Under ARC the only mode supported by properties and not instance variables is copy
- so if you need copy
use a property.
If you declare your private instance variables in the @implementation
section:
@implementation MyClass
{
// private instance vars
}
then it takes serious effort to access them from outside the class. As you say accessing a "private" property just takes guessing its name - or using the library calls which tell you.
Is it worth it for security? YMMV. But its a good coding practice regardless.
Addendum
As the comment trail shows there has been much discussion over my use of serious effort.
First let's be clear: Objective-C is in the C family of languages, they all allow the programmer to just about anything they choose while staying within the language[*] - these are not the languages of choice if you want strong typing, access restrictions, etc., etc. within your code.
Second, "effort" is not an absolute measure! So maybe I should have chosen the word "obvious" to qualify it rather than "serious". To access a private property just requires the use of a standard method call where the object has type id
- there is little clue in the code that the method being called is hidden. To access a private variable requires either an API call (a runtime function or KVC call) or some pointer manipulation - the resultant code looks nothing like a standard variable assignment. So its more obvious.
That said, apart from uses requiring copy
, under ARC there is no good reason to use a private property when a private instance variable will do. For a private variable fred
compare:
self.fred = 42; // property access, may involve a call (if not optimised out)
_fred = 42; // common way to bypass the accessors and get at the underlying var
fred = 42; // direct access
Take your pick, there is no right answer, but there isn't a wrong one either - this is the realm of opinion (and that is of course an opinion ;-)). I would often pick the last one, private variable - clean & simple. However @RobNapier in his answer prefers the use of properties.
[*] Note: once you consider linking to external code, say written in assembler, all bets are of in any language. At that point you have to look at the "hardware" (real or virtual) and/or "OS" to provide protection.