sized,` 1.3.0[src]

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

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

#### `fn cmp(&self, other: &ManuallyDrop<T>) -> Ordering`[src]

This method returns an `Ordering` between `self` and `other`. Read more

#### `#[must_use]fn max(self, other: Self) -> Self where
Self: Sized,` 1.21.0[src]

Compares and returns the maximum of two values. Read more

#### `#[must_use]fn min(self, other: Self) -> Self where
Self: Sized,` 1.21.0[src]

Compares and returns the minimum of two values. Read more

#### `#[must_use]fn clamp(self, min: Self, max: Self) -> Self where
Self: Sized,` [src]

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

Restrict a value to a certain interval. Read more

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

#### `fn eq(&self, other: &ManuallyDrop<T>) -> bool`[src]

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

#### `fn ne(&self, other: &ManuallyDrop<T>) -> bool`[src]

This method tests for `!=`.

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

#### `fn partial_cmp(&self, other: &ManuallyDrop<T>) -> Option<Ordering>`[src]

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

#### `fn lt(&self, other: &ManuallyDrop<T>) -> bool`[src]

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

#### `fn le(&self, other: &ManuallyDrop<T>) -> bool`[src]

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

#### `fn gt(&self, other: &ManuallyDrop<T>) -> bool`[src]

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

#### `fn ge(&self, other: &ManuallyDrop<T>) -> bool`[src]

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

### `impl<T: ?Sized> StructuralEq for ManuallyDrop<T>`[src]

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

## Auto Trait Implementations

### `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,`

## Blanket Implementations

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

#### `fn type_id(&Self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

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

#### `fn borrow(&Self) -> &T`[src]

Immutably borrows from an owned value. Read more

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

#### `fn borrow_mut(&mut Self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

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

#### `fn from(T) -> T`[src]

Performs the conversion.

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

#### `fn into(Self) -> U`[src]

Performs the conversion.

### `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.

#### `fn try_from(U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

Performs the conversion.

### `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.

#### `fn try_into(Self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

Performs the conversion.