happens-before in swift when mutiple thread

Using Swift
1

i have one question about swift thread safaty. class Test { public var boolValue = false }. in above code, the property is safe or not safe if it access in A thread happens-before B thread. then i read the property from B thread. can i got correct value in this case .
is there possible the property value cache into A thread register, B thread can not read correct value even though read the property after A thread done

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2

That should not be possible if the accesses are properly ordered by happens-before, i.e. if the threads appropriately synchronize between the events.

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3 
let variable = Test()

DispatchQueue.global().async {
    variable.boolValue = true
}

sleep(1)

DispatchQueue.global().async {
    print("current value \(variable.boolValue)")
}

in above code, write the property in A thread first and read the property in B thread one second later. is automic in swift ? i have a little experience about C++ , value of variables in C++ may cache into CPU Core Cache, so read and write varibles in mutiple thread must use synchronizes

maybe my C++ experience is wrong, correct me if i am wrong

4

That code would be wrong in Swift, as it would in C++. Really, in some sense that’s a hardware constraint that both languages inherit.

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