Okay I understand most of that, but I still feel it’s misleading to put `reserveCapacity()` on `CharacterView` and `UnicodeScalarView`. `reserveCapacity()` should live in a type where its meaning matches up with the meaning of the `.count` property, ideally the `UTF8View`. Otherwise it should at least be removed from `CharacterView` and `UnicodeScalarView` and only live in the parent `String` type.
On Thu, May 4, 2017 at 6:33 AM, Brent Royal-Gordon <br...@architechies.com> wrote: > On May 2, 2017, at 12:35 PM, Kelvin Ma via swift-users < > swift-users@swift.org> wrote: > > I’m wondering why the String.CharacterView structure has a > reserveCapacity(:) member? > > > Because it conforms to the RangeReplaceableCollection protocol, which > requires `reserveCapacity(_:)`. > > More broadly, because you can append characters to the collection, and so > you might want to pre-size it to reduce the amount of reallocating you > might need to do in the future. > > And even more strangely, why String itself has the same method? > > > Because it has duplicates of those `CharacterView` methods which don't > address individual characters. (In Swift 4, it will be merged with > CharacterView.) > > It’s even weirder that String.UnicodeScalarView has this method, but it > reserves `n` `UInt8`s of storage, instead of `n` `UInt32`s of storage. > > > Because the views are simply different wrappers around a single underlying > buffer type, which stores the string in 8-bit (if all characters are ASCII) > or 16-bit (if some are non-ASCII). That means that `UnicodeScalarView` > isn't backed by a UTF-32 buffer; it's backed by an ASCII or UTF-16 buffer, > but it only generates and accepts indices corresponding to whole > characters, not the second half of a surrogate pair. > > Why not allocate a larger buffer anyway? Most strings use no extraplanar > characters, and many strings use only ASCII characters. (Even when the user > works in a non-ASCII language, strings representing code, file system > paths, URLs, identifiers, localization keys, etc. are usually ASCII-only.) > By reserving only `n` `UInt8`s, Swift avoids wasting memory, at the cost of > sometimes having to reallocate and copy the buffer when a string contains > relatively rare characters. I believe Swift doubles the buffer size on each > allocation, so we're talking no more than one reallocation for a non-ASCII > string and two for an extraplanar string. That's quite acceptable. > > Also why String.UTF8View and String.UTF16View do not have this method, > when it would make more sense for them to have it than for String itself > and String.CharacterView to have it. > > > Because UTF8View and UTF16View are immutable. They don't conform to > RangeReplaceableCollection and cannot be used to modify the string (since > you could modify them to generate an invalid string). > > -- > Brent Royal-Gordon > Architechies > >
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