rust, smart pointer


rust, smart pointer

  • Box<>
    • Box::new
  • Rc<>
    • Rc.new
    • Weak<>
  • Cell<>
    • Cell.new
    • Cell get/set
  • RefCell<>
    • RefCell borrow/borrow_mut
  • Arc<>
    • Arc::new
  • Mutex<>
    • Mutex::new
    • Mutex lock/trylock/drop

Box<>

Like unique_ptr in C++.

Internally, it contains a Unqiue<> which holds the raw pointer.

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pub struct Box<T: ?Sized>(Unique<T>);

pub struct Unique<T: ?Sized> {
    pointer: *const T,
    // NOTE: this marker has no consequences for variance, but is necessary
    // for dropck to understand that we logically own a `T`.
    //
    // For details, see:
    // https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md#phantom-data
    _marker: PhantomData<T>,
}

Box::new

It uses ‘box’ to allocate T from heap.

‘box’ -> exchange_malloc by compiler.

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    pub fn new(x: T) -> Box<T> {
        box x
    }

Rc<>

Like shared_ptr in C++. It implements the reference-counting pointer for single-threaded environment.

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pub struct Rc<T: ?Sized> {
    ptr: NonNull<RcBox<T>>,
    phantom: PhantomData<RcBox<T>>,
}

pub struct NonNull<T: ?Sized> {
    pointer: *const T,
}

struct RcBox<T: ?Sized> {
    strong: Cell<usize>,
    weak: Cell<usize>,
    value: T,
}

RcBox holds counters for strong and weak references. Note these two counters are Cell<> which means it is interior mutable.

Rc.new

RcBox is created by ‘box’ to hold the value and counters.

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    pub fn new(value: T) -> Rc<T> {
        // There is an implicit weak pointer owned by all the strong
        // pointers, which ensures that the weak destructor never frees
        // the allocation while the strong destructor is running, even
        // if the weak pointer is stored inside the strong one.
        Self::from_inner(Box::into_raw_non_null(box RcBox {
            strong: Cell::new(1),
            weak: Cell::new(1),
            value,
        }))
    }

Weak<>

Weak is a version of [Rc] that holds a non-owning reference to the managed allocation. The allocation is accessed by calling [upgrade] on the Weak pointer, which returns an [Option]<[Rc]>.

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pub struct Weak<T: ?Sized> {
    // This is a `NonNull` to allow optimizing the size of this type in enums,
    // but it is not necessarily a valid pointer.
    // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
    // to allocate space on the heap.  That's not a value a real pointer
    // will ever have because RcBox has alignment at least 2.
    ptr: NonNull<RcBox<T>>,
}
    pub fn upgrade(&self) -> Option<Rc<T>> {
        let inner = self.inner()?;
        if inner.strong() == 0 {
            None
        } else {
            inner.inc_strong();
            Some(Rc::from_inner(self.ptr))
        }
    }
    pub fn downgrade(this: &Self) -> Weak<T> {
        this.inc_weak();
        // Make sure we do not create a dangling Weak
        debug_assert!(!is_dangling(this.ptr));
        Weak { ptr: this.ptr }
    }

Cell<>

Cell is a mutable memory location which means mutation inside an immutable struct, or interior mutability. Internally it contains a UnsafeCell<> which is a wrap to the T.

Strictly it is not a pointer.

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pub struct Cell<T: ?Sized> {
    value: UnsafeCell<T>,
}

Cell.new

It creates a new Cell containing the given value.

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    pub const fn new(value: T) -> Cell<T> {
        Cell { value: UnsafeCell::new(value) }
    }

Cell get/set

set will replace the value with new val and drop the old.
get will simple return the copy of the value. Note ‘get’ is under a constraint Copy trait.

