<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">My hesitation about "required" is that required initializers don't have to call the <i class="">same</i> initializer, which is something that NS_REQUIRES_SUPER <i class="">does</i> enforce. (In fact, this new attribute could very well apply to required initializers as well: if you implement this initializer, you must chain to the same initializer. I'm not quite sure when that would come up, but it's potentially useful.)</div><div class=""><br class=""></div><div class="">Jordan</div><br class=""><div><blockquote type="cite" class=""><div class="">On Dec 16, 2015, at 10:56 , Ian Ynda-Hummel via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">+1 for this if for nothing else but UIKit classes yelling at me to call super.viewDidLoad().<div class=""><br class=""></div><div class="">I think using the required keyword makes sense. The one possible caveat is overloading, as my knee jerk reaction is that methods of the same name would behave like initializers. So a method overriding a required method would have to call a required method of the same name on the superclass if one exists. I'm not convinced that's correct, though.<br class=""></div></div><br class=""><div class="gmail_quote"><div dir="ltr" class="">On Wed, Dec 16, 2015 at 1:40 PM Marc Knaup via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="">"required" also doesn't mean that a subclass has to implement the required initializer since it can be inherited.<div class="">Your example is an abstract function which should have it's own keyword (if we ever get abstract functions).<br class=""></div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Wed, Dec 16, 2015 at 7:37 PM, Vester Gottfried <span dir="ltr" class=""><<a href="mailto:vester.gottfried@gmail.com" target="_blank" class="">vester.gottfried@gmail.com</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="">I think reusing required would send the wrong message. Required would mean for me something like NSOperation subclasses maybe require to have a main() function, but that doesn't mean you have to call super. On the contrary, the documentation of NSOperation main() explicitly states not to call super. </div><div class=""><div class=""><div class="gmail_extra"><br class=""><div class="gmail_quote">On Wed, Dec 16, 2015 at 7:08 PM, Marc Knaup <span dir="ltr" class=""><<a href="mailto:marc@knaup.koeln" target="_blank" class="">marc@knaup.koeln</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="">What about re-using the "required" keyword in the superclass which already means something similar for initializers?<div class="">Subclass implementations are required to call super's implementation.<br class=""><div class="">If a subclass doesn't implemented the required method it could mean that it inherits the behavior from the superclass - just like initializers can be inherited too.</div></div></div><div class=""><div class=""><div class="gmail_extra"><br class=""><div class="gmail_quote">On Wed, Dec 16, 2015 at 7:02 PM, Jordan Rose <span dir="ltr" class=""><<a href="mailto:jordan_rose@apple.com" target="_blank" class="">jordan_rose@apple.com</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div class="">+1 from me. FWIW, the Objective-C one is syntactic.</div><div class=""><br class=""></div><div class="">Information from Radar: the request for this is <a class="">rdar://problem/17408107</a> (plus a few duplicates). One of the dups suggests a variation where a subclass method can be declared as "refine" instead of "override" so that you can document that your <i class="">own</i> method is expected to call super. In this model, "@requires_super" could become something like "imposed". I personally think this doesn't add enough, especially since we wouldn't be publishing refine-vs-override in a library's public interface.</div><span class=""><font color="#888888" class=""><div class=""><br class=""></div><div class="">Jordan</div></font></span><div class=""><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Dec 16, 2015, at 9:49 , Marc Knaup via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:</div><br class=""><div class=""><div dir="ltr" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="">Sounds reasonable since even the best flow analysis cannot ensure that all codepaths call the super implementation.<div class=""><br class=""></div><div class="">Some more edge cases:</div><div class=""><ul class=""><li class="">Calling super asynchronously by using it in a closure</li><li class="">Referring to the super implementation by assign it to a variable and call it later (is that really possible? never did that)</li></ul></div></div><div class="gmail_extra" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><br class=""><div class="gmail_quote">On Wed, Dec 16, 2015 at 6:25 PM, Vester Gottfried<span class=""> </span><span dir="ltr" class=""><<a href="mailto:vester.gottfried@gmail.com" target="_blank" class="">vester.gottfried@gmail.com</a>></span><span class=""> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr" class="">I would suggest that @requires_super only checks if a call to super is present at all. More detailed behaviour should be part of the functions documentation, because I think all possibilities cannot be checked easily by the compiler. For example a call to super my be required to happen early or late inside the function. But when too early or too late is can probably not been forseen by the compiler.</div><div class=""><div class=""><div class="gmail_extra"><br class=""><div class="gmail_quote">On Wed, Dec 16, 2015 at 5:46 PM, Marc Knaup<span class=""> </span><span dir="ltr" class=""><<a href="mailto:marc@knaup.koeln" target="_blank" class="">marc@knaup.koeln</a>></span><span class=""> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr" class="">+1 always had such issues with UIViewController's lifecycle methods.<div class=""><br class=""></div><div class="">But edge cases need to be considered like "throws" for example.</div><div class="">Do I need to call super before I throw something?</div></div><div class=""><div class=""><div class="gmail_extra"><br class=""><div class="gmail_quote">On Wed, Dec 16, 2015 at 5:41 PM, Matthew Johnson via swift-evolution<span class=""> </span><span dir="ltr" class=""><<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>></span><span class=""> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div style="word-wrap:break-word" class="">+1 to this. Anything that helps ensure inheritance is thought through carefully and used correctly is a win.<div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class=""><div class=""><div class="">On Dec 16, 2015, at 10:32 AM, Vester Gottfried via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:</div><br class=""></div></div><div class=""><div class=""><div class=""><div dir="ltr" class=""><div class="">Some class based libraries/frameworks expect the consumer to subclass certain classes and override specific method and require that the super implementation of an overridden method is being called.</div><div class=""><br class=""></div><div class="">Not calling the super implementation is a common source of bugs that may be prevented if the compiler checks if super is called, like it does in some cases of init().</div><div class=""><br class=""></div><div class="">Example:<br class=""></div><div class=""><br class=""></div><div class="">class Box {</div><div class=""> @requires_super<br class=""></div><div class=""> <span class=""> </span>func addStuff() { ... }</div><div class="">}<br class=""></div><div class=""><br class=""></div><div class="">Overriding class Box's addStuff without calling super.addStuff() should result in an error</div><div class=""><br class=""></div><div class="">class Chest : Box {</div><div class=""> <span class=""> </span>override addStuff() {<br class=""></div><div class=""> // ERROR: addStuff() requires call to super.addStuff()</div><div class=""> <span class=""> </span>...</div><div class=""> <span class=""> </span>}</div><div class="">}<br class=""></div><div class=""><br class=""></div><div class="">Objective-C developers know this as NS_REQUIRES_SUPER and I think its worth thinking about adapting it.</div><div class=""><div class=""><br class=""></div><div class="">I hope my proposal was clear and thanks for reading,</div><div class=""><br 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