# tokio::sync::RwLockWriteGuard - Rust
API documentation for the Rust `RwLockWriteGuard` struct in crate `tokio`.
Keywords: rust, rustlang, rust-lang, RwLockWriteGuard

# Struct tokio::sync::RwLockWriteGuard [−] [src]

    pub struct RwLockWriteGuard<'a, T: ?Sized> { /* fields omitted */ }

RAII structure used to release the exclusive write access of a lock when dropped.

This structure is created by the `write` and method on `RwLock`.

## Implementations

### `impl<'a, T: ?Sized> RwLockWriteGuard<'a, T>` [src]

#### `pub fn map<F, U: ?Sized>(this: Self, f: F) -> RwLockMappedWriteGuard<'a, U> where
F: FnOnce(&mut T) -> &mut U,` [src]

Make a new `RwLockMappedWriteGuard` for a component of the locked data.

This operation cannot fail as the `RwLockWriteGuard` passed in already locked the data.

This is an associated function that needs to be used as `RwLockWriteGuard::map(..)`. A method would interfere with methods of the same name on the contents of the locked data.

This is an asynchronous version of `RwLockWriteGuard::map` from the `parking_lot` crate.

# Examples

    use tokio::sync::{RwLock, RwLockWriteGuard};

    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    struct Foo(u32);

    let lock = RwLock::new(Foo(1));

    {
        let mut mapped = RwLockWriteGuard::map(lock.write().await, |f| &mut f.0);
        *mapped = 2;
    }

    assert_eq!(Foo(2), *lock.read().await);

#### `pub fn try_map<F, U: ?Sized>(
this: Self,
f: F
) -> Result<RwLockMappedWriteGuard<'a, U>, Self> where
F: FnOnce(&mut T) -> Option<&mut U>,` [src]

Attempts to make a new `RwLockMappedWriteGuard` for a component of the locked data. The original guard is returned if the closure returns `None`.

This operation cannot fail as the `RwLockWriteGuard` passed in already locked the data.

This is an associated function that needs to be used as `RwLockWriteGuard::try_map(...)`. A method would interfere with methods of the same name on the contents of the locked data.

This is an asynchronous version of `RwLockWriteGuard::try_map` from the `parking_lot` crate.

# Examples

    use tokio::sync::{RwLock, RwLockWriteGuard};

    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    struct Foo(u32);

    let lock = RwLock::new(Foo(1));

    {
        let guard = lock.write().await;
        let mut guard = RwLockWriteGuard::try_map(guard, |f| Some(&mut f.0)).expect("should not fail");
        *guard = 2;
    }

    assert_eq!(Foo(2), *lock.read().await);

#### `pub fn into_mapped(this: Self) -> RwLockMappedWriteGuard<'a, T>` [src]

Converts this `RwLockWriteGuard` into an `RwLockMappedWriteGuard`. This method can be used to store a non-mapped guard in a struct field that expects a mapped guard.

This is equivalent to calling `RwLockWriteGuard::map(guard, |me| me)`.

#### `pub fn downgrade(self) -> RwLockReadGuard<'a, T>` [src]

Atomically downgrades a write lock into a read lock without allowing any writers to take exclusive access of the lock in the meantime.

Note: This won't necessarily allow any additional readers to acquire locks, since `RwLock` is fair and it is possible that a writer is next in line.

Returns an RAII guard which will drop this read access of the `RwLock` when dropped.

# Examples

    let lock = Arc::new(RwLock::new(1));

    let n = lock.write().await;

    let cloned_lock = lock.clone();
    let handle = tokio::spawn(async move {
        *cloned_lock.write().await = 2;
    });

    let n = n.downgrade();
    assert_eq!(*n, 1, "downgrade is atomic");

    drop(n);
    handle.await.unwrap();
    assert_eq!(*lock.read().await, 2, "second writer obtained write lock");

## Trait Implementations

### `impl<'a, T: ?Sized> Debug for RwLockWriteGuard<'a, T> where
T: Debug,` [src]

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

Formats the value using the given formatter. Read more

### `impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T>` [src]

#### `type Target = T`

The resulting type after dereferencing.

#### `fn deref(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Dereferences the value.

### `impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T>` [src]

#### `fn deref_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Mutably dereferences the value.

### `impl<'a, T: ?Sized> Display for RwLockWriteGuard<'a, T> where
T: Display,` [src]

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

Formats the value using the given formatter. Read more

### `impl<'a, T: ?Sized> Drop for RwLockWriteGuard<'a, T>` [src]

#### `fn drop(&mut self)` [src]

Executes the destructor for this type. Read more

### `impl<T: ?Sized> Send for RwLockWriteGuard<'_, T> where
T: Send + Sync,` [src]

### `impl<T: ?Sized> Sync for RwLockWriteGuard<'_, T> where
T: Send + Sync,` [src]

## Auto Trait Implementations

### `impl<'a, T> !RefUnwindSafe for RwLockWriteGuard<'a, T>` [src]

### `impl<'a, T: ?Sized> Unpin for RwLockWriteGuard<'a, T>` [src]

### `impl<'a, T> !UnwindSafe for RwLockWriteGuard<'a, T>` [src]

## Blanket Implementations

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

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

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

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

#### `pub fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Immutably borrows from an owned value. Read more

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

#### `pub fn borrow_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Mutably borrows from an owned value. Read more

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

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

Performs the conversion.

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

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

Performs the conversion.

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

#### `pub default fn to_string(&self) -> String` [src]

Converts the given value to a `String`. Read more

### `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.

#### `pub fn try_from(value: 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.

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

Performs the conversion.