Struct qcell::QCellOwner

pub struct QCellOwner { /* fields omitted */ }

Borrowing-owner of zero or more QCell instances.

See crate documentation.

Implementations

impl QCellOwner

pub fn new() -> Self

Create an owner that can be used for creating many QCell instances. It will have a temporally unique ID associated with it to detect using the wrong owner to access a cell at runtime, which is a programming error. This call will panic if the limit of 2^31 owners active at the same time is reached. This is the slow and safe version that uses a mutex and a free list to allocate IDs. If speed of this call matters, then consider using fast_new() instead.

This safe version does successfully defend against all malicious and unsafe use, as far as I am aware. If not, please raise an issue. The same unique ID will later be allocated to someone else once you drop the returned owner, but this cannot be abused to cause unsafe access to cells because there will still be only one owner active at any one time with that ID. Also it cannot be used maliciously to access cells which don't belong to the new caller, because you also need a reference to the cells. So for example if you have a graph of cells that is only accessible through a private structure, then someone else getting the same owner ID makes no difference, because they have no way to get a reference to those cells. In any case, you are probably going to drop all those cells at the same time as dropping the owner, because they are no longer of any use without the owner ID.

pub unsafe fn fast_new() -> Self

Create an owner that can be used for creating many QCell instances. It will have a unique(-ish) ID associated with it to detect using the wrong owner to access a cell at runtime, which is a programming error.

Safety

This call is much faster than new() because it uses a simple atomic increment to get a new ID, but it could be used maliciously to obtain unsafe behaviour, so the call is marked as unsafe.

If used non-maliciously the chance of getting unsafe behaviour in practice is zero -- not just close to zero but actually zero. To get unsafe behaviour, you'd have to accidentally create exactly 2^31 more owners to get a duplicate ID and you'd also have to have a bug in your code where you try to use the wrong owner to access a cell (which should normally be rejected with a panic). Already this is vanishingly improbable, but then if that happened by accident on one run but not on another, your code would still panic and you would fix your bug. So once that bug in your code is fixed, the risk is zero. No amount of fuzz-testing could ever cause unsafe behaviour once that bug is fixed. So whilst strictly-speaking this call is unsafe, in practice there is no risk unless you really try hard to exploit it.

pub fn cell<T>(&self, value: T) -> QCell<T>

Create a new cell owned by this owner instance. See also QCell::new.

pub fn id(&self) -> QCellOwnerID

Get the internal owner ID. This may be used to create QCell instances without needing a borrow on this structure, which is useful if this structure is already borrowed.

pub fn ro<'a, T>(&'a self, qc: &'a QCell<T>) -> &'a T

Borrow contents of a QCell immutably (read-only). Many QCell instances can be borrowed immutably at the same time from the same owner. Panics if the QCell is not owned by this QCellOwner.

pub fn rw<'a, T>(&'a mut self, qc: &'a QCell<T>) -> &'a mut T

Borrow contents of a QCell mutably (read-write). Only one QCell at a time can be borrowed from the owner using this call. The returned reference must go out of scope before another can be borrowed. Panics if the QCell is not owned by this QCellOwner.

pub fn rw2<'a, T, U>(
    &'a mut self,
    qc1: &'a QCell<T>,
    qc2: &'a QCell<U>
) -> (&'a mut T, &'a mut U)

Borrow contents of two QCell instances mutably. Panics if the two QCell instances point to the same memory. Panics if either QCell is not owned by this QCellOwner.

pub fn rw3<'a, T, U, V>(
    &'a mut self,
    qc1: &'a QCell<T>,
    qc2: &'a QCell<U>,
    qc3: &'a QCell<V>
) -> (&'a mut T, &'a mut U, &'a mut V)

Borrow contents of three QCell instances mutably. Panics if any pair of QCell instances point to the same memory. Panics if any QCell is not owned by this QCellOwner.

Trait Implementations

impl Default for QCellOwner

fn default() -> Self

Returns the "default value" for a type.

impl Drop for QCellOwner

fn drop(&mut self)

Executes the destructor for this type.