Deep Copy and Shallow Copy

Question

I have read the answer for difference between deep copy and shallow copy from the post, What is the difference between a deep copy and a shallow copy? . Now I got some doubt

Answer 1

You can also call [[NSArray alloc] initWithArray:arraytoBeCopied copyItems:YES];

Answer 2

newArary = oldArray isn't a copy at all. You end up with two pointers pointing to the exact same memory location.

newArray = [NSMutableArray arrayWithArray:oldArray]; is a shallow copy. You end up with two distinct arrays, so if you were to remove or add items from one array, it wouldn't affect the other array. However, the items in the two arrays are identical. If the first element of oldArray were an NSMutableDictionary and you added a key to it, you'd see that change on the first element of newArray as well (since those two objects are the same).

To do a deep copy, you would have to make a new array, and each element of the new array would be a deep copy of the corresponding element of the old array. (Yes, that definition is recursive).

Answer 3

First of all, NSArray's don't have a deep copy function. However, you can make a deep copy function by doing the following:

@interface NSArray(deepCopy)

-(NSArray *) deepCopy;

@end

@implementation

-(NSArray *) deepCopy
{
    NSMutableArray *ret = [NSMutableArray array];

    for (id val in self)
    {
        if ([val conformsToProtocol:@protocol(NSCopying)])
        { 
            [ret addObject:[val copy]];
        }
        else
        {
           [ret addObject:val];
        }
    }

    return ret;
}

@end 

Second of all, newArray = oldArray does not copy the array. It simply makes newArray point to the array that oldArray points to.

Third, +arrayWithArray: does a shallow copy of the array, meaning the individual objects are NOT copied.

Answer 4

In Objective-C "Copy" keyword just increase the "Retain Count" of the object. So only use of "Copy" will not perform a copy.

But when we make a change in object, then Objective-C create a copy of the original object at that time.

Please correct me if i am wrong.

Thanks