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Sometimes you might wish to obtain an independent object copy You accomplish this using the copy attribute When you use the copy attribute, the setter creates a new object and the original object is duplicated This property is an independent object, not related to the original There are two copy types: shallow and deep A shallow copy is when you only duplicate an object s references to its instance variables, while a deep copy is when you make a copy of those instance variables as well Making a shallow copy is easy a deep copy, not so easy To copy your own custom class, your class must implement the NSCopying protocol You learn more about protocols later; a comprehensive discussion on writing your own class that implements the NSCopying protocol is beyond this chapter s scope and would needlessly complicate it However, in 15, this book does briefly discuss the NSCopying protocol Look up the NSCopying online documentation for more information However, copying a Cocoa class that already implements the NSCopying protocol is not beyond this chapter s scope For instance, copying an independent string seems a reasonable enough requirement Consider the class, Foo, in Listings 4-3 and 4-4, and a method that uses Foo (Listing 4-5) Listing 4-3 Foo s interface
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#import <Foundation/Foundationh> @interface Foo : NSObject { NSMutableString * myString; } @property(copy) NSMutableString *myString; @end
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Listing 4-4
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Foo s implementation
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#import "Fooh" @implementation Foo @synthesize myString; @end
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-(IBAction)giveWelcome { Foo * myFoo = [[Foo alloc] autorelease]; NSMutableString* message = [[[NSMutableString alloc] initWithString: @"A copied string"] autorelease]; myFoomyString = message; [message appendString:@" More added to end of string"]; NSLog(myFoomyString); }
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The giveWelcome method creates a Foo instance (myFoo), creates a new string (message), and then sets myFoo s myString property to message Because Foo s interface declared that myString uses copy, myFoo creates a copy of the new string when giveWelcome sets myString When giveWelcome changes the message string, myFoomyString remains the same value Had you used retain or assign, the myFoomyString s value would have changed as well
Releasing Properties
Remember, every class you define should ultimately inherit from the NSObject class The NSObject class contains a dealloc method You use this method to release any instance variables and perform other cleanup tasks When you declare properties in your class, you should always override this method by declaring your own dealloc method in your class You will see this over and over again in the remainder of this book In fact, to avoid memory leaks, remember this one rule: when using properties with the attributes nonatomic and retain, always release the properties in a dealloc method For instance, in Listing 4-3 you declare a property named myString in the class Foo To prevent a memory leak, Foo should have a dealloc method like Listing 4-6 As you progress through this book, the dealloc method should become second nature Listing 4-6 A simple dealloc method
-(void) dealloc { [myString release]; [super dealloc]; }
NOTE
A common technique you will see used is to set a property to nil rather than releasing it By setting the property to nil, the generated setter releases the previously allocated object instance and assigns the property to nil -(void) dealloc { selfmyString = nil; [super dealloc]; }
4:
Just Enough Objective-C: Part Two
Multiple-Argument Messages
As with Objective-C s other language constructs, multiple arguments will probably appear strange at first; however, once you become accustomed to them, you will find the syntax easier than Java, C++, and other dot-notation languages Why are we so confident that you will love Objective-C s syntax for multiple arguments In a word, readability How many times have you seen code like this in a Java program
objMyClassstartPlay("Adventures of Tom Thumb", 44, new CherryPie( ), "Jack Sprat", 77);
What exactly do the arguments mean What are you sending to the startPlay method in objMyClass Now consider the same method using Objective-C
[objMyClass startPlay: @"Adventures of Tom Thumb" audienceMembers:44 pie: [[CherryPie alloc] init] supportingActor:@"Jack Sprat" extrasNeeded:77];
You know exactly what the arguments sent to the method mean when using Objective-C You are starting a play entitled Adventures of Tom Thumb that has 44 members in the audience, needs a cherry pie, has a supporting actor named Jack Sprat, and requires 77 extras The signature of the method called in the previous message has a syntax as follows:
-(void) startPlay: (NSString*) title audienceMembers: (int) value pie: (CherryPie*) pievalue supportingActor: (NSString*) actorvalue extrasNeeded: (int) extrasvalue;
The first argument is unnamed The second and any further arguments are distinguished by a space followed by an argument name and colon, followed by the type in parentheses, followed by a parameter name to hold the value Now, here s the tricky part: When referring to a multiple-argument method, when calling the method, you refer to its named arguments An argument s named argument is the name prior to the argument s data type When using the argument within the method s implementation that the argument is a part of, you refer to the actual parameter name, not the argument name So, for instance, in the startPlay method s implementation, you refer to title, value, pievalue, actorvalue, and extrasvalue When calling the method, you refer to startPlay s named arguments: audienceMembers, pie, supportingActor, and extrasNeeded
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