Objective-C Fundamentals

188 CHAPTER 9 Memory management
Be careful of macro optimizations
When developing your application, be careful not to optimize for performance on a macro
level. Although there are performance-based overheads to using an NSAutorelease-
Pool, these overheads could be an insignificant factor in the overall performance of
your application.
It’s generally better to use performance-monitoring and analysis tools, such as those
mentioned in chapter 14, to determine where the bottlenecks occur in your application’s
performance than to apply general rules of thumb, such as “manual retain/
release is more efficient than retain/autorelease.”
inflating the memory consumption of your application. This may not be much of a
problem on a desktop, but it can be a big problem on a memory-constrained device
such as the iPhone. As an extreme case, consider the following code snippet:
for (int i = 0; i < 100; i++) {
for (int j = 0; j < 1000; j++) {
NSString *msg = [[NSString alloc] initWithFormat:
@"Message number %d is 'Hello, World!'",
i * 1000 + j];
[msg autorelease];
}
}
NSLog(@"%@", msg);
This code creates 100,000 string objects. At the end of each iteration through the
inner loop, an additional string is added to the autorelease pool even though that
string will never be referred to again by the code snippet. All 100,000 string objects
will pile up in the pool, only to be released when the entire code snippet completes
and returns to the application’s run loop. At least in theory…
If you run this code snippet, you’ll probably find that your application crashes well
before the 100,000th message is displayed with a message similar to the following:
RentalManager (4888) malloc: *** mmap(size=16777216) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
This indicates that the Objective-C runtime attempted to allocate some memory for a
new object but failed to find enough. This seems extreme considering the code interacts
with only a single string at any given point in time.
One solution is to explicitly create your own autorelease pool so you can release
the string objects earlier, as demonstrated by the following code snippet:
for (int i = 0; i < 100; i++) {
NSAutoreleasePool *myPool = [[NSAutoreleasePool alloc] init];
for (int j = 0; j < 1000; j++) {
NSString *msg = [[NSString alloc] initWithFormat:
@"Message number %d is 'Hello, World!'",