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1 Open the previous Try This project and implement dealloc, retain, release, and alloc in
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Simplem (Listing 3-16) Note that retain returns an id, and that all these methods are declared in NSObject and don t require you to add their signatures to Simpleh Listing 3-16 Simplem modified to include memory management methods
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#import "Simpleh" @implementation Simple + (void) sayGoodBye { NSLog(@"Goodbye"); } -(void) sayHello: (NSString *) name { NSMutableString *message = [[NSMutableString alloc] initWithString:@"Hello there"]; [message appendString:name]; NSLog(message); [message release]; } -(void) dealloc { NSLog(@"deallocating Simple"); [super dealloc]; }
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-(id) retain { NSLog(@"retaining Simple"); return [super retain]; } -(void) release { NSLog(@"releasing Simple"); [super release]; } +(id) alloc { NSLog(@"allocating Simple"); return [super alloc]; } @end 2 Modify mainm to write log statements of the Simple s retainCount (Listing 3-17)
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Listing 3-17
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The mainh file modified to include retainCount logging
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#import <UIKit/UIKith> #import "Simpleh" int main(int argc, char *argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; Simple * mySimple = [[Simple alloc] init]; NSLog(@"retainCount: %d", [mySimple retainCount]); [mySimple sayHello:@"James"]; [mySimple release]; [Simple sayGoodBye]; int retVal = UIApplicationMain(argc, argv, nil, nil); [pool release]; return retVal; } 3 Build and run the application The debugger includes the logging added in Listing 3-16
(Listing 3-18) Listing 3-18 Debugger console echoing memory management logging
2010-12-17 22:30:02 -0500] 2010-12-17 22:30:03894 allocating Simple 2010-12-17 22:30:03895 retaincount: 1 2010-12-17 22:30:03899 Hello there James 2010-12-17 22:30:03903 releasing Simple 2010-12-17 22:30:03904 deallocating Simple 2010-12-17 22:30:03904 Goodbye
[Session started at ChapThree[1062:20b] ChapThree[1062:20b] ChapThree[1062:20b] ChapThree[1062:20b] ChapThree[1062:20b] ChapThree[1062:20b]
3:
Just Enough Objective-C: Part One
In mainm, the main method first allocates a new Simple instance and assigns the pointer (mySimple) to point to the newly allocated and initialized object
Simple *mySimple = [[Simple alloc] init];
The reference count to the object mySimple points to is one, and the debug statement in Listing 3-17 prints a retainCount of one
Instance Variables and Memory
In 4, you will learn about properties You should use them and their accessor methods If you do, you avoid this section s complications But you should still understand a little about instance variables and how they are handled in memory Suppose you have an instance variable, personName, you wish to set, as in Listing 3-19 Listing 3-19 An instance variable in Simpleh
#import <Foundation/Foundationh> @interface Simple : NSObject { NSString * personName; } -(void) sayGoodBye; -(void) sayName; -(void) sayHello: (NSString *) name; @end
Now suppose you modified sayHello to set personName, as in Listing 3-20 You must retain the variable; otherwise, when the caller of sayHello releases the string, it will go away and the personName instance variable will be pointing to unallocated memory Listing 3-20 Retaining an instance variable
-(void) sayHello: (NSString*) name { NSMutableString *message = [[NSMutableString alloc] initWithString:@"Hello there "]; [message appendString:name]; NSLog(message); personName = [name retain]; [message release]; }
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Note that by retaining name, you are increasing its reference count by one, returning it, and then setting personName to it This ensures that the string pointed to by personName will not be deallocated until Simple is finished with it Not retaining a variable when assigning it to another variable, as in Listing 3-20, is a good example of the type of problem you might encounter when not using properties When the name variable pointer is passed to sayHello, assume there is only one other pointer pointing to name (a retainCount of one) Then, after assigning personName to name, the retainCount remains one The personName pointer is now at the mercy of the pointer that originally pointed to name outside the sayHello method When the pointer external to Simple releases the object name points to, the object is deallocated So the personName pointer now points to deallocated memory space and an error occurs To correct this problem, you call retain on the instance variable as in Listing 3-20 Anytime you set an instance variable, you should retain it That way, you ensure that it will not reach a zero reference count while the instance variable still points to the object Of course, the better solution is to always use accessor methods combined with properties You learn about accessor methods and properties in the next chapter
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