How do copy and mutableCopy apply to NSArray and NSMutableArray?

Question

What is the difference between copy and mutableCopy when used on either an NSArray or an NSMutableArray?

This is

Answer 1

The "copy" method returns the object created by implementing NSCopying protocols copyWithZone:

If you send NSString a copy message:

NSString* myString;

NSString* newString = [myString copy];

The return value will be an NSString (not mutable)

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The mutableCopy method returns the object created by implementing NSMutableCopying protocol's mutableCopyWithZone:

By sending:

NSString* myString;

NSMutableString* newString = [myString mutableCopy];

The return value WILL be mutable.

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In all cases, the object must implement the protocol, signifying it will create the new copy object and return it to you.

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In the case of NSArray there is an extra level of complexity regarding shallow and deep copying.

A shallow copy of an NSArray will only copy the references to the objects of the original array and place them into the new array.

The result being that:

NSArray* myArray;

NSMutableArray* anotherArray = [myArray mutableCopy];

[[anotherArray objectAtIndex:0] doSomething];

Will also affect the object at index 0 in the original array.

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A deep copy will actually copy the individual objects contained in the array. This done by sending each individual object the "copyWithZone:" message.

NSArray* myArray;

NSMutableArray* anotherArray = [[NSMutableArray alloc] initWithArray:myArray
                                                       copyItems:YES];

Edited to remove my wrong assumption about mutable object copying