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| </pre><pre class="rust"><code><span class="kw">use </span>std::mem; |
| <span class="kw">use </span>std::ptr; |
| <span class="kw">use </span>std::rc::Rc; |
| <span class="kw">use </span>std::sync::Arc; |
| |
| <span class="doccomment">/// A trait describing smart reference counted pointers. |
| /// |
| /// Note that in a way [`Option<Arc<T>>`][Option] is also a smart reference counted pointer, just |
| /// one that can hold NULL. |
| /// |
| /// The trait is unsafe, because a wrong implementation will break the [ArcSwapAny] |
| /// implementation and lead to UB. |
| /// |
| /// This is not actually expected for downstream crate to implement, this is just means to reuse |
| /// code for [Arc] and [`Option<Arc>`][Option] variants. However, it is theoretically possible (if |
| /// you have your own [Arc] implementation). |
| /// |
| /// It is also implemented for [Rc], but that is not considered very useful (because the |
| /// [ArcSwapAny] is not `Send` or `Sync`, therefore there's very little advantage for it to be |
| /// atomic). |
| /// |
| /// # Safety |
| /// |
| /// Aside from the obvious properties (like that incrementing and decrementing a reference count |
| /// cancel each out and that having less references tracked than how many things actually point to |
| /// the value is fine as long as the count doesn't drop to 0), it also must satisfy that if two |
| /// pointers have the same value, they point to the same object. This is specifically not true for |
| /// ZSTs, but it is true for `Arc`s of ZSTs, because they have the reference counts just after the |
| /// value. It would be fine to point to a type-erased version of the same object, though (if one |
| /// could use this trait with unsized types in the first place). |
| /// |
| /// Furthermore, the type should be Pin (eg. if the type is cloned or moved, it should still |
| /// point/deref to the same place in memory). |
| /// |
| /// [Arc]: std::sync::Arc |
| /// [Rc]: std::rc::Rc |
| /// [ArcSwapAny]: crate::ArcSwapAny |
| </span><span class="kw">pub unsafe trait </span>RefCnt: Clone { |
| <span class="doccomment">/// The base type the pointer points to. |
| </span><span class="kw">type </span>Base; |
| |
| <span class="doccomment">/// Converts the smart pointer into a raw pointer, without affecting the reference count. |
| /// |
| /// This can be seen as kind of freezing the pointer ‒ it'll be later converted back using |
| /// [`from_ptr`](#method.from_ptr). |
| /// |
| /// The pointer must point to the value stored (and the value must be the same as one returned |
| /// by [`as_ptr`](#method.as_ptr). |
| </span><span class="kw">fn </span>into_ptr(me: <span class="self">Self</span>) -> <span class="kw-2">*mut </span><span class="self">Self</span>::Base; |
| |
| <span class="doccomment">/// Provides a view into the smart pointer as a raw pointer. |
| /// |
| /// This must not affect the reference count ‒ the pointer is only borrowed. |
| </span><span class="kw">fn </span>as_ptr(me: <span class="kw-2">&</span><span class="self">Self</span>) -> <span class="kw-2">*mut </span><span class="self">Self</span>::Base; |
| |
| <span class="doccomment">/// Converts a raw pointer back into the smart pointer, without affecting the reference count. |
| /// |
| /// This is only called on values previously returned by [`into_ptr`](#method.into_ptr). |
| /// However, it is not guaranteed to be 1:1 relation ‒ `from_ptr` may be called more times than |
| /// `into_ptr` temporarily provided the reference count never drops under 1 during that time |
| /// (the implementation sometimes owes a reference). These extra pointers will either be |
| /// converted back using `into_ptr` or forgotten. |
| /// |
| /// # Safety |
| /// |
| /// This must not be called by code outside of this crate. |
| </span><span class="kw">unsafe fn </span>from_ptr(ptr: <span class="kw-2">*const </span><span class="self">Self</span>::Base) -> <span class="self">Self</span>; |
| |
| <span class="doccomment">/// Increments the reference count by one. |
| /// |
| /// Return the pointer to the inner thing as a side effect. |
| </span><span class="kw">fn </span>inc(me: <span class="kw-2">&</span><span class="self">Self</span>) -> <span class="kw-2">*mut </span><span class="self">Self</span>::Base { |
| <span class="self">Self</span>::into_ptr(<span class="self">Self</span>::clone(me)) |
| } |
| |
| <span class="doccomment">/// Decrements the reference count by one. |
| /// |
| /// Note this is called on a raw pointer (one previously returned by |
| /// [`into_ptr`](#method.into_ptr). This may lead to dropping of the reference count to 0 and |
| /// destruction of the internal pointer. |
| /// |
| /// # Safety |
| /// |
| /// This must not be called by code outside of this crate. |
| </span><span class="kw">unsafe fn </span>dec(ptr: <span class="kw-2">*const </span><span class="self">Self</span>::Base) { |
