On Sep 12, 2009, at 9:43 AM, John Love wrote:
Basic question: if a parent object is retained, why isn't each sibling component object of that parent retained??
It's not a component object in the implementation. Whenever you call - path, the NSURL implementation copies out and decodes the path portion of the URL string, puts it in a new NSString object, autoreleases it, and returns it. If you don't retain that NSString, it will go away when the current autorelease pool exits. That's why your app crashed.
The moral is that you have no way of knowing the natural lifespan of an object returned by an accessor method. It might be, as you assumed, kept in an instance variable of the parent object and lives as long as the parent. But it equally well might be created on the fly. Or there might be a more complex relationship, where the property object is cached for a while but sometimes released. You have no way of telling.
The point is that if you follow the rules it won't matter to you. (And it shouldn't matter, because a property that's implemented one way in one release might change its implementation in the next, and you wouldn't want that to break your app.)
If you get an object returned from a method (other than +alloc, -copy and a few other exceptions) and you want to hang onto it, you must retain or copy it. Period. (Or you could make your app garbage collected, assuming it's not on iPhone, and not worry about these rules.)
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