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    pub fn set(&self, val: T) {
        let old = self.replace(val);
        drop(old);
    }
    pub fn replace(&self, val: T) -> T {
        // SAFETY: This can cause data races if called from a separate thread,
        // but `Cell` is `!Sync` so this won't happen.
        mem::replace(unsafe { &mut *self.value.get() }, val)
    }    
    pub fn get(&self) -> T {
        // SAFETY: This can cause data races if called from a separate thread,
        // but `Cell` is `!Sync` so this won't happen.
        unsafe { *self.value.get() }
    }

RefCell<>

RefCell is a mutable memory location with dynamically checked borrow rules. Not like Cell<>, borrow rules are checked at runtime. In this case, it will panic!() if any violations found.

Roughly, RefCell<> is just like Cell<>, but provides reference/de-reference. To get access to the value in RefCell, borrow/borrow_mut must be used.

BorrowFlag is used to track the borrow relationship, and value is held by UnsafeCell<>.

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pub struct RefCell<T: ?Sized> {
    borrow: Cell<BorrowFlag>,
    value: UnsafeCell<T>,
}

RefCell borrow/borrow_mut

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    pub fn borrow_mut(&self) -> RefMut<'_, T> {
        self.try_borrow_mut().expect("already borrowed")
    }  
    pub fn borrow(&self) -> Ref<'_, T> {
        self.try_borrow().expect("already mutably borrowed")
    }

Arc<>

A thread-safe reference-counting pointer. ‘Arc’ stands for ‘Atomically Reference Counted’.

Internally it uses ArcInner to hold the value and counters.

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pub struct Arc<T: ?Sized> {
    ptr: NonNull<ArcInner<T>>,
    phantom: PhantomData<ArcInner<T>>,
}

struct ArcInner<T: ?Sized> {
    strong: atomic::AtomicUsize,

    // the value usize::MAX acts as a sentinel for temporarily "locking" the
    // ability to upgrade weak pointers or downgrade strong ones; this is used
    // to avoid races in `make_mut` and `get_mut`.
    weak: atomic::AtomicUsize,

    data: T,
}

Arc::new

Use box to allocate memory for ArcInner.

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    pub fn new(data: T) -> Arc<T> {
        // Start the weak pointer count as 1 which is the weak pointer that's
        // held by all the strong pointers (kinda), see std/rc.rs for more info
        let x: Box<_> = box ArcInner {
            strong: atomic::AtomicUsize::new(1),
            weak: atomic::AtomicUsize::new(1),
            data,
        };
        Self::from_inner(Box::into_raw_non_null(x))
    }

Mutex<>

Mutex is a mutual exclusion primitive useful for protecting shared data.

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pub struct Mutex<T: ?Sized> {
    // Note that this mutex is in a *box*, not inlined into the struct itself.
    // Once a native mutex has been used once, its address can never change (it
    // can't be moved). This mutex type can be safely moved at any time, so to
    // ensure that the native mutex is used correctly we box the inner mutex to
    // give it a constant address.
    inner: Box<sys::Mutex>,
    poison: poison::Flag,
    data: UnsafeCell<T>,
}

The sys::Mutex is boxed as shown above.

Mutex::new

inner is a boxed sys::Mutex.

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    pub fn new(t: T) -> Mutex<T> {
        let mut m = Mutex {
            inner: box sys::Mutex::new(),
            poison: poison::Flag::new(),
            data: UnsafeCell::new(t),
        };
        unsafe {
            m.inner.init();
        }
        m
    }

Mutex lock/trylock/drop

drop will unlock the mutex.

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    pub fn lock(&self) -> LockResult<MutexGuard<'_, T>> {
        unsafe {
            self.inner.raw_lock();
            MutexGuard::new(self)
        }
    }

    pub fn try_lock(&self) -> TryLockResult<MutexGuard<'_, T>> {
        unsafe {
            if self.inner.try_lock() {
                Ok(MutexGuard::new(self)?)
            } else {
                Err(TryLockError::WouldBlock)
            }
        }
    }
    fn drop(&mut self) {
        // This is actually safe b/c we know that there is no further usage of
        // this mutex (it's up to the user to arrange for a mutex to get
        // dropped, that's not our job)
        //
        // IMPORTANT: This code must be kept in sync with `Mutex::into_inner`.
        unsafe { self.inner.destroy() }
    }