Memory Management in Objective-C [duplicate]

Answer 1

Something to be aware of if you use an C-style array to store objects and you decide to use garbage collection is you'll need to allocate that memory with NSAllocateCollectable(sizeof(id)*size, NSScannedOption) and tag that variable as __strong.

This way the collector knows that it holds objects and will treat objects stored there as roots during that variables lifetime.

Answer 2

For instance, is it completely illegal to use a pointer to an Objective C object and treat it as an array?

If it's not an array, then yes.

Are you forced to use NSArray and NSMutableArray for data structures?

No. You can use C arrays, and you should be able to use C++ STL vectors (although I don't use C++, so I don't know specifics of how).

But there's no reason not to use NS{,Mutable}Array. Fear not the Cocoa frameworks, for they are your friend.

And don't forget the other collection types, such as NS{,Mutable}Set and NS{,Mutable}Dictionary.

Answer 3

Here are the rules:

1. If you create an object by calling alloc or copy, you own it and must release it when you're done.
2. If you didn't create an object, but want it to ensure it sticks around before control returns to the run loop (or, to keep things simple, your method returns), send it a retain message and then release it later when you're done.
3. If you create an object and want to return it from your method, you are obligated to release it, but you don't want to destroy it before the caller gets a chance to see it. So you send it autorelease instead, which puts it in the Autorelease Pool, which is emptied once control gets back to the program's event loop. If nobody else retains the object, it will be deallocated.

Regarding arrays, you are free to do something like this:

NSObject *threeObjects[3];

threeObjects[0] = @"a string";
threeObjects[1] = [NSNumber numberWithInt:2];
threeObjects[2] = someOtherObject;

Reasons to use NSArray anyway:

• NSArray will take care of retaining objects as you add them and releasing them as you remove them, whereas in a plain C array you will have to do that yourself.
• If you are passing an array as a parameter, an NSArray can report the count of objects it contains, with a plain C array you'll need to pass a count along too.

• Mixing square bracket meanings on one line feels weird:

  [threeObjects[0] length]

Answer 4

It's probably also useful to note that for class messages like NSString + (NSString *)stringWithFormat: (basically, helper messages that allocate an object for you rather than requiring you to allocate the object yourself), the resulting object is auto-released unless you explicitly retain it.