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POObjective-C Returning Autoreleased Copies
 

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    <p>A strange issue I've come across trying to understand Apple's Memory Management standards. Lets say I have a method that returns a copy without letting the user know that it is a copy.</p> <pre><code>+(Point2D*) Add:(Point2D*)a To:(Point2D*)b { Point2D * newPoint = [a copy]; [newPoint Add:b]; // Actually perform the arithmetic. return [newPoint autorelease]; } </code></pre> <p>The problem is that Xcode's Analyze function flags this as an object being sent too many -autorelease calls. I'm assuming this is because -copy implicitly assumes that you are taking ownership, and thus the possibility of +0 retain count is likely. But I'm not entirely sure.</p> <p>Xcode's Analyze Information</p> <pre><code>+(Point2D*) Add:(Point2D*)a To:(Point2D*)b { Point2D * newPoint = [a copy]; // &lt;- 1. Method returns an Objective-C object with a +0 retain count. [newPoint Add:b]; return [newPoint autorelease]; // &lt;- 2. Object sent -autorelease method. // &lt;- 3. Object returned to caller with a +0 retain count. // &lt;- 4. Object over -autoreleased: object was sent -autorelease but the object has zero (locally visible) retain counts. } </code></pre> <p>Any tips or hints on why this is happening? Unless I'm missing something, the code should work fine because the autorelease wont trigger until a safe time (i.e. it works kind of like a convenience constructor, user has time to retain.)</p> <p>As per request, -copyWithZone: would be implemented as such:</p> <pre><code>-(id)copyWithZone:(NSZone *)zone { return [[Point2D allocWithZone:zone] initX:x Y:y Z:z]; } </code></pre>
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    1. COYour code looks right. You should be autoreleasing the copy. What version of Xcode are you using? On mine, I can't even name a type called "Point", because it conflicts with the one in QuickTime: https://developer.apple.com/library/mac/#documentation/QuickTime/Reference/QTRef_DataTypes/Reference/reference.html#//apple_ref/doc/uid/TP40003454-GlobalQuickTimeAPITypes-Point
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    2. COYou're correct, I used a general case and simply named it Point arbitrarily. In my code it occurs in my Matrix3x3, Matrix4x4, Vector3, Vector4 classes. I was trying to cite a general case for the purposes of this question. - I have edited the question to use Point2D to avoid further confusion. Also, I am using Xcode 4.2 Build: 4C199
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    3. COIt must be a bug in your Xcode. Try Xcode 4.3
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