Objective-C Lecture 5

Memory managementMemory management in OC is semi-

automatic:The programmer must allocate memory for objects either a) explicitly (alloc) or b) indirectly using a convenience constructorNo need to deallocate

AllocationAllocation 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 so called 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 (examples in the lecture)

Sending the retain message to the object increases the reference counter by 1.

Sending the release message decreases the reference counter by 1.

reference counter = retain counter

• When the reference counter reaches zero, the object is automatically deallocated. The programmer does not deallocate.

• The programmer only does:allocretainrelease

Rules for memory management 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. The calling code can do (retain – release) but this is not a must.

Autorelease pool

• 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 studentWithName:@"Johnnie" 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 initAn autorelease pool must be in place