<div style="white-space:pre-wrap">+1<br><br>* I think swift expose too much informations inside compiler to the user。<br><br>* I can't imagine, a modern, swift language can't get the size of T/t with same API:<br> MemoryLayout<T>.size<br> MemoryLayout.size(ofValue:t)<br><br><br><br>Karl via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>>于2016年8月6日 周六23:23写道：<br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><br><div><blockquote type="cite"><div>On 6 Aug 2016, at 11:05, Adrian Zubarev via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br><div><div style="font-family:Helvetica,Arial;font-size:13px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(254,254,254)"><p style="margin:15px 0px">What’s so confusing about<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">MemoryLayout<T.Type>.size</code>?</p><p style="margin:15px 0px"><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">size</code><span> </span>in this context gives you the size for the metatype of<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">T</code>, not the size of<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">T</code>.</p><p style="margin:15px 0px">If I memorize correctly:</p><ul style="margin:15px 0px"><li style="margin:15px 0px">Size of metatypes of value types and functions/closures is 0.</li><li style="margin:15px 0px">Size for class metatypes is 8.</li><li style="margin:15px 0px">Size for protocols metatypes (in your case it’s<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">P.Protocol</code>, not<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">P.Type</code>) is 16 except for the (old) empty protocol<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">Any</code><span> </span>which is 8.</li></ul><p style="margin:15px 0px">You may ask why these metatypes have these sizes?! I can’t answer this question, because I don’t know the technical reason, but it doesn’t matter in our case.</p><p style="margin:15px 0px">Furthermore there is a difference between<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">T.Type</code><span> </span>and what you get from<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">T.self</code>.</p><p style="margin:15px 0px"><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">T.self</code><span> </span>returns a concrete metatype for<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">T</code>.<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">T.Type</code><span> </span>is a metatype supertype for all subtypes of<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">T</code><span> </span>if there exist a subtype relationship.</p><p style="margin:15px 0px">That’s why something like this works fine:</p><pre style="margin:15px 0px;font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(204,204,204);overflow:auto;padding:4px 8px;word-break:normal;word-wrap:normal"><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:0px;margin:0px;padding:0px;word-break:normal;word-wrap:normal">let a1: Any.Type = Int.self // `Int.self` is 0 where `Any.self` is 8
let a2: Any.Type = P.self // same story with different sizes
</code></pre><p style="margin:15px 0px">We’re revising our proposal to provide a clear distinction of this behavior and get rid of<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">.Protocol</code><span> </span>and<span> </span><code style="font-family:Menlo,Consolas,'Liberation Mono',Courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);color:inherit;border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">.Type</code>.</p><div><br></div></div></div></blockquote><br></div></div><div style="word-wrap:break-word"><div>I think the underlying issue isn’t that MemoryLayout<T> vs MemoryLayout<T.Type> is confusing, it’s that our metatype system is a little awkward, so the way you construct a MemoryLayout<X> can lead to non-obvious results unless you’re an expert. Given time constraints, the most practical solution which gives us the best product for Swift 3.0 is to revise the MemoryLayout interface, but I think most users would be happy to see some simplification of the metatype system for Swift >3.0.</div><div><br></div><div>Personally I only sporadically remember which name means what in a given context. Oh, the dark corners I would have been lost in without code-completion…</div><div><br></div><div><br></div></div>_______________________________________________<br>
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