The Detroit Post
Saturday, 16 October, 2021

Rust Arc

author
David Lawrence
• Thursday, 07 January, 2021
• 9 min read

When the last Arc pointer to a given allocation is destroyed, the value stored in that allocation (often referred to as “inner value”) is also dropped. Shared references in Rust disallow mutation by default, and Arc is no exception: you cannot generally obtain a mutable reference to something inside an Arc.

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Contents

If you need to mutate through an Arc, use Mute, Rowlock, or one of the Atomic types. Unlike RC, Arc uses atomic operations for its reference counting.

The disadvantage is that atomic operations are more expensive than ordinary memory accesses. If you are not sharing reference-counted allocations between threads, consider using RC for lower overhead.

The key is this: Arc makes it thread safe to have multiple ownership of the same data, but it doesn't add thread safety to its data. But then we'd have a problem: Recall is not thread safe; it keeps track of the borrowing count using non-atomic operations.

In the end, this means that you may need to pair Arc with some sort of std::sync type, usually Mute. The downgrade method can be used to create a non-owning Weak pointer.

A Weak pointer can be upgrade d to an Arc, but this will return None if the value stored in the allocation has already been dropped. In other words, Weak pointers do not keep the value inside the allocation alive; however, they do keep the allocation (the backing store for the value) alive.

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For example, a tree could have strong Arc pointers from parent nodes to children, and Weak pointers from children back to their parents. Creating a new reference from an existing reference-counted pointer is done using the Clone trait implemented for Arc and Weak.

Arc automatically differences to T (via the Dark trait), so you can call T's methods on a value of type Arc . Arc 's implementations of traits like Clone may also be called using fully qualified syntax.

Pub FN new (data: T) Arc Attempting to upgrade the weak reference before this function returns will result in a None value.

However, the weak reference may be cloned freely and stored for use at a later time. Constructs a new Arc with uninitialized contents, with the memory being filled with 0 bytes.

If T does not implement Unpin, then data will be pinned in memory and unable to be moved. Returns the inner value, if the Arc has exactly one strong reference.

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This will succeed even if there are outstanding weak references. Constructs a new atomically reference-counted slice with uninitialized contents.

Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being filled with 0 bytes. As with MaybeUninit::assume_init, it is up to the caller to guarantee that the inner value really is in an initialized state.

Calling this when the content is not yet fully initialized causes immediate undefined behavior. As with MaybeUninit::assume_init, it is up to the caller to guarantee that the inner value really is in an initialized state.

Calling this when the content is not yet fully initialized causes immediate undefined behavior. Consumes the Arc, returning the wrapped pointer.

The pointer is valid for as long as there are strong counts in the Arc. The raw pointer must have been previously returned by a call to Arc ::into_raw where U must have the same size and alignment as T.

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Note that if U is not T but has the same size and alignment, this is basically like transmuting references of different types. See me::transmute for more information on what restrictions apply in this case.

The user of from_raw has to make sure a specific value of T is only dropped once. This function is unsafe because improper use may lead to memory unsafely, even if the returned Arc is never accessed.

This method by itself is safe, but using it correctly requires extra care. Another thread can change the weak count at any time, including potentially between calling this method and acting on the result.

Gets the number of strong (Arc) pointers to this allocation. This method by itself is safe, but using it correctly requires extra care.

Another thread can change the strong count at any time, including potentially between calling this method and acting on the result. Increments the strong reference count on the Arc associated with the provided pointer by one.

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The pointer must have been obtained through Arc ::into_raw, and the associated Arc instance must be valid (i.e. the strong count must be at least 1) for the duration of this method. Decrements the strong reference count on the Arc associated with the provided pointer by one.

The pointer must have been obtained through Arc ::into_raw, and the associated Arc instance must be valid (i.e. the strong count must be at least 1) when invoking this method. This method can be used to release the final Arc and backing storage, but should not be called after the final Arc has been released.

Returns true if the two Arc s point to the same allocation (in a vein similar to PTR::EQ). If there are other Arc or Weak pointers to the same allocation, then make_mut will create a new allocation and invoke clone on the inner value to ensure unique ownership.

