Struct state::TypeMap

pub struct TypeMap<K: Kind> { /* private fields */ }

A type map storing values based on types.

A type map stores at most one instance of given type as well as n thread-local instances of a given type.

Type Bounds

A TypeMap can store values that are both Send + Sync, just Send, or neither. The TypeMap! macro is used to specify the kind of type map:

use state::TypeMap;

// Values must implement `Send + Sync`. The type_map itself is `Send + Sync`.
let type_map: TypeMap![Send + Sync] = <TypeMap![Send + Sync]>::new();
let type_map: TypeMap![Sync + Send] = <TypeMap![Sync + Send]>::new();

// Values must implement `Send`. The type_map itself is `Send`, `!Sync`.
let type_map: TypeMap![Send] = <TypeMap![Send]>::new();

// Values needn't implement `Send` nor `Sync`. `TypeMap` is `!Send`, `!Sync`.
let type_map: TypeMap![] = <TypeMap![]>::new();

Setting State

Global state is set via the set() method and retrieved via the get() method. The type of the value being set must meet the bounds of the TypeMap.

use state::TypeMap;

fn f_send_sync<T: Send + Sync + Clone + 'static>(value: T) {
    let type_map = <TypeMap![Send + Sync]>::new();
    type_map.set(value.clone());

    let type_map = <TypeMap![Send]>::new();
    type_map.set(value.clone());

    let type_map = <TypeMap![]>::new();
    type_map.set(value.clone());
}

fn f_send<T: Send + Clone + 'static>(value: T) {
    // This would fail to compile since `T` may not be `Sync`.
    // let type_map = <TypeMap![Send + Sync]>::new();
    // type_map.set(value.clone());

    let type_map = <TypeMap![Send]>::new();
    type_map.set(value.clone());

    let type_map = <TypeMap![]>::new();
    type_map.set(value.clone());
}

fn f<T: 'static>(value: T) {
    // This would fail to compile since `T` may not be `Sync` or `Send`.
    // let type_map = <TypeMap![Send + Sync]>::new();
    // type_map.set(value.clone());

    // This would fail to compile since `T` may not be `Send`.
    // let type_map = <TypeMap![Send]>::new();
    // type_map.set(value.clone());

    let type_map = <TypeMap![]>::new();
    type_map.set(value);
}

// If `TypeMap` is `Send + Sync`, it can be `const`-constructed.
static TYPE_MAP: TypeMap![Send + Sync] = <TypeMap![Send + Sync]>::new();

TYPE_MAP.set(String::new());
TYPE_MAP.get::<String>();

Freezing

By default, all get, set, get_local, and set_local calls result in synchronization overhead for safety. However, if calling set or set_local is no longer required, the overhead can be eliminated by freezing the TypeMap. A frozen type map can only be read and never written to. Attempts to write to a frozen type map will be ignored.

To freeze a TypeMap, call freeze(). A frozen map can never be thawed. To check if a type map is frozen, call is_frozen().

Thread-Local State

Thread-local state on a Send + Sync type map is set via the set_local() method and retrieved via the get_local() method. The type of the value being set must be transferable across thread boundaries but need not be thread-safe. In other words, it must satisfy Send + 'static but not necessarily Sync. Values retrieved from thread-local state are exactly that: local to the current thread. As such, you cannot use thread-local state to synchronize across multiple threads.

Thread-local state is initialized on an as-needed basis. The function used to initialize the thread-local state is passed in as an argument to set_local. When the state is retrieved from a given thread for the first time, the function is executed to generate the initial value. The function is executed at most once per thread. The same function is used for initialization across all threads.

Note: Rust reuses thread IDs across multiple threads. This means that is possible to set thread-local state in thread A, have that thread die, start a new thread B, and access the state set in tread A in thread B.

Example

Set and later retrieve a value of type T:

use state::TypeMap;

static TYPE_MAP: TypeMap![Send + Sync] = <TypeMap![Send + Sync]>::new();

TYPE_MAP.set_local(|| T::new());
TYPE_MAP.get_local::<T>();

Implementations

impl TypeMap<SendSync>

pub const fn new() -> Self

Creates a new type map with no stored values.

Example

Create a globally available type map:

use state::TypeMap;

static TYPE_MAP: TypeMap![Send + Sync] = <TypeMap![Send + Sync]>::new();

pub fn set<T: Send + Sync + 'static>(&self, state: T) -> bool

Sets the global state for type T if it has not been set before and self is not frozen.

If the state for T has previously been set or self is frozen, the state is unchanged and false is returned. Otherwise true is returned.

Example

Set the state for AtomicUsize. The first set is succesful while the second fails.

use state::TypeMap;

static TYPE_MAP: TypeMap![Send + Sync] = <TypeMap![Send + Sync]>::new();

assert_eq!(TYPE_MAP.set(AtomicUsize::new(0)), true);
assert_eq!(TYPE_MAP.set(AtomicUsize::new(1)), false);

impl TypeMap<Send>

pub fn new() -> Self

Creates a new type map with no stored values.

Example

use std::cell::Cell;

use state::TypeMap;

let type_map = <TypeMap![Send]>::new();

let value: Cell<u8> = Cell::new(10);
type_map.set(value);
assert_eq!(type_map.get::<Cell<u8>>().get(), 10);

type_map.get::<Cell<u8>>().set(99);
assert_eq!(type_map.get::<Cell<u8>>().get(), 99);

pub fn set<T: Send + 'static>(&self, state: T) -> bool

Sets the global state for type T if it has not been set before and self is not frozen.

If the state for T has previously been set or self is frozen, the state is unchanged and false is returned. Otherwise true is returned.

Example

Set the state. The first set is succesful while the second fails.

use state::TypeMap;

let type_map = <TypeMap![Send]>::new();
assert!(type_map.set(AtomicUsize::new(0)));
assert!(!type_map.set(AtomicUsize::new(1)));

impl TypeMap<Neither>

pub fn new() -> Self

Creates a new type_map with no stored values.

Example

use std::cell::Cell;
use state::TypeMap;

let type_map = <TypeMap![]>::new();

let value: Cell<u8> = Cell::new(10);
type_map.set(value);
assert_eq!(type_map.get::<Cell<u8>>().get(), 10);

type_map.get::<Cell<u8>>().set(99);
assert_eq!(type_map.get::<Cell<u8>>().get(), 99);

pub fn set<T: 'static>(&self, state: T) -> bool

Sets the global state for type T if it has not been set before and self is not frozen.

If the state for T has previously been set or self is frozen, the state is unchanged and false is returned. Otherwise true is returned.

Example

Set the state. The first set is succesful while the second fails.

use std::cell::Cell;
use state::TypeMap;

let type_map = <TypeMap![]>::new();
assert!(type_map.set(Cell::new(10)));
assert!(!type_map.set(Cell::new(17)));

impl<K: Kind> TypeMap<K>

pub fn try_get<T: 'static>(&self) -> Option<&T>

Attempts to retrieve the global state for type T.

Returns Some if the state has previously been set. Otherwise returns None.

Example

use state::TypeMap;

static TYPE_MAP: TypeMap![Send + Sync] = <TypeMap![Send + Sync]>::new();

struct MyState(AtomicUsize);

// State for `T` is initially unset.
assert!(TYPE_MAP.try_get::<MyState>().is_none());

TYPE_MAP.set(MyState(AtomicUsize::new(0)));

let my_state = TYPE_MAP.try_get::<MyState>().expect("MyState");
assert_eq!(my_state.0.load(Ordering::Relaxed), 0);

pub fn get<T: 'static>(&self) -> &T

Retrieves the global state for type T.

Panics

Panics if the state for type T has not previously been set(). Use try_get() for a non-panicking version.

Example

use state::TypeMap;

static TYPE_MAP: TypeMap![Send + Sync] = <TypeMap![Send + Sync]>::new();

struct MyState(AtomicUsize);

TYPE_MAP.set(MyState(AtomicUsize::new(0)));

let my_state = TYPE_MAP.get::<MyState>();
assert_eq!(my_state.0.load(Ordering::Relaxed), 0);

pub fn freeze(&mut self)

Freezes the type_map. A frozen type_map disallows writes allowing for synchronization-free reads.

Example

use state::TypeMap;

// A new type_map starts unfrozen and can be written to.
let mut type_map = <TypeMap![Send + Sync]>::new();
assert_eq!(type_map.set(1usize), true);

// While unfrozen, `get`s require synchronization.
assert_eq!(type_map.get::<usize>(), &1);

// After freezing, calls to `set` or `set_local `will fail.
type_map.freeze();
assert_eq!(type_map.set(1u8), false);
assert_eq!(type_map.set("hello"), false);

// Calls to `get` or `get_local` are synchronization-free when frozen.
assert_eq!(type_map.try_get::<u8>(), None);
assert_eq!(type_map.get::<usize>(), &1);

pub fn is_frozen(&self) -> bool

Returns true if the type_map is frozen and false otherwise.

Example

use state::TypeMap;

// A new type_map starts unfrozen and is frozen using `freeze`.
let mut type_map = <TypeMap![Send]>::new();
assert_eq!(type_map.is_frozen(), false);

type_map.freeze();
assert_eq!(type_map.is_frozen(), true);

pub fn len(&self) -> usize

Returns the number of distinctly typed values in self.

Example

use state::TypeMap;

let type_map = <TypeMap![Send + Sync]>::new();
assert_eq!(type_map.len(), 0);

assert_eq!(type_map.set(1usize), true);
assert_eq!(type_map.len(), 1);

assert_eq!(type_map.set(2usize), false);
assert_eq!(type_map.len(), 1);

assert_eq!(type_map.set(1u8), true);
assert_eq!(type_map.len(), 2);

pub fn is_empty(&self) -> bool

Returns true if self contains zero values.

Example

use state::TypeMap;

let type_map = <TypeMap![Send + Sync]>::new();
assert!(type_map.is_empty());

assert_eq!(type_map.set(1usize), true);
assert!(!type_map.is_empty());

Trait Implementations

impl<K: Kind> Debug for TypeMap<K>

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

Formats the value using the given formatter.

impl Send for TypeMap<Send>

impl Send for TypeMap<SendSync>

impl Sync for TypeMap<SendSync>