Beginning Objective-C pdf - EBook Free Download

CHAPTER 2: Object-Oriented Programming 39Additionally, an object’s instance variables can also be declared within this block in a mannersimilar to that used within the @interface block seen earlier. Note, however, all variables mustbe placed together in the same place. You can’t put some variables in the @interface block andsome in the @implementation. Listing 2-12 shows this in action.Listing 2-12. A Sample Object Implementation@implementation MyObject{int aValue;}@synthesize value = aValue;+ (id) objectWithValue: (int) value{return ( [[self alloc] initWithValue: value] );}- (id) initWithValue: (int) value{self = [super init];if ( self == nil )return ( nil );aValue = value;}return ( self );- (id) init{// call through to the designated initializerreturn ( [self initWithValue: 0] );}@endHere you can see the definitions of a class method and two initializers, as well as a few newkeywords.First is the @synthesize statement: this is the counterpart to a @property declaration in theclass’s interface. Following the keyword is either a comma-separated list of property names or alist of property-name/variable-name pairs. In the former case, the compiler pairs the property toan identically-named instance variable. If one has been specified already, it uses that; otherwiseit will create one for you at runtime. In the latter case, an instance variable has already beendeclared, and the @synthesize statement simply tells the compiler to use that variable to storethe property’s value. These must have the exact same type or the compiler will complain. InListing 2-12 the value property is told to use the aValue instance variable as its storage.Most programmers prefer to explicitly name the backing variables for their properties, using@synthesize myVar = _myVar or similar. We recommend using this only when exposing an internalwww.it-ebooks.info