To retain or not to release - maybe a bug?

I agree that this looks very suspicious - however I think it's unlikely to be a bug because this kind of fundamental thing in Obj/C are unlikely to have such an obvious error.  I also can't see how the image you allocated wouldn't be dealloc'd when you released it. So several thoughts come to mind:

1) Do you have garbage collection enabled?
2) Can you also printing the retainCount on [image retainCount] in the NSLog(@"2..."); statement
3) Subclass NSImage as MyImage and put NSLog's in its dealloc/release/retain methods to monitor.

This is rather curious.

Objective C Garbage Collection is set to "Unsupported" in Target->Build->GCC 4.2 Code Generation.

So i think it is disabled.

bool isImage2(NSData *data){
  NSLog(@"data: %i", [data retainCount]);

  NSImage *image = [[NSImage alloc] initWithData:data];

  NSLog(@"data: %i, image: %i", [data retainCount], [image retainCount]);

  bool ret = false;

  if(image)
     ret = true;

  NSLog(@"data: %i, image: %i", [data retainCount], [image retainCount]);

  [image release];

  //NSLog(@"data: %i, image: %i", [data retainCount], [image retainCount]); // CRASH!!!
  NSLog(@"data: %i", [data retainCount]);

  [data release];

  NSLog(@"data: %i", [data retainCount]);


  return ret;
}

This gives:

2010-05-12 09:59:23.903 ImageOrganizer02[1439:903] data: 1
2010-05-12 09:59:23.903 ImageOrganizer02[1439:903] data: 2, image: 1
2010-05-12 09:59:23.907 ImageOrganizer02[1439:903] data: 2, image: 1
2010-05-12 09:59:23.908 ImageOrganizer02[1439:903] data: 2
2010-05-12 09:59:23.908 ImageOrganizer02[1439:903] data: 1

Very odd behavior.

Thanks for running these tests. This is really curious - I'll look at this after work this evening.  I had something like this a few weeks ago - and I exploded on the equivalent of your final NSLog (because data really was released!).  I wonder who's got the unexplained retain.  This is curious.

I've found a fix, however I'm not certain why it works.  For sure it has something to with the NSAutoreleasePool.  I've modified your code a little and run from a command-line program.  The result is correct, the retain count is 1 at the end.

I think while you have an autorelease pool in place, you shouldn't release objects that you alloc.  You can see that I get a warning from main() when I alloc.

There is a paper on apple.com about memory management in cocoa.  I think we're both going to have to visit that to unpuzzle this.  http://developer.apple.com/mac/library/documentation/cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html

bool isImage(NSData *data)
{
bool result = false ;
if ( !data ) return result ;

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    NSLog(@"1: %i", [data retainCount]); //This prints 1
    NSImage* image = [[NSImage alloc] initWithData:data];
    NSLog(@"2: %i", [data retainCount]); //This prints 2, NSImage holds reference

    if( image )
{
result = true;
[image release];
}
    NSLog(@"3: %i", [data retainCount]); //This prints 1, had to release it myself
    [pool drain];

    return result;
}

int main (int argc, const char * argv[])
{
//    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
int result = 0 ;
if ( argc < 2 ) {
printf("syntax: TestImageRetain pathToImage\n") ;
result = 1 ;
} else {
const char* filename = argv[1] ;

NSString* path = [NSString stringWithFormat:@"%s",filename];
NSString* imagePath = [path stringByExpandingTildeInPath];
NSData* data = [NSData dataWithContentsOfFile:imagePath];

NSLog(@"imagePath = %@ result = %s",imagePath,isImage(data)?"YES":"NO") ;

NSLog(@"4: %i", [data retainCount]); //This prints 2, NSImage is gone but retain count is the same 
[data release];
}
//    [pool drain];
    return 0;
}

2010-05-12 23:16:44.104 TestImageRetain[26287:a0f] *** __NSAutoreleaseNoPool(): Object 0x1001152a0 of class NSCFString autoreleased with no pool in place - just leaking
2010-05-12 23:16:44.107 TestImageRetain[26287:a0f] *** __NSAutoreleaseNoPool(): Object 0x100119050 of class NSPathStore2 autoreleased with no pool in place - just leaking
2010-05-12 23:16:44.113 TestImageRetain[26287:a0f] *** __NSAutoreleaseNoPool(): Object 0x1001156d0 of class NSConcreteData autoreleased with no pool in place - just leaking
2010-05-12 23:16:44.113 TestImageRetain[26287:a0f] 1: 1
2010-05-12 23:16:44.132 TestImageRetain[26287:a0f] 2: 2
2010-05-12 23:16:44.132 TestImageRetain[26287:a0f] representations count = 1
2010-05-12 23:16:44.133 TestImageRetain[26287:a0f] 3: 2
2010-05-12 23:16:44.133 TestImageRetain[26287:a0f] imagePath = /Users/rmills/R.jpg result = YES
2010-05-12 23:16:44.134 TestImageRetain[26287:a0f] 4: 1