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

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

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W`
`fn deref_mut(&mut self) -> &mut T`[src]

Mutably dereferences the value.

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

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

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

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

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

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

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

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

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

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

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

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

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

Compares and returns the maximum of two values. Read more

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

Compares and returns the minimum of two values. Read more

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

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

Restrict a value to a certain interval. Read more

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

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

#### `type Target = T`

The resulting type after dereferencing.

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W`
`fn deref(&self) -> &T`[src]

Dereferences the value.

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

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

Formats the value using the given formatter. Read more

### `impl<T> Clone for ManuallyDrop<T> where
T: Clone + ?Sized,` [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> PartialOrd<ManuallyDrop<T>> for ManuallyDrop<T> where
T: PartialOrd<T> + ?Sized,` [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> StructuralPartialEq for ManuallyDrop<T> where
T: ?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]

#### `fn from(t: T) -> T`[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(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[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> 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.

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

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W`
`fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

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

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W`
`fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `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> ToOwned for T where
T: Clone,` [src]

#### `type Owned = T`

The resulting type after obtaining ownership.

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

Creates owned data from borrowed data, usually by cloning. Read more

#### `fn clone_into(&self, target: &mut T)`[src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into` #41263)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more