LocalKey in std::thread - Rust
Description: A thread local storage (TLS) key which owns its contents.

## LocalKey

# Struct LocalKey

1.0.0 · Source

    pub struct LocalKey<T: 'static> { /* private fields */ }

Expand description

A thread local storage (TLS) key which owns its contents.

This key uses the fastest implementation available on the target platform. It is instantiated with the `thread_local!` macro and the primary method is the `with` method, though there are helpers to make working with `Cell` types easier.

The `with` method yields a reference to the contained value which cannot outlive the current thread or escape the given closure.

## §Initialization and Destruction

Initialization is dynamically performed on the first call to a setter (e.g. `with`) within a thread, and values that implement `Drop` get destructed when a thread exits. Some platform-specific caveats apply, which are explained below. Note that, should the destructor panic, the whole process will be aborted. On platforms where initialization requires memory allocation, this is performed directly through `System`, allowing the global allocator to make use of thread local storage.

A `LocalKey`’s initializer cannot recursively depend on itself. Using a `LocalKey` in this way may cause panics, aborts, or infinite recursion on the first call to `with`.

## §Single-thread Synchronization

Though there is no potential race with other threads, it is still possible to obtain multiple references to the thread-local data in different places on the call stack. For this reason, only shared (`&T`) references may be obtained.

To allow obtaining an exclusive mutable reference (`&mut T`), typically a `Cell` or `RefCell` is used (see the `std::cell` for more information on how exactly this works). To make this easier there are specialized implementations for `LocalKey<Cell<T>>` and `LocalKey<RefCell<T>>`.

## §Examples

    use std::cell::Cell;
    use std::thread;

    // explicit `const {}` block enables more efficient initialization
    thread_local!(static FOO: Cell<u32> = const { Cell::new(1) });

    assert_eq!(FOO.get(), 1);
    FOO.set(2);

    // each thread starts out with the initial value of 1
    let t = thread::spawn(move || {
        assert_eq!(FOO.get(), 1);
        FOO.set(3);
    });

    // wait for the thread to complete and bail out on panic
    t.join().unwrap();

    // we retain our original value of 2 despite the child thread
    assert_eq!(FOO.get(), 2);

## §Platform-specific behavior

Note that a “best effort” is made to ensure that destructors for types stored in thread local storage are run, but not all platforms can guarantee that destructors will be run for all types in thread local storage. For example, there are a number of known caveats where destructors are not run:

1. On Unix systems when pthread-based TLS is being used, destructors will not be run for TLS values on the main thread when it exits. Note that the application will exit immediately after the main thread exits as well.
2. On all platforms it’s possible for TLS to re-initialize other TLS slots during destruction. Some platforms ensure that this cannot happen infinitely by preventing re-initialization of any slot that has been destroyed, but not all platforms have this guard. Those platforms that do not guard typically have a synthetic limit after which point no more destructors are run.
3. When the process exits on Windows systems, TLS destructors may only be run on the thread that causes the process to exit. This is because the other threads may be forcibly terminated.

### §Synchronization in thread-local destructors

On Windows, synchronization operations (such as `JoinHandle::join`) in thread local destructors are prone to deadlocks and so should be avoided. This is because the loader lock is held while a destructor is run. The lock is acquired whenever a thread starts or exits or when a DLL is loaded or unloaded. Therefore these events are blocked for as long as a thread local destructor is running.

## Implementations§

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### impl<T: 'static> LocalKey<T>

1.0.0 · Source

#### pub fn with<F, R>(&'static self, f: F) -> R
where F: FnOnce(&T) -> R,

Acquires a reference to the value in this TLS key.

This will lazily initialize the value if this thread has not referenced this key yet.

##### §Panics

This function will `panic!()` if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    thread_local! {
        pub static STATIC: String = String::from("I am");
    }

    assert_eq!(
        STATIC.with(|original_value| format!("{original_value} initialized")),
        "I am initialized",
    );

1.26.0 · Source

#### pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError>
where F: FnOnce(&T) -> R,

Acquires a reference to the value in this TLS key.

This will lazily initialize the value if this thread has not referenced this key yet. If the key has been destroyed (which may happen if this is called in a destructor), this function will return an `AccessError`.

##### §Panics

This function will still `panic!()` if the key is uninitialized and the key’s initializer panics.

##### §Examples

    thread_local! {
        pub static STATIC: String = String::from("I am");
    }

    assert_eq!(
        STATIC.try_with(|original_value| format!("{original_value} initialized")),
        Ok(String::from("I am initialized")),
    );

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### impl<T: 'static> LocalKey<Cell<T>>

1.73.0 · Source

#### pub fn set(&'static self, value: T)

Sets or initializes the contained value.

Unlike the other methods, this will not run the lazy initializer of the thread local. Instead, it will be directly initialized with the given value if it wasn’t initialized yet.

##### §Panics

Panics if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    use std::cell::Cell;

    thread_local! {
        static X: Cell<i32> = panic!("!");
    }

    // Calling X.get() here would result in a panic.

