[Biococoa-dev] BCSequenceRecord

[Charles Parnot]

On Jul 10, 2005, at 1:16 AM, Alexander Griekspoor wrote:
> <snip>...Anyway, I would have no problem to make the public method  
> return an NSData but the private property be an NSMutableData, also  
> for the immutable sequence....<snip>

I think we should not do that. This is very dangerous. In most cases,  
it will go unnoticed, but in certain configurations, this is the path  
to very weird bugs in programs using the framework.

Let me give you an example where that would be bad. The user  
implements a very rudimentary undo, simply by saving the contents of  
the sequence at each manipulation (let's say she does not know about  
the NSUndoManager!!).

So every time the user changes something:
NSData *currentSequence = [mySequence data];
[myMemoryStack addObject:currentSequence withKey:[NSDate date]];


and then later wants to go back to the sequence 2 hours ago:
NSData *previousSequence = [myMemoryStack objectForKey:twoHoursAgoDate];
oldSequence = [BCSequence sequenceWithData:previousSequence];

Well, the data retrieved then will actually be the data corresponding  
to the sequence as of right now, because the pointer was actually to  
the actual data object in the sequence, which has been mutated since...

You only return a mutable object instead of a non-mutable, when you  
just created it on the fly, do not share it with any other object,  
and you are (auto)releasing it anyway and are not going to modify it.

Of course, for performance, you could still return the muutable data  
object, and do a lazy copy of the data if needed, later, when:
* the data is asked by another object
* the data  is being modified
(so you need some sort of flag for that).



> But if there is a way to have the immutable bcsequence have an  
> NSData property and the mutable version have an NSMutableData then  
> that would be more elegant obviously...
> Cheers,
> Alex

You can set an ivar to be NSData, but make it an NSMutableData  
anyway. With the proper casts, the compiler won't tell anything, and  
the runtime won't bother... As long as you don't call the wrong  
methods on the wrong type!!

Another option to avoid the casts is to type the ivar as an 'id'. But  
then you don't get any compiler checking, of course,...

charles

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Charles Parnot
charles.parnot at gmail.com