Memory management in Objective-C is semi-automatic, requiring the programmer to allocate memory for objects using alloc or convenience constructors, but not requiring de-allocation. Every object has a reference counter that tracks the number of references retaining it; sending retain increments the counter, while release decrements it. When the reference counter reaches zero, the object is automatically de-allocated. The programmer must follow rules to balance allocations, retains and releases. Autorelease pools are used to defer releasing of objects until the pool is drained. Newer versions of Xcode use Automatic Reference Counting (ARC) to automate more of memory management.

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Memory Management In iOS

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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

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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.

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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.

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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

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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.

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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];

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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];
}

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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

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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 init
An autorelease pool must be in place

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End of MemoryEnd 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!

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