2006-05-07

Beware the nib

Say you have an object, X, and a nib containing two other objects, Y and Z. X has two outlets, y and z, with typical accessors for each.

X, or another object on behalf of X, loads the nib with X as its owner. NSNib, naturally, sets the y and z properties of X to the two objects in the nib, using the accessors since they exist.

Now, when an object is instantiated, it has a retain count of 1; it is implicitly owned by the object that created it. If you release that object, the retain count goes down to 0 and the object dies.

Such is the case of the objects Y and Z when they are unarchived from the nib: After they come out of deep freeze, they have that starting retain count of 1. But then they are passed to the accessors of X, which, if written in the usual way, retain Y and Z. Now the retain counts are 2 each.

When X is finished with Y and Z, it releases them, of course (perhaps by calling its accessors with nil). But the implicit retains still exist. Therefore, so do Y and Z.

This isn't a nib bug, though; the bug is in X. Outlets aren't supposed to be retained. So, if you do implement accessors, they should be non-retaining. Or, if you can, omit the accessors; NSNib will do the Right Thing for you.

Example of the bug: NibLeak (source code with Xcode 2.2.1 project).

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