Question

Is there any reason to declare a private ivar in @interface instead of @implementation?

I see code like this all over the internet (including documentation provided by Apple):

Foo.h

@interface Foo : NSObject {
@private
    id _foo;
}
@end

Foo.m

@implementation Foo
// do something with _foo
@end

The header file defines the public interface of a class, whereas a private ivar is… well… private. So why not declare it like this?

Foo.h

@interface Foo : NSObject
@end

Foo.m

@implementation Foo {
@private
    id _foo;
}

// do something with _foo
@end

Answer 1

Declaring instance variables in the @implementation is a recent feature of Obj-C, this is why you see a lot of code with them in the @interface – there was no other choice.

If you are using a compiler which supports declaring instance variables in the implementation declaring them there is probably the best default – only put them in the interface if they need to be accessed by others.

Edit: Additional Info

Instance variables declared in the implementation are implicitly hidden (effectively private) and the visibility cannot be changed – @public, @protected and @private do not produce compiler errors (with the current Clang at least) but are ignored.