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BOOK: Beginning iOS 5 Application Development
This is the forum to discuss the Wrox book Beginning iOS 5 Application Development by Wei-Meng Lee; ISBN: 978-1-1181-4425-1
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Old June 7th, 2012, 10:09 AM
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Question Empty Application Chapter4 Page 81

@synthesize window = _window;
@synthesize viewController = _viewController;
I understand that we are initializing accessor methods for window and viewController but whats the _windows & _viewController?
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Old June 9th, 2012, 02:18 PM
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Proper memory management is best handled by using accessors.The pattern used in the project is useful in helping the programmer avoid unintentional direct setting of instance variables. It also also prevents data hiding when local variables have the same name as instance variables.

From the example in the book the header file
#import <UIKit/UIKit.h>

@class HelloWorldViewController;

@interface EmptyAppAppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (strong, nonatomic) HelloWorldViewController *viewController;    

could have had in the .m file the following
@synthesize window;

@synthesize viewController;
This would have allowed both of the following

viewController = [[[HelloWorldViewController alloc] initWithNibName:@"HelloWorldViewController" bundle:nil] autorelease];

self.viewController = [[[HelloWorldViewController alloc] initWithNibName:@"HelloWorldViewController" bundle:nil] autorelease];
These are not the same.
The first one sets the variable directly and the second uses the accessor. The first version has an object ownership problem, since the object will be deallocated when autorelease is called and viewController will probably end up pointing to garbage. The second one would have a synthesized setter similar to the following:

-(void)setViewController:(HelloWorldViewController *)aViewController
    [aViewController retain];
    [viewController release];
    viewController = aViewController;
Where the variable in is retained to balance the autorelease. The old viewController is released so that it is deallocated unless retained by another object somewhere else, and the viewController stores the pointer to variable in.
The @synthesize variable = _variable; pattern will result in a compiler error if an attempt to directly set the variable e.g.
variable = newVariable;
is attempted.
On the rare occasion where the variable does need to be acted on directly you call the variable with the underscore
_variable = newVariable;
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