# Struct std::thread::LocalKey 1.0.0 [−] [src]

    pub struct LocalKey<T: 'static> {
        // some fields omitted
    }

A thread local storage key which owns its contents.

This key uses the fastest possible implementation available to it for the target platform. It is instantiated with the `thread_local!` macro and the primary method is the `with` method.

The `with` method yields a reference to the contained value which cannot be sent across threads or escape the given closure.

# Initialization and Destruction

Initialization is dynamically performed on the first call to `with()` within a thread, and values support destructors which will be run when a thread exits.

# Examples

fn main() { use std::cell::RefCell; use std::thread; thread_local!(static FOO: RefCell<u32> = RefCell::new(1)); FOO.with(|f| { assert_eq!(*f.borrow(), 1); *f.borrow_mut() = 2; }); // each thread starts out with the initial value of 1 thread::spawn(move|| { FOO.with(|f| { assert_eq!(*f.borrow(), 1); *f.borrow_mut() = 3; }); }); // we retain our original value of 2 despite the child thread FOO.with(|f| { assert_eq!(*f.borrow(), 2); }); }

    use std::cell::RefCell;
    use std::thread;

    thread_local!(static FOO: RefCell<u32> = RefCell::new(1));

    FOO.with(|f| {
        assert_eq!(*f.borrow(), 1);
        *f.borrow_mut() = 2;
    });

    // each thread starts out with the initial value of 1
    thread::spawn(move|| {
        FOO.with(|f| {
            assert_eq!(*f.borrow(), 1);
            *f.borrow_mut() = 3;
        });
    });

    // we retain our original value of 2 despite the child thread
    FOO.with(|f| {
        assert_eq!(*f.borrow(), 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. On OSX, initializing TLS during destruction of other TLS slots can sometimes cancel all destructors for the current thread, whether or not the slots have already had their destructors run or not.

## Methods

### `impl<T: 'static> LocalKey<T>`
[src]

#### `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.

#### `fn state(&'static self) -> LocalKeyState`

Unstable (`thread_local_state` #27716)

: state querying was recently added

Query the current state of this key.

A key is initially in the `Uninitialized` state whenever a thread starts. It will remain in this state up until the first call to `with` within a thread has run the initialization expression successfully.

Once the initialization expression succeeds, the key transitions to the `Valid` state which will guarantee that future calls to `with` will succeed within the thread.

When a thread exits, each key will be destroyed in turn, and as keys are destroyed they will enter the `Destroyed` state just before the destructor starts to run. Keys may remain in the `Destroyed` state after destruction has completed. Keys without destructors (e.g. with types that are `Copy`), may never enter the `Destroyed` state.

Keys in the `Uninitialized` state can be accessed so long as the initialization does not panic. Keys in the `Valid` state are guaranteed to be able to be accessed. Keys in the `Destroyed` state will panic on any call to `with`.

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