| drop(<span class="self">Self</span>::from_ptr(ptr)); |
| } |
| } |
| |
| <span class="kw">unsafe impl</span><T> RefCnt <span class="kw">for </span>Arc<T> { |
| <span class="kw">type </span>Base = T; |
| <span class="kw">fn </span>into_ptr(me: Arc<T>) -> <span class="kw-2">*mut </span>T { |
| Arc::into_raw(me) <span class="kw">as </span><span class="kw-2">*mut </span>T |
| } |
| <span class="kw">fn </span>as_ptr(me: <span class="kw-2">&</span>Arc<T>) -> <span class="kw-2">*mut </span>T { |
| <span class="comment">// Slightly convoluted way to do this, but this avoids stacked borrows violations. The same |
| // intention as |
| // |
| // me as &T as *const T as *mut T |
| // |
| // We first create a "shallow copy" of me - one that doesn't really own its ref count |
| // (that's OK, me _does_ own it, so it can't be destroyed in the meantime). |
| // Then we can use into_raw (which preserves not having the ref count). |
| // |
| // We need to "revert" the changes we did. In current std implementation, the combination |
| // of from_raw and forget is no-op. But formally, into_raw shall be paired with from_raw |
| // and that read shall be paired with forget to properly "close the brackets". In future |
| // versions of STD, these may become something else that's not really no-op (unlikely, but |
| // possible), so we future-proof it a bit. |
| |
| // SAFETY: &T cast to *const T will always be aligned, initialised and valid for reads |
| </span><span class="kw">let </span>ptr = Arc::into_raw(<span class="kw">unsafe </span>{ std::ptr::read(me) }); |
| <span class="kw">let </span>ptr = ptr <span class="kw">as </span><span class="kw-2">*mut </span>T; |
| |
| <span class="comment">// SAFETY: We got the pointer from into_raw just above |
| </span>mem::forget(<span class="kw">unsafe </span>{ Arc::from_raw(ptr) }); |
| |
| ptr |
| } |
| <span class="kw">unsafe fn </span>from_ptr(ptr: <span class="kw-2">*const </span>T) -> Arc<T> { |
| Arc::from_raw(ptr) |
| } |
| } |
| |
| <span class="kw">unsafe impl</span><T> RefCnt <span class="kw">for </span>Rc<T> { |
| <span class="kw">type </span>Base = T; |
| <span class="kw">fn </span>into_ptr(me: Rc<T>) -> <span class="kw-2">*mut </span>T { |
| Rc::into_raw(me) <span class="kw">as </span><span class="kw-2">*mut </span>T |
| } |
| <span class="kw">fn </span>as_ptr(me: <span class="kw-2">&</span>Rc<T>) -> <span class="kw-2">*mut </span>T { |
| <span class="comment">// Slightly convoluted way to do this, but this avoids stacked borrows violations. The same |
| // intention as |
| // |
| // me as &T as *const T as *mut T |
| // |
| // We first create a "shallow copy" of me - one that doesn't really own its ref count |
| // (that's OK, me _does_ own it, so it can't be destroyed in the meantime). |
| // Then we can use into_raw (which preserves not having the ref count). |
| // |
| // We need to "revert" the changes we did. In current std implementation, the combination |
| // of from_raw and forget is no-op. But formally, into_raw shall be paired with from_raw |
| // and that read shall be paired with forget to properly "close the brackets". In future |
| // versions of STD, these may become something else that's not really no-op (unlikely, but |
| // possible), so we future-proof it a bit. |
| |
| // SAFETY: &T cast to *const T will always be aligned, initialised and valid for reads |
| </span><span class="kw">let </span>ptr = Rc::into_raw(<span class="kw">unsafe </span>{ std::ptr::read(me) }); |
| <span class="kw">let </span>ptr = ptr <span class="kw">as </span><span class="kw-2">*mut </span>T; |
| |
| <span class="comment">// SAFETY: We got the pointer from into_raw just above |
| </span>mem::forget(<span class="kw">unsafe </span>{ Rc::from_raw(ptr) }); |
| |
| ptr |
| } |
| <span class="kw">unsafe fn </span>from_ptr(ptr: <span class="kw-2">*const </span>T) -> Rc<T> { |
| Rc::from_raw(ptr) |
| } |
| } |
| |
| <span class="kw">unsafe impl</span><T: RefCnt> RefCnt <span class="kw">for </span><span class="prelude-ty">Option</span><T> { |
| <span class="kw">type </span>Base = T::Base; |
| <span class="kw">fn </span>into_ptr(me: <span class="prelude-ty">Option</span><T>) -> <span class="kw-2">*mut </span>T::Base { |
| me.map(T::into_ptr).unwrap_or_else(ptr::null_mut) |
| } |
| <span class="kw">fn </span>as_ptr(me: <span class="kw-2">&</span><span class="prelude-ty">Option</span><T>) -> <span class="kw-2">*mut </span>T::Base { |
| me.as_ref().map(T::as_ptr).unwrap_or_else(ptr::null_mut) |
| } |
| <span class="kw">unsafe fn </span>from_ptr(ptr: <span class="kw-2">*const </span>T::Base) -> <span class="prelude-ty">Option</span><T> { |
| <span class="kw">if </span>ptr.is_null() { |
| <span class="prelude-val">None |
| </span>} <span class="kw">else </span>{ |
| <span class="prelude-val">Some</span>(T::from_ptr(ptr)) |
| } |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="arc_swap" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
