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61 lines
2.0 KiB
61 lines
2.0 KiB
# Cloning
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Now that we've got some basic code set up, we'll need a way to clone the `Arc`.
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Basically, we need to:
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1. Get the `ArcInner` value of the `Arc`
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2. Increment the atomic reference count
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3. Construct a new instance of the `Arc` from the inner pointer
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Next, we can update the atomic reference count as follows:
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```rust,ignore
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self.inner().rc.fetch_add(1, Ordering::Relaxed);
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```
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As described in [the standard library's implementation of `Arc` cloning][2]:
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> Using a relaxed ordering is alright here, as knowledge of the original
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> reference prevents other threads from erroneously deleting the object.
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>
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> As explained in the [Boost documentation][1]:
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> > Increasing the reference counter can always be done with
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> > memory_order_relaxed: New references to an object can only be formed from an
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> > existing reference, and passing an existing reference from one thread to
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> > another must already provide any required synchronization.
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>
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> [1]: https://www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html
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[2]: https://github.com/rust-lang/rust/blob/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/alloc/src/sync.rs#L1171-L1181
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We'll need to add another import to use `Ordering`:
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```rust,ignore
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use std::sync::atomic::Ordering;
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```
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It is possible that in some contrived programs (e.g. using `mem::forget`) that
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the reference count could overflow, but it's unreasonable that would happen in
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any reasonable program.
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Then, we need to return a new instance of the `Arc`:
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```rust,ignore
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Self {
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ptr: self.ptr,
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_marker: PhantomData
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}
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```
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Now, let's wrap this all up inside the `Clone` implementation:
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```rust,ignore
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use std::sync::atomic::Ordering;
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impl<T> Clone for Arc<T> {
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fn clone(&self) -> Arc<T> {
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// Using a relaxed ordering is alright here as knowledge of the original
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// reference prevents other threads from wrongly deleting the object.
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self.inner().rc.fetch_add(1, Ordering::Relaxed);
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Self {
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ptr: self.ptr,
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_marker: PhantomData,
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}
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}
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}
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```
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