Struct Arena 

pub struct Arena<T: Sized> { /* private fields */ }

An arena

It can be sent to other threads but is not Sync.
Pointers to the elements in the Arena are shareable between threads.

use shared_arena::Arena;

let arena = Arena::new();
let foo = arena.alloc(1);

let bar = std::thread::spawn(move || {
    let bar = arena.alloc(100);
    std::mem::drop(arena);
    bar
});

// The values are still valid, even if the arena has been dropped
// in the other thread
assert_eq!(*bar.join().unwrap() + *foo, 101);

Implementations

impl<T: Sized> Arena<T>

pub fn with_capacity(cap: usize) -> Arena<T>

Constructs a new Arena capable of holding at least cap elements

Because the arena allocate by page of 63 elements, it might be able to hold more elements than cap.

The Arena will reallocate itself if there is not enough space when allocating (with alloc* functions)

Example

let arena = Arena::with_capacity(2048);

pub fn new() -> Arena<T>

Constructs a new Arena capable of holding exactly 63 elements

The Arena will reallocate itself if there is not enough space when allocating (with alloc* functions)

Example

let arena = Arena::new();

pub fn alloc(&self, value: T) -> ArenaBox<T>

Writes a value in the arena, and returns an ArenaBox pointing to that value.

Example

let arena = Arena::new();
let my_num: ArenaBox<u8> = arena.alloc(0xFF);

assert_eq!(*my_num, 255);

pub fn alloc_with<F>(&self, initializer: F) -> ArenaBox<T>

where F: Fn(&mut MaybeUninit<T>) -> &T,

Finds an empty space in the arena and calls the function initializer with its argument pointing to that space. It returns an ArenaBox pointing to the newly initialized value.

The difference with alloc is that it has the benefit of avoiding intermediate copies of the value.

Safety

It is the caller responsability to initialize properly the value.
initializer must return &T, this is a way to ensure that its parameter &mut MaybeUninit<T> has been “consumed”.

If initializer returns a different reference than its parameter, the function will panic.

When the ArenaBox is dropped, the value is also dropped. If the value is not initialized correctly, it will drop an unitialized value, which is undefined behavior.

Example

struct MyData {
    a: usize
}

fn initialize_data<'a>(uninit: &'a mut MaybeUninit<MyData>, source: &MyData) -> &'a MyData {
    unsafe {
        let ptr = uninit.as_mut_ptr();
        ptr::copy(source, ptr, 1);
        &*ptr
    }
}

let arena = Arena::<MyData>::new();
let source = MyData { a: 101 };

let data = arena.alloc_with(|uninit| {
    initialize_data(uninit, &source)
});
assert!(data.a == 101);

pub fn alloc_arc(&self, value: T) -> ArenaArc<T>

Writes a value in the arena, and returns an ArenaArc pointing to that value.

Example

let arena = Arena::new();
let my_num: ArenaArc<u8> = arena.alloc_arc(0xFF);

assert_eq!(*my_num, 255);

pub fn alloc_arc_with<F>(&self, initializer: F) -> ArenaArc<T>

where F: Fn(&mut MaybeUninit<T>) -> &T,

Finds an empty space in the arena and calls the function initializer with its argument pointing to that space. It returns an ArenaArc pointing to the newly initialized value.

The difference with alloc_arc is that it has the benefit of avoiding intermediate copies of the value.

Safety

It is the caller responsability to initialize properly the value.
initializer must return &T, this is a way to ensure that its parameter &mut MaybeUninit<T> has been “consumed”.

If initializer returns a different reference than its parameter, the function will panic.

When all ArenaArc pointing that value are dropped, the value is also dropped. If the value is not initialized correctly, it will drop an unitialized value, which is undefined behavior.

Example

struct MyData {
    a: usize
}

fn initialize_data<'a>(uninit: &'a mut MaybeUninit<MyData>, source: &MyData) -> &'a MyData {
    unsafe {
        let ptr = uninit.as_mut_ptr();
        ptr::copy(source, ptr, 1);
        &*ptr
    }
}

let arena = Arena::<MyData>::new();
let source = MyData { a: 101 };

let data = arena.alloc_arc_with(|uninit| {
    initialize_data(uninit, &source)
});
assert!(data.a == 101);

pub fn alloc_rc(&self, value: T) -> ArenaRc<T>

Writes a value in the arena, and returns an ArenaRc pointing to that value.

Example

let arena = Arena::new();
let my_num: ArenaRc<u8> = arena.alloc_rc(0xFF);

assert_eq!(*my_num, 255);

pub fn alloc_rc_with<F>(&self, initializer: F) -> ArenaRc<T>

where F: Fn(&mut MaybeUninit<T>) -> &T,

Finds an empty space in the arena and calls the function initializer with its argument pointing to that space. It returns an ArenaRc pointing to the newly initialized value.

The difference with alloc_rc is that it has the benefit of avoiding intermediate copies of the value.

Safety

It is the caller responsability to initialize properly the value.
initializer must return &T, this is a way to ensure that its parameter &mut MaybeUninit<T> has been “consumed”.

If initializer returns a different reference than its parameter, the function will panic.

When all ArenaRc pointing that value are dropped, the value is also dropped. If the value is not initialized correctly, it will drop an unitialized value, which is undefined behavior.

Example

struct MyData {
    a: usize
}

fn initialize_data<'a>(uninit: &'a mut MaybeUninit<MyData>, source: &MyData) -> &'a MyData {
    unsafe {
        let ptr = uninit.as_mut_ptr();
        ptr::copy(source, ptr, 1);
        &*ptr
    }
}

let arena = Arena::<MyData>::new();
let source = MyData { a: 101 };

let data = arena.alloc_rc_with(|uninit| {
    initialize_data(uninit, &source)
});
assert!(data.a == 101);

pub fn shrink_to_fit(&self) -> bool

Shrinks the capacity of the arena as much as possible.

It will drop all pages that are unused (no Arena{Box,Arc,Rc} points to it).
If there is still one or more references to a page, the page won’t be dropped.

This is a slow function and it should not be called in a hot path.

The dedicated memory will be deallocated during this call.

Example

let mut arena = Arena::with_capacity(2048);
let mut values = Vec::new();

assert_eq!(arena.stats(), (0, 2079));

for _ in 0..80 {
    values.push(arena.alloc(0xFF));
}

arena.shrink_to_fit();

let (used, free) = arena.stats();
assert!(used == 80, free == 46);

pub fn stats(&self) -> (usize, usize)

Returns a tuple of non-free and free spaces in the arena

This is a slow function and it should not be called in a hot path.

Example

let arena = Arena::new();
let item = arena.alloc(1);
let (used, free) = arena.stats();
assert!(used == 1 && free == 62);

Trait Implementations

impl<T> Debug for Arena<T>

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

Formats the value using the given formatter.

impl<T: Sized> Default for Arena<T>

fn default() -> Arena<T>

Returns the “default value” for a type.

impl<T: Sized> Drop for Arena<T>

fn drop(&mut self)

Executes the destructor for this type.

impl<T: Sized> Send for Arena<T>