You can't access instance variables of an object from any code that is outside of the @implementation of that object's class. You can access those ivars anywhere within that @implementation—*even within a C function*.

gistfile1.m

#import <Foundation/Foundation.h>

@class Foo;

static int geti1(Foo *foo);
static int geti2(Foo *foo);

@interface Foo: NSObject
@end

int main(int argc, char *argv[]) {
	@autoreleasepool {
		Foo *foo = [Foo new];
		NSLog(@"%d, %d", geti1(foo), geti2(foo));
	}
}

@implementation Foo
{
	int i;
}

static int geti1(Foo *foo) {
	return foo->i;
}

@end

static int geti2(Foo *foo) {
	return foo->i;
}