Question

I understand what read and write do when used as a properties but I’m confused with the rest.

• What is nonatomic and what is it used for?
• What is retain and what is it used for?
• Why is copy used? Wouldn’t it be equivalent to “read” since read also returns the value of the variables?
• What is assign used for? Wouldn’t it be equivalent to “write” since both set copies of the variable?
• Properties basically create getter and setter functions. Do they however create actual functions or do I just access the variable as in [class variable]? (instead of [class getVariable] or [class setVariable:int variable]. If I am just directly accessing the variable isn’t this the equivalent of making the variable public?

Answer 1

You need to know which flags are alternatives to each other:

• nonatomic is in a group by itself
• retain, copy, assign
• readonly, readwrite

By default, getters and setters are thread-safe which also incurs a performance penalty. nonatomic tells the compiler to not worry about thread-safety considerations when writing getters and setters for that property. It will be faster, and should be preferred if you will only access that property from a given thread, usually the main thread.

assign tells the compiler to generate a setter that does not retain the new value. You should use it for primitive properties (int, BOOL, etc). copy creates a new copy of the object when it is assigned. retain calls [newValue retain] on the new value of the object and [oldValue release] on the old value of the object in the setter.

For completeness, readonly tells the compiler to only generate a getter, not a setter, whereas readwrite generates both.

When you access the variable using [class variable], you are calling a function with the same name as the property. This lets you implement the getter or setter at a future point with your own implementation, without having to modify your code in numerous other places.