<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=""><br class=""><div><blockquote type="cite" class=""><div class="">Le 28 janv. 2017 à 05:34, Torin Kwok via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> a écrit :</div><br class="Apple-interchange-newline"><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><p style="-webkit-print-color-adjust: exact; margin-right: 0px; margin-bottom: 15px; margin-left: 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; margin-top: 0px !important;" class="">Hello guys,</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><strong style="-webkit-print-color-adjust: exact;" class="">Note:</strong> This issue has been <a href="https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20170123/004552.html" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">originally presented in <strong style="-webkit-print-color-adjust: exact;" class="">swift-users</strong> mailling list</a>. And then I post it again here at the <a href="https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20170123/004561.html" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">suggestion</a> of Jordan Rose:</p><blockquote style="-webkit-print-color-adjust: exact; margin: 15px 0px; border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221); padding: 0px 15px; color: rgb(119, 119, 119); font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><div style="-webkit-print-color-adjust: exact; margin: 0px;" class="">It might be reasonable to change this behavior, but it probably deserves a bit of discussion on swift-evolution; it's not 100%, for-sure a bug.</div></blockquote><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><strong style="-webkit-print-color-adjust: exact;" class="">--- the original content follows this line ---</strong></p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">I encountered a strange behavior when I declared a property with the <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">@NSCopying</code> attribute:</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class=""><code class="language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">// `Person` class inherits from `NSObject` class and conforms to `NSCopying` protocol
@NSCopying var employee: Person</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">and then assigned an external instance of <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">Person</code> class protocol to this property within the designated init methods:</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class=""><code class="language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">// Designated initializer of `Department` class
init( employee externalEmployee: Person ) {
self.employee = externalEmployee
super.init()
// Assertion would fail since Swift do not actually copy the value assigned to this property
// even though `self.employee` has been marked as `@NSCoyping`
// assert( self.employee !== externalEmployee )
}</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">If I indeed require the deep copying behavior during the init process, instead of taking advantage of <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">@NSCopying</code> attribute, I would have to invoke the <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">copy()</code> method manually:</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class=""><code class="language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">init( employee externalEmployee: Person ) {
// ...
self.employee = externalEmployee.copy() as! Person
// ...
}</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">In fact, what really makes me confusing is that <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">@NSCopying</code> semantic does work properly within the other parts of the class definition such as normal instance methods, or external scope. For instance, if we're assigning an external instance of <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">Person</code> to the <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">self.employee</code> proper of <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">Department</code> directly through setter rather than initializer:</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class=""><code class="language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">department.employee = johnAppleseed</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">then <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">self.employee</code> property and <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">johnAppleseed</code> variable will no longer share the same underlying object now. In the other words, <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">@NSCopying</code> attribute makes sense.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">After I looked through a great deal of results given by Google, and dicussions on StackOverflow, I finally end up with nothing helpful — the vast majority of articles, documentations as well as issues talking about this similar topics only focus on the basic concepts and effects of <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">@NSCopying</code> itself but do not mentioned this strange behavior at all — besides one radar descriping the same problem (<a href="rdar://21383959" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">rdar://21383959</a>) and a final conclusion mentioned in a guy's Gist comment: <strong style="-webkit-print-color-adjust: exact;" class="">... values set during initialization are not cloned ...</strong></p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">That is, <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">@NSCopying</code> semantic has no effect in initializers.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Then, what I want to figure out is the reason why <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">@NSCopying</code> semantic will become effectless implicitly whithin initializers of a class, and the special considerations behind this behavior, if any.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><strong style="-webkit-print-color-adjust: exact;" class="">--- END ---</strong></p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Jordan:</p><blockquote style="-webkit-print-color-adjust: exact; margin: 15px 0px; border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221); padding: 0px 15px; color: rgb(119, 119, 119); font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><div style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Your observation is correct: @NSCopying currently does not affect initializers. This is because accessing a property in an initializer always does direct access to the storage rather than going through the setter.</div></blockquote><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">I have tested the identical logic in Objective-C and the <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px;" class="">NSCopying</code> semantic works perfectly within Obj-C's class initializer.</p></div></div></blockquote><div>This is because Obj-C guarantee that all ivars are zero initialized and does not enforce initializer safety (but forcing initialization of ivars before calling other methods).</div><div><br class=""></div><div>Calling a setter (like any other method) before full class initialization is unsafe as the setter may be overridden or simply customized, and may need to access to the class or subclasses ivars.</div><div><br class=""></div><div>That said, I’m not sure what is the best way to solve that inconsistency.</div><div><br class=""></div></div><br class=""></body></html>