    X.set(123); // But X.set() is fine, as it skips the initializer above.

    assert_eq!(X.get(), 123);

1.73.0 · Source

#### pub fn get(&'static self) -> T
where T: Copy,

Returns a copy of the contained value.

This will lazily initialize the value if this thread has not referenced this key yet.

##### §Panics

Panics if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    use std::cell::Cell;

    thread_local! {
        static X: Cell<i32> = const { Cell::new(1) };
    }

    assert_eq!(X.get(), 1);

1.73.0 · Source

#### pub fn take(&'static self) -> T
where T: Default,

Takes the contained value, leaving `Default::default()` in its place.

This will lazily initialize the value if this thread has not referenced this key yet.

##### §Panics

Panics if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    use std::cell::Cell;

    thread_local! {
        static X: Cell<Option<i32>> = const { Cell::new(Some(1)) };
    }

    assert_eq!(X.take(), Some(1));
    assert_eq!(X.take(), None);

1.73.0 · Source

#### pub fn replace(&'static self, value: T) -> T

Replaces the contained value, returning the old value.

This will lazily initialize the value if this thread has not referenced this key yet.

##### §Panics

Panics if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    use std::cell::Cell;

    thread_local! {
        static X: Cell<i32> = const { Cell::new(1) };
    }

    assert_eq!(X.replace(2), 1);
    assert_eq!(X.replace(3), 2);

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#### pub fn update(&'static self, f: impl FnOnce(T) -> T)
where T: Copy,

🔬This is a nightly-only experimental API. (`local_key_cell_update` #143989)

Updates the contained value using a function.

##### §Examples

    #![feature(local_key_cell_update)]
    use std::cell::Cell;

    thread_local! {
        static X: Cell<i32> = const { Cell::new(5) };
    }

    X.update(|x| x + 1);
    assert_eq!(X.get(), 6);

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### impl<T: 'static> LocalKey<RefCell<T>>

1.73.0 · Source

#### pub fn with_borrow<F, R>(&'static self, f: F) -> R
where F: FnOnce(&T) -> R,

Acquires a reference to the contained value.

This will lazily initialize the value if this thread has not referenced this key yet.

##### §Panics

Panics if the value is currently mutably borrowed.

Panics if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    use std::cell::RefCell;

    thread_local! {
        static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
    }

    X.with_borrow(|v| assert!(v.is_empty()));

1.73.0 · Source

#### pub fn with_borrow_mut<F, R>(&'static self, f: F) -> R
where F: FnOnce(&mut T) -> R,

Acquires a mutable reference to the contained value.

This will lazily initialize the value if this thread has not referenced this key yet.

##### §Panics

Panics if the value is currently borrowed.

Panics if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    use std::cell::RefCell;

    thread_local! {
        static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
    }

    X.with_borrow_mut(|v| v.push(1));

    X.with_borrow(|v| assert_eq!(*v, vec![1]));

1.73.0 · Source

#### pub fn set(&'static self, value: T)

Sets or initializes the contained value.

Unlike the other methods, this will not run the lazy initializer of the thread local. Instead, it will be directly initialized with the given value if it wasn’t initialized yet.

##### §Panics

Panics if the value is currently borrowed.

Panics if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    use std::cell::RefCell;

    thread_local! {
        static X: RefCell<Vec<i32>> = panic!("!");
    }

    // Calling X.with() here would result in a panic.

    X.set(vec![1, 2, 3]); // But X.set() is fine, as it skips the initializer above.

    X.with_borrow(|v| assert_eq!(*v, vec![1, 2, 3]));

1.73.0 · Source

#### pub fn take(&'static self) -> T
where T: Default,

Takes the contained value, leaving `Default::default()` in its place.

This will lazily initialize the value if this thread has not referenced this key yet.

##### §Panics

Panics if the value is currently borrowed.

Panics if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    use std::cell::RefCell;

    thread_local! {
        static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
    }

    X.with_borrow_mut(|v| v.push(1));

    let a = X.take();

    assert_eq!(a, vec![1]);

    X.with_borrow(|v| assert!(v.is_empty()));

1.73.0 · Source

#### pub fn replace(&'static self, value: T) -> T

Replaces the contained value, returning the old value.

##### §Panics

Panics if the value is currently borrowed.

Panics if the key currently has its destructor running, and it may panic if the destructor has previously been run for this thread.

##### §Examples

    use std::cell::RefCell;

    thread_local! {
        static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
    }

    let prev = X.replace(vec![1, 2, 3]);
    assert!(prev.is_empty());

    X.with_borrow(|v| assert_eq!(*v, vec![1, 2, 3]));

## Trait Implementations§

1.16.0 · Source§

### impl<T: 'static> Debug for LocalKey<T>

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

Formats the value using the given formatter. Read more

## Auto Trait Implementations§

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### impl<T> Freeze for LocalKey<T>

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### impl<T> RefUnwindSafe for LocalKey<T>

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### impl<T> Send for LocalKey<T>

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### impl<T> Sync for LocalKey<T>

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### impl<T> Unpin for LocalKey<T>

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### impl<T> UnwindSafe for LocalKey<T>

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.