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Memory management in Objective-C(iPhone)

One of the biggest obstacles most people have to get their head around when they first start Objective C / Cocoa development is the memory management – and it’s actually very beautiful when it finally dawns on you how simple it really is (though this may take a couple of proper projects!).

Memory management in C/C++

With traditional C/C++, determining responsibility for clearing up after unused objects is a bit of a nightmare. There is nothing in the language that specifies how this should be approached and it all depends on the communication and conventions used by individual programmers.

Imagine programmer A writes some code (Class A) which creates and allocates an object called Data and passes this object to programmer B for him to use in his code (Class B). Both programmer A and programmer B are using the same object Data. Who should delete the object Data when it is no longer used?

If Class A only creates and initializes the Data and no longer needs it, then class B can safely delete the object after he is done with Data. However, how can he be sure that class A no longer needs the Data? If Class A does still need the Data while Class B is using it, how do we known when to delete it? If Class A deletes the pointer when he’s done, Class B might try to reference it and you get a crash. If Class B deletes the pointer when he’s done, Class A might try to reference it and you get another crash. Of course if Class C is introduced which needs to share the same Data, it gets even more complicated.

There are conventions and patterns to handle these situations, essentially a form of communication saying “Hang on, don’t delete that object, I’m still using it” or “Ok, I’m done with it, do with it whatever you will”. That is exactly the reason reference counting was developed, and is the main memory management technique used in Objective C.

Memory management in Objective C

Objective C uses ‘reference counting’ as its main memory management technique (wikipedia.org/wiki/Reference_counting). Every object keeps an internal count of how many times it’s ‘needed’. The system makes sure that objects that are needed are not deleted, and when an object is not needed it is deleted. This may sound like automatic garbage collection (the way it works in Java, AS3 (wikipedia.org/wiki/Garbage_collection_(computer_science)), but it is not. The main difference is that in automatic GC (Java, AS3 etc.), a seperate chunk of code periodically runs in the background to see what objects are being referenced and which ones are not needed. It then deletes any unused objects automatically with no special handling required by the programmer (apart from making sure all references to objects are removed when not needed). In the reference counting method, the programmer has the responsibility of declaring when he needs the object and when he’s done with the object, and object deletion takes place immediately when the object is no longer used, i.e. reference count drops to zero (See the above wikipedia links for more on the matter).

Note: Objective C 2.0 also has an option to enable automatic garbage collection. However, garbage collection is not an option when developing for iPhone so its still important to understand reference counts, object ownership etc..

Object Ownership

It’s important to understand the concept of object ownership. In Objective C, an object owner is someone (or piece of code) that has explicitly said “Right, I need this object, don’t delete it”. This could be the person (or code) that created the object (e.g. Class A in the example above). Or it could be another person (or code) that received the object and needs it (e.g. Class B in the example above). Thus an object can have more than one owner. The number of owners an object has, is also the reference count.

Object owners have the responsibility of telling the object when they are done with it. When they do, they are no longer an owner and the object’s reference count is decreased by one. When an object has no owners left (i.e. no one needs it, reference count is zero), it is deleted.

You can still use an object without being its owner (using it temporarily), but bear in mind that the object might (and probably will) get deleted in the near future. So if you need an object long-term, you should take ownership. If you only need the object there and then (e.g. within the function that you received it, or within that event loop – more on autorelease pools later), then you don’t need to take ownership.

Messages

The main messages you can send an object (regarding memory management) are:

alloc (e.g. [NSString alloc]): This allocates an instance of the object (in this case NSString). It also sets the reference count to 1. You are the only owner of this object. You must release the object when you are done with it. (OFFTOPIC: remember to call an init function on the newly allocated object: [[NSString alloc] init] or [[NSString alloc] initWithFormat:] etc.)

new (e.g. [NSString new]): This is basically a shorthand way of writing [[NSString alloc] init]. So same rules apply as alloc.

retain (e.g. [aString retain]): You call this when you are passed an existing object (memory has already been allocated for it elsewhere), and you want to tell the object that you need it as well. This is like saying “Ok, I need this object, don’t delete it till I’m done.”. The reference count is increased by one, and you are the new owner of this object (along with any other previous owners). You must release the object when you are done with it.

release (e.g. [aString release]): You call this when you are done using an object. You are no longer the owner of the object so the reference count is decreased by one. If reference count is now zero (i.e. no owners left), then the object is automatically deleted and the memory is freed, otherwise the object stays in memory for other owners to use. It is like saying “Right, I’m done with this object, you can delete it if no one else is using it”. If you are not the owner of an object, you should not call this. If you are the owner of an object, you must call this when you are done.

autorelease (e.g. [aString autorelease]). This means that you need the object temporarily, and does not make you an owner. It’s like saying “Ok, I need this object for now, keep it in memory while I do a few things with it, then you can delete it”. More on this later in the ‘autorelease pools’ section.

So when dealing with Objective C pointers/objects, it’s important to remember to send the correct messages. The general rule of thumb is: If you own an object (allocate or retain it), you release it. If you don’t own it (came via convenience method or someone else allocated it), you don’t release it.

