<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div><blockquote type="cite" class=""><div class="">On May 2, 2017, at 12:35 PM, Kelvin Ma via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><div class="">I’m wondering why the String.CharacterView structure has a reserveCapacity(:) member?</div></div></div></blockquote><div><br class=""></div><div>Because it conforms to the RangeReplaceableCollection protocol, which requires `reserveCapacity(_:)`.</div><div><br class=""></div><div>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.</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><div class="">And even more strangely, why String itself has the same method?<br class=""></div></div></div></blockquote><div><br class=""></div><div>Because it has duplicates of those `CharacterView` methods which don't address individual characters. (In Swift 4, it will be merged with CharacterView.)</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">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.</div></div></blockquote><div><br class=""></div><div>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.</div><div><br class=""></div><div>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.</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">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.</div></div></blockquote><br class=""></div><div>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).</div><br class=""><div class="">
<span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div class=""><div style="font-size: 12px; " class="">-- </div><div style="font-size: 12px; " class="">Brent Royal-Gordon</div><div style="font-size: 12px; " class="">Architechies</div></div></span>
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