Question

What is the difference between declaring variables in the interface section vs. in the implementation section outside of a method.

Answer 1

Do you mean in the ivar block in the implementation, as in

@implementation MyClass {
    id someIvar;
}
// methods go here
@end

If so, then the only difference is visibility to other code. At runtime, the ivar will be indistinguishable from one declared in the @interface section. However, code outside the class can see ivars declared in @interfaces, and unless those ivars are marked with @protected or @private, then the other classes can reach in and twiddle the ivars. But ivars declared in the @implementation are not even visible to code outside, so they cannot touch the ivars.

For the most part, this is just a code cleanliness issue. Nothing should go in the header file unless it’s meant to be public. So why put ivars there?

As Josh Caswell noted, ivars declared in this fashion require a recent version of Clang.

---

The alternative interpretation of your question is you have code like

@implementation MyClass

- (void)someMethod { /* ... */ }

NSString *var;

- (void)otherMethod { /* ... */ }
@end

If this is what you meant, then the answer is, don’t do that. In this code snippet we’ve declared a global variable named var rather than an instance variable. The location of the variable inside the @implementation block is irrelevant, it’s exactly identical to a global variable in C (because that’s what it is).