A coding pattern that does not work under Garbage Collection
A coding pattern that does not work under Garbage Collection
- Subject: A coding pattern that does not work under Garbage Collection
- From: Rick Hoge <email@hidden>
- Date: Fri, 9 Nov 2007 10:39:28 -0500
I've been testing some of our older libraries under GC, and have found
one coding pattern (that may well have been bad practice anyway) that
really does not work well under GC.
I used to use convenience constructors for NSMutableData objects as a
kind of lazy replacement for malloc.
That is, an assignment like this:
float *myPointer = [[NSMutableData
dataWithLength:size*sizeof(float)] mutableBytes]; // Memory freed at
end of event loop
could be used in place of
float *myPointer = (float*)malloc(size*sizeof(float)); // Needs a
free() later
The advantage of the first, under traditional memory management, was
that the memory would presumably be cleaned up at the end of the
current event loop (whereas the second would require a malloc
downstream in the code).
It seems like this (not surprisingly) doesn't work reliably under GC,
as the collector obviously has no way of knowing that you are later
looking at the memory originally addressed by myPointer. It keeps
track of objects and not pointers.
One possible solution is to replace the first assignment with
NSMutableData *myData = [NSMutableData
dataWithLength:size*sizeof(float)];
and use
((float*)[myData mutableBytes]) in place of myPointer.
The other option is to just use malloc/free pairs - which I don't mind
to a certain extent but the autoreleased convenience constructor was
nice when there were a lot of little chunks of data to allocate which
would not be needed after the end of the event loop. I guess the
concept of a convenience constructor no longer has the same
significance it did pre-GC.
As there are probably better ways of approaching this kind of
situation, I'd be interested in any suggestions or comments. The way
I was doing it before was probably just bad... still this explained a
few spooky bugs in one of our apps under GC.
Cheers,
Rick
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