Note that this differs from the behavior of RC::make_mut which disassociates any remaining Weak pointers. Returns None otherwise, because it is not safe to mutate a shared value.

Returns a mutable reference into the given Arc, without any check. Any other Arc or Weak pointers to the same allocation must not be referenced for the duration of the returned borrow.

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This is trivially the case if no such pointers exist, for example immediately after Arc ::new. Attempt to downcast the Arc to a concrete type.

This creates another pointer to the same allocation, increasing the strong reference count. Takes each element in the Iterator and collects it into an Arc <>.

When your Iterator implements Trusted Len and is of an exact size, a single allocation will be made for the Arc <>. #FN max (self, other: Self) Self 1.21.0 Compares and returns the maximum of two values.

#FN min (self, other: Self) Self 1.21.0 Compares and returns the minimum of two values. #FN clamp (self, min: Self, max: Self) Self This is a nightly-only experimental API.

Pub FN EQ (self, other: & Arc ) built Two Arc s are equal if their inner values are equal, even if they are stored in different allocation. Pub FN né (self, other: & Arc ) built Two Arc s are unequal if their inner values are unequal.

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If T also implements EQ (implying reflexivity of equality), two Arcs s that point to the same value are never unequal. Invoking clone on Arc produces a new pointer to the same value in the heap.

Shared references in Rust disallow mutation by default, and Arc is no exception: you cannot generally obtain a mutable reference to something inside an Arc. If you need to mutate through an Arc, use Mute, Rowlock, or one of the Atomic types.

Unlike RC, Arc uses atomic operations for its reference counting This means that it is thread-safe. The disadvantage is that atomic operations are more expensive than ordinary memory accesses.

If you are not sharing reference-counted values between threads, consider using RC for lower overhead. The key is this: Arc makes it thread safe to have multiple ownership of the same data, but it doesn't add thread safety to its data.

But then we'd have a problem: Recall is not thread safe; it keeps track of the borrowing count using non-atomic operations. In the end, this means that you may need to pair Arc with some sort of std::sync type, usually Mute.

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The downgrade method can be used to create a non-owning Weak pointer. A Weak pointer can be upgrade d to an Arc, but this will return None if the value has already been dropped.

For example, a tree could have strong Arc pointers from parent nodes to children, and Weak pointers from children back to their parents. The Arc ::clone(from) syntax is the most idiomatic because it conveys more explicitly the meaning of the code.

In the example above, this syntax makes it easier to see that this code is creating a new reference rather than copying the whole content of foo. Returns the contained value, if the Arc has exactly one strong reference.

Consumes the Arc, returning the wrapped pointer. The raw pointer must have been previously returned by a call to an Arc ::into_raw.

This function is unsafe because improper use may lead to memory problems. This method by itself is safe, but using it correctly requires extra care.

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Another thread can change the weak count at any time, including potentially between calling this method and acting on the result. Gets the number of strong (Arc) pointers to this value.

This method by itself is safe, but using it correctly requires extra care. Another thread can change the strong count at any time, including potentially between calling this method and acting on the result.

Fn max (self, other: Self) Self Compares and returns the maximum of two values. Fn min (self, other: Self) Self Compares and returns the minimum of two values.

Sources
1 www.fulcrumre.com - https://www.fulcrumre.com/
2 porch.com - https://porch.com/tacoma-wa/property-managers/fulcrum-residential/pp
3 www.fulcrumcres.com - https://www.fulcrumcres.com/
4 www.indeed.com - https://www.indeed.com/cmp/Fulcrum-Real-Estate-Services/reviews
5 www.indeed.com - https://www.indeed.com/cmp/Fulcrum-Real-Estate-Services/reviews
6 www.bbb.org - https://www.bbb.org/us/wa/olympia/profile/real-estate/fulcrum-real-estate-services-inc-1296-22607191
7 www.homes.com - https://www.homes.com/tacoma-wa/98446/