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.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="bloop_markdown"><p>You’ve got two things wrong there:</p>
<ol>
<li><code>Derived: Base</code> otherwise you cannot cast at all.</li>
<li><code>w === x //=> FALSE</code></li>
</ol>
<p>The reason why this does not work is because it’s impossible to cast a type to an different type with the help of an initializer. As I cleaned up the current implementation I though about replacing <code>init?(casting: Type<T>?)</code> with something like this:</p>
<pre><code class="swift">public static func cast<U>(from optionalType: Type<U>?) -> Type<T>? {
guard let otherType = optionalType else { return nil }
// Check if we can up- or downcast the metatype from `otherType` to `Metatype<T>`
// Bug: SR-2085
// Workaround: Check explicitly if `T` is `Any`
//
// let isTAny = _uniqueIdentifierOf(Any.metatype) == _uniqueIdentifierOf(T.metatype)
// guard isTAny || otherType._metatype is Metatype<T> else {
// return nil
// }
guard otherType._metatype is Metatype<T> else {
return nil
}
// `T` implicitly converted to `Type<T>()`
return unsafeBitCast(otherType, to: T)
}
</code></pre>
<p>Your example would become this:</p>
<pre><code class="swift">let x = Type<Derived>()
// downcast
let y = Type<Base>.cast(from: x)!
y === x //=> true
// this is safe - upcast
let z = unsafeBitCast(y, to: Type<Derived>())
z === y //=> true
z === x //=> true
let w = Type<Derived>.cast(from: y)!
w === x //=> true
w === y //=> true
w === z //=> true
</code></pre>
<p>Global dictionary is not involved here at all. We can change this too and put the reference for all different <code>Type<T></code> into the dictionary from the designated initializer.</p>
<p>Remember that the global dictionary does only contain unique (dynamic) types, which means we could have this.</p>
<p><code>[1: Type<Base>(), 2: Type<Derived>()]</code></p>
<ul>
<li><p><code>Type<Derived>.sharedInstance</code> will give you (2) which you can downcast to <code>Base</code> or <code>Any</code></p>
<ul>
<li>The reference will remain pointing to (2) in the global dictionary, which is the correct behavior.</li>
</ul></li>
<li><p><code>Type<Base>.sharedInstance</code> will give you (1) which you only can downcast to <code>Any</code> but not upcast to <code>Derived</code></p></li>
</ul>
<p>The reason for this is the (dynamic) metatype inside each <code>Type<T></code> instance.</p>
<p>Here is an example how it works in Swift 2.2:</p>
<pre><code class="swift">class Base {}
class Derived: Base {}
let baseMetatype = Base.self
(baseMetatype is Base.Type) == true
(baseMetatype is Any.Type) == true
(baseMetatype is Derived.Type) == false
let derivedMetatype = Derived.self
(derivedMetatype is Base.Type) == true
(derivedMetatype is Any.Type) == true
(derivedMetatype is Derived.Type) == true
</code></pre>
<p>Problem solved? Lets tackle next one.</p>
<p>PS: I removed <code>_identifier</code> because now <code>_metatype</code> does the most work.</p>
<p></p></div><div class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div> <br> <div id="bloop_sign_1468596267880691968" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 15. Juli 2016 um 17:12:29, Anton Zhilin (<a href="mailto:antonyzhilin@gmail.com">antonyzhilin@gmail.com</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div><div class="gmail_quote" style="color: rgb(0, 0, 0); font-family: 'helvetica Neue', helvetica; font-size: 14px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div>Here you are, full example:<br></div><br><div><font face="monospace, monospace">let x = Type<Derived>()</font></div><div><font face="monospace, monospace"><br></font></div><div><font face="monospace, monospace">// Type<Derived> is not a subtype of Type<Base>, we must construct a new instance of Type<Base></font></div><div><font face="monospace, monospace">let y = Type<Base>(casting: </font><span style="font-family: monospace, monospace;">x</span><font face="monospace, monospace">)!</font></div><div><font face="monospace, monospace"><br></font></div><div><span style="font-family: monospace, monospace;">y === x //=> false</span><font face="monospace, monospace"><br></font></div><div><font face="monospace, monospace"><br></font></div><div><font face="monospace, monospace">// Type<T> are reference types. z will point to the same instance as y</font></div><div><font face="monospace, monospace">let z = unsafeBitCast(y, to: Type<Derived>())</font></div></div><div class="gmail_quote" style="color: rgb(0, 0, 0); font-family: 'helvetica Neue', helvetica; font-size: 14px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><br><div><font face="monospace, monospace">z === y //=> true</font></div><div><font face="monospace, monospace">z === x //=> false</font></div><div><font face="monospace, monospace"><br></font></div><div><font face="monospace, monospace">// Static and dynamic types are equal here, so we decide to draw from global dictionary</font></div><div><font face="monospace, monospace">let w = Type<Derived>(casting: y)!</font></div><div><font face="monospace, monospace"><br></font></div><div><font face="monospace, monospace">w === x //=> true</font></div><div><font face="monospace, monospace">w === y //=> false</font></div><div><font face="monospace, monospace">w === z //=> false</font></div></div></div></span></blockquote></div><div class="bloop_markdown"><p></p></div></body></html>