Convenience methods

Many classes in Cocoa have whats known as convenience methods. These are static methods used for allocating and initializing objects directly. You are not the owner of the returned objects and they are deleted automatically when the autorelease pool is popped (generally at the end of the event loop, but this depends on you or the app).

E.g. the explicit way of allocating and initializing an NSNumber is:

NSNumber *aNumber = [[NSNumber alloc] initWithFloat:5.0f];

This creates a new instance of NSNumber, initializes it with the ‘initWithFloat’ method, and parameter 5.0f.
aNumber has a reference count of 1. You are the owner of aNumber and you must release it when you are done.
Using a convenience method would be:

NSNumber *aNumber = [NSNumber numberWithFloat:5.0f];

This also creates a new instance of NSNumber, initializes it with the ‘numberWithFloat’ method, and parameter 5.0f.

It also has a reference count of 1. But you are not the owner of aNumber and should not release it. The owner is the NSNumber class and the object will be deleted automatically at the end of the current scope – defined by the autorelease pool, more on this later – for now its safe to say you should not release the object, but keep in mind the object will not hang around for long.

Convenience methods generally have the same name as the relevant init function, but with the init replaced by the type of object. E.g. for NSNumber: initWithFloat -> numberWithFloat, initWithInt -> numberWithInt.

An example with NSString:


NSString *aString1 = [[NSString alloc] initWithFormat:@"Results are %i and %i", int1, int2]; // explicit allocation, you are the owner, you must release when you are done

NSString *aString2 = [NSString stringWithFormat:@"Results are %i and %i", int1, int2]; // convenience method, you are not the owner, the object will be deleted when the autorelease pool is popped.

Autorelease Pools

An autorelease pool is an instance of NSAutoreleasePool and defines a scope for temporary objects (objects which are to be autoreleased). Any objects which are to be autoreleased (e.g. objects you send the autorelease message to or created with convenience methods) are added to the current autorelease pool. When the autorelease pool is popped (released) all objects that were added to it are also automatically released. This is a simple way of managing automatic release for objects which are needed temporarily.

E.g. You want to create a bunch of objects for temporary calculations, and instead of keeping track of all the local variables you define and then calling release for all of them at the end of your function, you can create them all with autorelease (or convenience methods) safe in the knowledge that they are going to be released next time the autorelease pool is popped. Note: there is a downside to this which I’ll discuss in the Convenience vs Explicit section.

Autorelease pools can be nested, in which case autorelease objects are added to the latest autorelease pool to be created (the pools are stacked in a Last In First Out type stack).

Example in the Autorelease, Convenience vs Explicit section.

Arrays, Dictionaries etc.

Arrays, dictionaries etc. generally retain any objects added to them. (When dealing with 3rd party collection type objects, always check the documentation to see if they retain or not). This means that these collections will take ownership of the object, and you do not need to retain before adding.
E.g. The following code will create a leak:

-(void) addNumberToArray:(float)aFloat
 NSNumber *aNumber = [[NSNumber alloc] initWithFloat:aFloat]; // reference count is now 1, you are the owner
 [anArray addObject: aNumber]; // reference count is now 2, the array is also an owner as well as you.
}

You need to release the number after you’ve added it if you no longer need it elsewhere other than the array. The following code is correct:

-(void) addNumberToArray:(float)aFloat {
 NSNumber *aNumber = [[NSNumber alloc] initWithFloat:aFloat]; // reference count is now 1, you are the owner
 [anArray addObject: aNumber]; // reference count is now 2, the array is also an owner as well as you.
 [aNumber release]; // reference count is now 1, you are not the owner anymore
}

Now, when the array is released, or the object is removed from the array, the reference count is dropped once more as the array delcares itself as no longer owner of the object, so the object is deleted.

Of course another way of doing the above safely is:

-(void) addNumberToArray:(float)aFloat {
 NSNumber *aNumber = [NSNumber numberWithFloat:aFloat]; // reference count is now 1, NSNumber is the owner, you are not
 [anArray addObject: aNumber]; // reference count is now 2, the array is also an owner as well as NSNumber.
}

Now when the autorelease pool is popped the NSNumber loses ownership and reference count drops to 1 (now only the array owns the number). When the array is released, or the object is removed from the array the reference count drops to zero and the number is deleted.

You may wonder which is a better way of doing this? Method 1 (explicitly using alloc and release), or method 2 (the convenience method). I generally preferred method 2 on OSX, because it looks simpler and is less code. The functionality looks identical – but it is not. It is actually better practise to use method 1 (especially when developing for iPhone) or use method 2 with your own autorelease pools, more on this below.

