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    <p>If I understand correctly, you're asking why someone might implement a cache as</p> <pre><code>void DoSomethingWith(CGColorRef c) { static CGColorRef cached_c = NULL; static CFTypeID cached_colorID; if (c == cached_c &amp;&amp; c-&gt;colorID == cached_colorID) ... </code></pre> <p>instead of merely</p> <pre><code>void DoSomethingWith(CGColorRef c) { static CFTypeID cached_colorID = 0; if (c-&gt;colorID == cached_colorID) ... </code></pre> <p>? Well, two obvious reasons are</p> <ul> <li><p><a href="http://highscalability.com/blog/2013/6/13/busting-4-modern-hardware-myths-are-memory-hdds-and-ssds-rea.html" rel="nofollow">Random access memory isn't random access.</a> Dereferencing <code>c</code> is probably a slow operation (a cache miss wastes many nanoseconds), so if we can save those nanoseconds 90% of the time by prepending a cheap pointer comparison, let's do it.</p></li> <li><p>How do you initialize <code>cached_colorID</code>? In the first implementation above, if we assume that the user respects the API contract and always passes in a non-null <code>c</code>, then once we know <code>c</code> == <code>cached_c</code>, then we also know that <code>cached_c != NULL</code> and therefore we have a meaningful value in <code>cached_colorID</code>. In the second implementation, if the first <code>c</code> the user passes in happens to have <code>c-&gt;colorID == 0</code>, then we'll incorrectly believe that we've seen it before, and madcap hijinks ensue.</p></li> </ul> <p>I don't know if either of these is the reason Apple did what you're looking at... but they seem like solid possibilities, don't they?</p>
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    1. CO* First the second reason, it is unrelated because c->colorID it starts from 1. * And about first reason, it is fairly sensible. When both refs and ids are equal, there will be an extra comparison in first implementation but i assume a comparison is far less cheaper than a memory read so its negligible!..(am i right?) When refs are not equal, in first implementation there will be only one comparison and memory will not be read. However, i don't know if it's worth to optimize out a single memory read while there will be a bunch of memory reads by transformation calculations.
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