I have a question what's the difference between these two methods on initializing an array?
I am assuming copyItems will provide a deep copy?
When would you use one versus the other?
Thank you!
2 Answers
The documentation for these methods is the first result (for me) with a little Google search.
initWithArray: copyItems:
has this documentation:
Initializes a newly allocated array using anArray as the source of data objects for the array.
Parameters
array
An array containing the objects with which to initialize the new array.flag
If YES, each object in array receives a copyWithZone: message to create a copy of the object—objects must conform to the NSCopying protocol. In a managed memory environment, this is instead of the retain message the object would otherwise receive. The object copy is then added to the returned array. If NO, then in a managed memory environment each object in array simply receives a retain message when it is added to the returned array.
whereas initWithArray: has this documentation:
Initializes a newly allocated array by placing in it the objects contained in a given array.
e.g. Note: Since NSArray isn't mutable, my corresponding implementations aren't directly usable
`array2 = [[NSArray alloc] initWithArray:array1 copyItems:YES]`
//would correspond to:
array2 = @[
[array1[0] copy],
[array1[1] copy],
[array1[2] copy],
...
[array1[n] copy],
]
whereas
array2 = [[NSArray alloc] initWithArray:array1]
//would correspond to:
array2 = @[
array1[0],
array1[1],
array1[2],
...
array1[n],
]
//or
array2[0] = array1[0];
array2[1] = array1[1];
array2[2] = array1[2];
...
array2[n] = array1[n];
3 Comments
Alan
I did read this, but I'm still not sure what that means. Could you give me an example of when you would use one or the other?
Van Du Tran
What about array2 = array1; ?
James Webster
That would just make the pointer array2 point to the same memory as array1. Not copy.
initWithArray: initializes a new array and places in it all the objects contained in a given array. This means that each object in the given array will receive a retain. Hence, if you edit an object in the new array, you will modify that object even in the given array. (see shallow copy)
On the other hand, initWithArray:copyItems:, if YES is passed as second argument, will provide a deep copy.
Beware that if you need to deeply copy an entire nested data structure then this approach will not suffice. (see the Apple documentation)
Example:
NSMutableString *s = [[NSMutableString alloc] initWithString:@"hello"];
NSArray *a = @[s];
NSArray *b = [[NSArray alloc] initWithArray:a];
[a[0] appendString:@" there"];
after these lines the arrays a and b will contain the mutable string "hello there"
NSMutableString *s = [[NSMutableString alloc] initWithString:@"hello"];
NSArray *a = @[s];
NSArray *c = [[NSArray alloc] initWithArray:a copyItems:YES];
[a[0] appendString:@" there"];
while after these lines the array c will contain the mutable string "hello" and the array a will contain the mutable string "hello there"
4 Comments
joakim
Actually in the second example it is exactly the opposite: 'a' will contain "hello there" and 'c' will contain "hello" :)
Luca Torella
are you sure you read the example correctly? I just run the code and it works asa expected.
joakim
I just pasted the second example as-is. 'a' logs "hello there" and 'c' logs "hello" and that's the expected result. 'c' holds a copy i.e. a NEW object entirely. The append only affects the 'original'. developer.apple.com/library/ios/documentation/Cocoa/Conceptual/…
Luca Torella
Ah yes, sorry I didn't read my description I just read my code. Yes, clearly I inverted them, but the example is doing what I wanted to show. Now I'll fix the answer, thanks
initWithArray