Question about storeIfNilThenLoad(_:) and sample code

Related Projects Swift Atomics
1

I'm trying to implement a thread-safe lazily initialized reference. I see this sample code here:

class Image {
  var _histogram: UnsafeAtomicLazyReference<Histogram> = ...

  // This is safe to call concurrently from multiple threads.
  var atomicLazyHistogram: Histogram {
    if let histogram = _histogram.load() { return foo }
    // Note that code here may run concurrently on
    // multiple threads, but only one of them will get to
    // succeed setting the reference.
    let histogram = ...
    return _histogram.storeIfNilThenLoad(foo)
}

But that code seems wrong to me. There seem to be a few issues with that code. Where is foo defined? Also can't a race happen between the call to load() and storeIfNilThenLoad(...)?

2

I think the sample code is simply incorrect. I've submitted a PR to fix it: