[ios] Objective C - Assign, Copy, Retain

I'm new to Objective C. I have basic knowledge in C, including the concept of pointers. I have two basic questions:

1. Can someone explain the difference between assign,copy, and retain with some analogy?
2. How do you handle a function which returns pointer variable, and how do you perform messaging through a return pointer?

The Memory Management Programming Guide from the iOS Reference Library has basics of assign, copy, and retain with analogies and examples.

copy Makes a copy of an object, and returns it with retain count of 1. If you copy an object, you own the copy. This applies to any method that contains the word copy where “copy” refers to the object being returned.

retain Increases the retain count of an object by 1. Takes ownership of an object.

release Decreases the retain count of an object by 1. Relinquishes ownership of an object.

1. assign

   • assign is a default property attribute
   • assign is a property attribute tells the compiler how to synthesize the property’s setter implementation

2. copy:

   • copy is required when the object is mutable
   • copy returns an object which you must explicitly release (e.g., in dealloc) in non-garbage collected environments
   • you need to release the object when finished with it because you are retaining the copy

3. retain:

   • specifies the new value should be sent “-retain” on assignment and the old value sent “-release”
   • if you write retain it will auto work like strong
   • Methods like “alloc” include an implicit “retain”

NSMutableArray *array = [[NSMutableArray alloc] initWithObjects:@"First",@"Second", nil];
NSMutableArray *copiedArray = [array mutableCopy];
NSMutableArray *retainedArray = [array retain];

[retainedArray addObject:@"Retained Third"];
[copiedArray addObject:@"Copied Third"];

NSLog(@"array = %@",array);
NSLog(@"Retained Array = %@",retainedArray);
NSLog(@"Copied Array = %@",copiedArray);

array = (
    First,
    Second,
    "Retained Third"
)
Retained Array = (
    First,
    Second,
    "Retained Third"
)
Copied Array = (
    First,
    Second,
    "Copied Third"
)