Basic objective C variable declaration

Question

I am puzzled by how is variable declared in objective C.

1: I see @property and @synthesize statement being used. My question regarding that is, what are these 2 statement for? Why are they always used together? I guess @synthesize is a shortcut to create the getter and setter?

2:Say, I want to declare an NSMutableArray that would be only be accessible inside the class that was declared in. I have to perform myArray = [[NSMutableArray alloc] init] before using the addObject method to write something to it. When do I release the array then?

3:Is there a different way to declaring a variable that is only accessible only at the class it was declared to being accessible at all classes?

4:Similar to question 2, but now the variable is an NSString. Why I don't have to alloc & init it to share the same variable within its own class? Whats the different between self.myString = @""; to myString = @"";

Thanks a lot.

Answer 1

For your first question:

@property (nonatomic, retain) NSString * someProperty;

This declares a property of the class. It becomes part of the public contract of the class but still lacks something important - actual implementation

@synthesize someProperty;

This is compiler sugar, its creates a getter and setter method for your property. To wit, this is the implementation that is needed in order to actually use your property in your class and from other classes.

You will in almost all situations, always have a @synthesize for each @property you declare.

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For your second question:

You are correct about how to initialize your array property. In order to release it you would do the following in your classes dealloc method:

- (void) dealloc {
    self.myarray = nil;

    [super dealloc];
}

This effectively releases the array (assuming you declared your property with the keyword retain).

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And for your last question:

Properties of a class are always available from other classes. In order to create a globally accessible variable you would declare it as static.

Answer 2

Ad 1: a property is a construct to control access an ivar (usually private) by getters and setters. Actually, a property doesn't even have to have a supporting ivar. Yes, @synthesize generates getter and setter (and ivar).

Ad 2: You release it when you don't need it anymore. When that is depends on the logic of your code.

Ad 3: If I understand that correcttly, you want @private ivars. Normally, ivars are protected, i.e. only accessible inside the class or in derived classes. Private ivars are only accessible inside the class itself. Properties are publicly accessible.

Ad 4: myString = @"" writes to the ivar directly, while self.myString = @"" uses the property setter.