Objective-C Fundamentals

Memory zones
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Memory zones have additional advantages
Although not a consideration for the iPhone and iPad, which don’t implement paging
to a secondary storage mechanism (such as disk) in their virtual memory implementations,
another use of custom memory zones is to ensure the locality of related data.
Custom memory zones are commonly used in audio buffer or streaming-related code,
for example, because they help prevent performance impacts caused by swapping
buffers in and out of disk-based storage if two commonly used buffers happen to be
allocated on different memory pages.
The first argument to NSCreateZone specifies how much memory to initially reserve
for the pool, and the second argument specifies the granularity of future memory
allocations for the pool. In this example, 2 megabytes of memory are preallocated for
use by the pool; if all of this memory is eventually consumed, a request to allocate one
additional byte will expand the memory zone by a further 1 megabyte. This helps
reduce fragmentation and helps ensure the locality of different objects allocated from
the same zone.
Once a memory zone is created, you can allocate a new object using memory from
the zone by sending an allocWithZone: message:
NSString *myString = [[NSString allocWithZone:myZone] init];
Incidentally, another way to allocate an object from the default zone is as follows. Seeing
this example should also help cement the relationship between init and init-
WithZone: for you:
NSZone *defaultZone = NSDefaultMallocZone();
NSString *myString = [[NSString allocWithZone:defaultZone] init];
Sending a class an alloc message (or passing nil to allocWithZone:) causes the
object to be allocated from the default memory zone, which can also be obtained via a
call to the NSDefaultMallocZone function.
Which zone does my object belong to?
To determine which zone an object is allocated to, you can send it a zone message
and compare the value to a list of known memory zones:
NSZone *zoneOfObject = [myObject zone];
if (zoneOfObject == NSDefaultMallocZone()) {
NSLog(@"The object was allocated from the default zone");
}
Objects allocated to a custom memory zone respond to standard memory management
methods such as retain, release, and autorelease. But there are a couple of
additional functions worth mentioning. When an object is deallocated, the zone