Objective-C Memory management Memory management is semi-automatic: The programmer must allocate memory for objects either a) explicitly (alloc) or b) indirectly using a convenience constructor No need to de-allocate

Allocation Allocation happens through the class method alloc. The message ‘alloc’ is sent to the class of the requested object. Alloc is inherited from NSObject. Every alloc creates a new instance (=object) [HelloWorld alloc]; The class creates the object with all zeros in it and returns a pointer to it. HelloWorld *p = [HelloWorld alloc]; The pointer p now points to the new instance. Now we send messages to the instance through p.

The reference counter Every instance has a reference counter. It counts how many references are retaining the object. The counter is 1 after allocation. It does not count how many references exist to the object Sending the retain message to the object increases the reference counter by 1. Sending the release message decreases the reference counter by 1. No need to do either if using ARC.

reference counter = retain counter When the reference counter reaches zero, the object is automatically de-allocated. The programmer does not de-allocate. The programmer only does: alloc retain release

Rules for memory management With no ARC: The method that does an alloc or a retain must also do a release, it must maintain the balance between: (alloc or retain) and (release) If a method does alloc and returns a pointer to the created object then the method must do an autorelease instead of release.

Autorelease pool With no ARC: For outorelease to work the programmer must create an autorelease pool, using: NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; When Cocoa is used then the autorelease pool is created automatically – the programmer does not need to do it. To release the pool and all objects in it, do: [pool release];

Convenience Constructors This is a class method that allocates and initializes an object. The programmer is neither doing alloc nor init. Example: +(id)studentWithName :(NSString*)name AndGpa:(float)gpa { id newInst = [[self alloc]initStudent:name :gpa]; return [newInst autorelease]; }

Convenience Constructors Essential: the method sends alloc to self which is the Student class object Essential: the method autoreleases the instance, because it returns a pointer to the created instance Not essential: This example uses an existing initializer, it could use something else or initialize the Student data directly

Convenience Constructors Calling the convenience constructor: id stud = [Student AndGpa: 3.8]; The message is sent to the Student class object and returns a pointer to it, the pointer is assigned to stud The calling code does neither alloc nor init An autorelease pool must be in place

End of Memory With newer versions of Xcode, the memory management is becoming more and more automatic. Use Automatic Reference Counting ARC to take advantage of the latest changes and improvements with memory management!