Re: Best Practice for Returning Immutable Objects?
Re: Best Practice for Returning Immutable Objects?
- Subject: Re: Best Practice for Returning Immutable Objects?
- From: "Shawn Erickson" <email@hidden>
- Date: Tue, 3 Jun 2008 09:08:51 -0700
On Tue, Jun 3, 2008 at 8:56 AM, Karl Moskowski
<email@hidden> wrote:
> I have a few methods that return NSData objects, but the objects are created
> and manipulated as NSMutableData, and then copied to an immutable version
> along these lines:
> NSMutableData *myData = [NSMutableData data];
> :
> :
> return [NSData dataWithData:myData];
>
> Would it be sufficient to cast, like this?
> :
> return (NSData *)myData;
>
> Does this generalize to other non-collection classes, e.g., NSString?
It fully depends what the caller expects (aka how you document it). If
they expect the data instance they get back to not change on them
while they hold a reference (what you imply by saying you return an
immutable object) then you need to ensure that holds true either by
copying it before you return it or by not modifying it during some
well defined (documented) scope/window of time.
If you populate the mutable object and never touch it again then you
likely can happily return that instance cast as an NSData since your
API states that the caller should consider it immutable ... however be
aware that the caller may make assumptions about the cost of copying
(no-op for immutable objects) and that assumption could be
invalidated when you return a mutable object instead of the promised
immutable object.
-Shawn
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