Question

1. What is the difference between using
   @class & #import? I had a situation
   where i was getting a build error,
   and the solution was to use @class
   instead of #import to import my
   class.

2. What does nonatomic mean? When do i
   use nonatomic to define a property,
   and when do i avoid it?

Answer 1

@class allows you to create a stub for a class that you will later define. For example:

MyOtherClass.h

@class MyClass;

@interface MyOtherClass : NSObject {
    MyClass *myObject;
}

MyOtherClass.m

#include "MyOtherClass.h"

@interface MyClass : NSObject {
    NSUInteger myInt;
}

#define is used to define strings that will be replaced by the preprocessor. For example:

#define MY_INT 5

x = MY_INT;

will be rewritten by the pre-processor as:

x = 5;