Objective-C Fundamentals

182 CHAPTER 9 Memory management
As an example, the following listing explicitly sends the msg object two additional
retain messages, so three release messages must be sent in order to ensure the object
eventually returns to a retain count of zero.
Listing 9.1
Correctly matching calls to retain and release
NSMutableString *msg =
[[NSMutableString alloc] initWithString:@"Hello, world!"];
[msg retain];
[msg retain];
NSLog(@"The message '%@' has %d characters", msg, [msg length]);
[msg release];
[msg release];
[msg release];
At first glance, the code in listing 9.1 may appear incorrect. The msg object is sent
three release messages, yet there are only two calls to retain. The calls seem unbalanced.
But remember from our previous discussions that a call to alloc (as shown on
the first line) implicitly makes you the owner of the created object. You can consider
alloc as making an internal call to retain on your behalf, and as such, it must be
matched with a call to release.
9.2.3 Determining the current retain count
For diagnostic purposes, it may be desirable to query an object to determine how
many owners it currently has. This can be done by sending it a retainCount message,
as demonstrated by the following listing. This listing is the same as listing 9.1 except
for additional calls to NSLog, which log out the current retain count after each statement
is executed.
Listing 9.2
Querying the retain count of an object during its lifecycle
NSMutableString *msg = [[NSMutableString alloc]
initWithString: @"Hello, world!"];
NSLog(@"After alloc: retain count is %d", [msg retainCount]);
[msg retain];
NSLog(@"After retain: retain count is %d", [msg retainCount]);
NSLog(@"The message '%@' has %d characters", msg, [msg length]);
[msg release];
NSLog(@"After 1st release: retain count is %d", [msg retainCount]);
[msg release];
// NSLog(@"After 2nd release: retain count is %d", [msg retainCount]);
Notice that the log statements report the same values as those manually placed in listing
9.1 and that the last call to NSLog is commented out. This is because the last
release message caused the object to return to a retain count of zero and hence
become deallocated. Once the object is deallocated, it’s impossible to communicate
with it.