Autorelease, Convenience vs Explicit

You may be wondering what exactly the difference and/or benefit is of the following two approaches:

Explicit:

-(void) doStuff:(float)aFloat {
 NSNumber *aNumber = [[NSNumber alloc] initWithFloat:aFloat]; // refcount is 1, you are owner
 /// ... do a bunch of stuff with aNumber...
 ...
 [aNumber release]; // release aNumber
}

Autoreleased:

-(void) doStuff:(float)aFloat {
 NSNumber *aNumber = [NSNumber numberWithFloat:aFloat];// refcount is 1, you are not ownder, will be automatically release
 /// ... do a bunch of stuff with aNumber...
 ...
}

With the explicit approach, aNumber is released immediately at the end of doStuff and the memory is deallocated there and then. With the Autoreleased approach, the aNumber is released when the autorelease pool is popped, and generally that happens at the end of the event loop. So if you create quite a lot of autorelease objects during an event loop, they are all going to add up and you may run out of memory. In the above example it isn’t that clear but let me give another example:

Explicit:

-(void) doStuff:(float)aFloat {
 for(int i=0; i<100; i++) {
 NSNumber *aNumber = [[NSNumber alloc] initWithFloat:aFloat]; // refcount is 1, you are owner
 /// ... do a bunch of stuff with aNumber...
 ...
 [aNumber release]; // release aNumber
 }
}

Autoreleased:

-(void) doStuff:(float)aFloat {
 for(int i=0; i<100; i++) {
 NSNumber *aNumber = [NSNumber numberWithFloat:aFloat];// refcount is 1, you are not owner, will be automatically released
 /// ... do a bunch of stuff with aNumber...
 ...
 }
}

Now you can see, in the first example we never have more than a single NSNumber in memory (the NSNumber is allocated at the beginning of, and deallocated at the end of each for loop). Whereas in the second example with each for loop, a new NSNumber is created while the old one is still hanging around in memory waiting for the autorelease pool to be released. On desktop systems with a lot of ram, you may have the luxury to decide which method you’d like to go for, but on limited memory platforms such as iPhone it’s pretty important to make sure objects are deleted as soon as they become unnecessary and not hang around.

Of course another option is to create your own autorelease pool, which would be especially useful if you are using lots of temporary objects and can’t be bothered to release them all individually. Consider the following code:

Explicit:

-(void) doStuff {
 for(int i=0; i<100; i++) {
 NSNumber *aNumber1 = [[NSNumber alloc] initWithFloat:1]; // refcount is 1, you are owner
 NSNumber *aNumber2 = [[NSNumber alloc] initWithFloat:2]; // refcount is 1, you are owner
 NSNumber *aNumber3 = [[NSNumber alloc] initWithFloat:3]; // refcount is 1, you are owner
 NSNumber *aNumber4 = [[NSNumber alloc] initWithFloat:4]; // refcount is 1, you are owner
 NSNumber *aNumber5 = [[NSNumber alloc] initWithFloat:5]; // refcount is 1, you are owner
 NSNumber *aNumber6 = [[NSNumber alloc] initWithFloat:6]; // refcount is 1, you are owner

 // ... do a bunch of stuff with all objects above.
 ...

 // release all objects
 [aNumber1 release];
 [aNumber2 release];
 [aNumber3 release];
 [aNumber4 release];
 [aNumber5 release];
 [aNumber6 release];
 }
}

Autoreleased:

-(void) doStuff {
 for(int i=0; i<100; i++) {
 NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; // create your own little autorelease pool

 // these objects get added to the autorelease pool you created above
 NSNumber *aNumber1 = [NSNumber numberWithFloat:1]; // refcount is 1, you are not owner, will be automatically released
 NSNumber *aNumber2 = [NSNumber numberWithFloat:2]; // refcount is 1, you are not owner, will be automatically released
 NSNumber *aNumber3 = [NSNumber numberWithFloat:3]; // refcount is 1, you are not owner, will be automatically released
 NSNumber *aNumber4 = [NSNumber numberWithFloat:4]; // refcount is 1, you are not owner, will be automatically released
 NSNumber *aNumber5 = [NSNumber numberWithFloat:5]; // refcount is 1, you are not owner, will be automatically released
 NSNumber *aNumber6 = [NSNumber numberWithFloat:6]; // refcount is 1, you are not owner, will be automatically released

 // ... do a bunch of stuff with all objects above.
 ...

 [pool release]; // all objects added to this pool (the ones above) are released
 }
}

In this case, both chunks of code essentially behave the same. In the first example 6 NSNumbers are created at the beginning of every for loop, and they are explicitly released at the end of each for loop (you own them). There is never more than 6 NSNumbers in memory.

In the second example you don’t own any of the NSNumbers, but by creating your own autorelease pool, you control their lifespan. Because you create and release an autorelease pool in the loop, the NSNumbers only live the duration of the for loop, so you never have more than 6 NSNumbers in memory. Had you not created the autorelease pool, at the end of every for loop you’d have 6 NSNumbers waiting to be deleted, and by the end of the function there’d be 6×100=600 NSNumbers hanging around in memory. Combine that with other autorelease objects allocated in other functions and you can have an awful lot of unused objects which are going to be released soon (so no memory leak), but potentially you may hit your memory limits if you don’t release as you go.

Category: iPhone

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