[−][src]Module qcell::doctest_qcell
This tests the QCell
implementation.
It should be impossible to copy a QCellOwner:
let mut owner1 = QCellOwner::new(); let mut owner2 = owner1; let rc = Rc::new(QCell::new(&owner1, 100u32)); // Compile fail
It should be impossible to clone a QCellOwner:
let mut owner1 = QCellOwner::new(); let owner2 = owner1.clone(); // Compile fail
Two different owners can't borrow each other's cells immutably:
let mut owner1 = QCellOwner::new(); let mut owner2 = QCellOwner::new(); let c1 = Rc::new(QCell::new(&owner1, 100u32)); let c1ref = owner2.ro(&c1); // Panics here println!("{}", *c1ref);
Or mutably:
let mut owner1 = QCellOwner::new(); let mut owner2 = QCellOwner::new(); let c1 = Rc::new(QCell::new(&owner1, 100u32)); let c1mutref = owner2.rw(&c1); // Panics here println!("{}", *c1mutref);
You can't have two separate mutable borrows active on the same owner at the same time:
let mut owner = QCellOwner::new(); let c1 = Rc::new(QCell::new(&owner, 100u32)); let c2 = Rc::new(QCell::new(&owner, 200u32)); let c1mutref = owner.rw(&c1); let c2mutref= owner.rw(&c2); // Compile error *c1mutref += 1; *c2mutref += 2;
However with rw2()
you can do two mutable borrows at the
same time, since this call checks at runtime that the two
references don't refer to the same memory:
let (c1mutref, c2mutref) = owner.rw2(&c1, &c2); *c1mutref += 1; *c2mutref += 2; assert_eq!(303, owner.ro(&c1) + owner.ro(&c2)); // Success!
You can't have a mutable borrow at the same time as an immutable borrow:
let c1ref = owner.ro(&c1); let c1mutref = owner.rw(&c1); // Compile error println!("{}", *c1ref);
Not even if it's borrowing a different object:
let c1mutref = owner.rw(&c1); let c2ref = owner.ro(&c2); // Compile error *c1mutref += 1;
Many immutable borrows at the same time is fine:
let c1ref = owner.ro(&c1); let c2ref = owner.ro(&c2); let c1ref2 = owner.ro(&c1); let c2ref2 = owner.ro(&c2); assert_eq!(600, *c1ref + *c2ref + *c1ref2 + *c2ref2); // Success!
Whilst a reference is active, it's impossible to drop the Rc
:
let c1ref = owner.ro(&c1); drop(c1); // Compile error println!("{}", *c1ref);
Also, whilst a reference is active, it's impossible to call
anything else that uses the owner
in an incompatible way,
e.g. &mut
when there's a &
reference:
fn test(o: &mut QCellOwner) {} let c1ref = owner.ro(&c1); test(&mut owner); // Compile error println!("{}", *c1ref);
Or &
when there's a &mut
reference:
fn test(o: &QCellOwner) {} let c1mutref = owner.rw(&c1); test(&owner); // Compile error *c1mutref += 1;
QCellOwner
and QCell
should be both Send
and Sync
by default:
fn is_send_sync<T: Send + Sync>() {} is_send_sync::<QCellOwner>(); is_send_sync::<QCell<()>>();
So for example we can share a cell ref between threads (Sync), and pass an owner back and forth (Send):
let mut owner = QCellOwner::new(); let cell = QCell::new(&owner, 100_i32); *owner.rw(&cell) += 1; let cell_ref = &cell; let mut owner = crossbeam::scope(move |s| { s.spawn(move |_| { *owner.rw(cell_ref) += 2; owner }).join().unwrap() }).unwrap(); *owner.rw(&cell) += 4; assert_eq!(*owner.ro(&cell), 107);
However you can't send a cell that's still borrowed:
let owner = QCellOwner::new(); let cell = QCell::new(&owner, 100); let val_ref = owner.ro(&cell); std::thread::spawn(move || { assert_eq!(*owner.ro(&cell), 100); }).join(); assert_eq!(*val_ref, 100);
If the contained type isn't Sync
, though, then QCell
shouldn't
be Sync
either:
fn is_sync<T: Sync>() {} is_sync::<QCell<Cell<i32>>>(); // Compile fail
let owner = QCellOwner::new(); let cell = QCell::new(&owner, Cell::new(100)); // This would be a data race if the compiler permitted it, but it doesn't std::thread::spawn(|| owner.ro(&cell).set(200)); // Compile fail owner.ro(&cell).set(300);
If the contained type isn't Send
, the QCell
should be neither
Sync
nor Send
:
fn is_sync<T: Sync>() {} is_sync::<QCell<Rc<()>>>(); // Compile fail
fn is_send<T: Send>() {} is_send::<QCell<Rc<()>>>(); // Compile fail
let owner = QCellOwner::new(); let cell = QCell::new(&owner, Rc::new(100)); // We aren't permitted to move the Rc to another thread std::thread::spawn(move || { // Compile fail assert_eq!(100, **owner.ro(&cell)); }).join();