How to make a real private instance variable?

I want to make an instance variable that can't be accessed from outside. Is something like that possible in objective-c? I remember Apple has private variables and stuff like that, but if people know about them, they can use them. Apple calls that "private API", but obviously others can access that stuff if they find out what's in there.

Until now I believed that something like this creates a private instance variable:

@interface MyClass : NSObject {
    CGFloat weight;
}

No @property, no @synthesize, just the declaration above.

Also I know Apple adds a _inFrontOfTheirPrivateInstanceVariables, but they said somewhere that they don't like to see others doing that because they might override accidently hidden instance variables when doing that.

What's the trick here?

Solution 1:

You can use the @private keyword inside the {} to make all subsequent variable declarations private. The default visibility is @protected (which is similar to protected in Java) and that generally works well. You'd have to specifically declare a variable as @public for it to be directly accessible outside the class.

This Apple documentation has further details about variable scope and visibility.

There is also a difference between "private API" and private variables. In Objective-C, you cannot make methods private — anyone can call any method. There are several ways to create "secret" methods, but that's somewhat out of the scope of this question. Here are a few related SO questions:

• About private instance variables in Objective-C
• What does “@private” mean in Objective-C?
• Is it possible to declare a method as private in Objective-C?
• Best way to define private methods for a class in Objective-C

As far as the leading _ in front of variables, be aware that Apple also reserves this prefix for "private" methods. The best way to guarantee you avoid problems is to use normal naming conventions for your own variables and methods. However, unless you subclass something from Cocoa (other than NSObject) you can be fairly confident that you won't run into problems.

Solution 2:

With the new LLVM Compiler available in XCode 4 and later, you can declare @private variables in default categories inside your implementation (.m) file:

@interface ClassName()
{
@private
// private variables here
}
@end

@implementation ClassName
// you can use private variables here
@end

I find this convenient, as I hate the pollution private variables bring into my header files.