withContiguousMutableStorageIfAvailable(_:)

Return Value 戻り値

The value returned from body, unless the collection doesn’t support contiguous storage, in which case the method ignores body and returns nil.

Discussion 解説

This method calls body(buffer), where buffer provides access to the contiguous mutable storage of the entire collection. If the contiguous storage doesn’t exist, the collection creates it. If the collection doesn’t support an internal representation in the form of contiguous mutable storage, this method doesn’t call body — it immediately returns nil.

The optimizer can often eliminate bounds- and uniqueness-checking within an algorithm. When that fails, however, invoking the same algorithm on the buffer argument may let you trade safety for speed.

Always perform any necessary cleanup in the closure, because the method makes no guarantees about the state of the collection if the closure throws an error. Your changes to the collection may be absent from the collection after throwing the error, because the closure could receive a temporary copy rather than direct access to the collection’s storage.

Successive calls to this method may provide a different pointer on each call. Don’t store buffer outside of this method.

A Collection that provides its own implementation of this method must provide contiguous storage to its elements in the same order as they appear in the collection. This guarantees that it’s possible to generate contiguous mutable storage to any of its subsequences by slicing buffer with a range formed from the distances to the subsequence’s startIndex and endIndex, respectively.