<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><blockquote type="cite" class="">On Jul 22, 2016, at 3:15 AM, Dave Abrahams via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class="">I wrote that subclassability is not an important element of safety<br class="">**independent of overriding**. If you don't allow any overriding,<br class="">your code is always “resilient” enough to handle subclassing.<br class=""></blockquote><br class=""><div class="">Code can make assumptions about a type having a fixed set of subclasses known at compile time:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 10.5px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">switch</span><span style="font-variant-ligatures: no-common-ligatures" class=""> foo {</span></div><div style="margin: 0px; font-size: 10.5px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">is</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #587ea8" class="">YinFoo</span><span style="font-variant-ligatures: no-common-ligatures" class="">:</span></div><div style="margin: 0px; font-size: 10.5px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> <font color="#587ea8" class="">...</font></span></div><div style="margin: 0px; font-size: 10.5px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">is</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #587ea8" class="">YangFoo</span><span style="font-variant-ligatures: no-common-ligatures" class="">:</span></div><div style="margin: 0px; font-size: 10.5px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> <font color="#587ea8" class="">...</font></span></div><div style="margin: 0px; font-size: 10.5px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">default</span><span style="font-variant-ligatures: no-common-ligatures" class="">:</span></div><div style="margin: 0px; font-size: 10.5px; line-height: normal; font-family: Menlo; color: rgb(132, 62, 100);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #587ea8" class="">fatalError</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">(</span><span style="font-variant-ligatures: no-common-ligatures" class="">"only two kinds of Foo known"</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">)</span></div><div style="margin: 0px; font-size: 10.5px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class="">Granted, code like this is usually a sign of a flawed design. Reasonable uses for the “fixed, known set of subtypes” pattern are rare.</div><div class=""><br class=""></div><div class="">Design quality questions aside, however, it is not strictly true that preventing all member overrides guarantees that code is resilient to unexpected subclassing.</div><div class=""><br class=""></div><div class="">Cheers,</div><div class=""><br class=""></div><div class="">Paul</div><div class=""><br class=""></div></body></html>