about memory managing--dealloc & autorelease

Discussion in 'Mac Programming' started by Howiieque, Feb 14, 2009.

  1. macrumors regular

    Joined:
    Feb 1, 2009
    #1
    Code:
    //  Controller.h
    
    #import <Cocoa/Cocoa.h>
    
    
    @interface Controller : NSObject {
        IBOutlet NSImageView *imageView;
        NSMutableArray * images;
        int currentImage;
    }
    - (IBAction)nextImage:(id)sender;
    
    @end
    
    //  Controller.m
    
    #import "Controller.h"
    
    @implementation Controller
    
    - (void)awakeFromNib
    {
        NSBundle * mainBundle = [NSBundle mainBundle];
        NSArray * imagePaths = [mainBundle pathsForResourcesOfType:@"jpg"
    												   inDirectory:nil];
        images = [[NSMutableArray alloc] init];
        int count = [imagePaths count];
        int i;
        for (i = 0; i < count; i++) {
            NSImage * image = [[NSImage alloc] initWithContentsOfFile:
    						   [imagePaths objectAtIndex:i]];
            [images addObject:image];
            [image release];
        }
        currentImage = 0;
        [imageView setImage:[images objectAtIndex:currentImage]];
    }
    
    - (IBAction)nextImage:(id)sender
    {
        currentImage++;
        if (currentImage == [images count]) {                              // a
            currentImage = 0;
        }
        [imageView setImage:[images objectAtIndex:currentImage]];          // b
    }
    
    @end
    this example is from learning cocoa with objective-c 2nd. it is an application shows the picture in the bundle and has a button to show the next image.

    My question is :
    1 there is a instance variable that owned by this class. It is allocated in the awakefromnib method. Should I override the dealloc method to release this instance variable?
    2 If I did so, it seems that the application did not invoke the dealloc method at all. I wonder why.
    3 if i add a [images autorelease] in the awakefromnib method for later release, an error occurred when i hit the next button. That means the images has been released. but it should be added to the autoreleasepool and then get released when application terminate. things work properly in command line utility. why didn't it work here?
     
  2. macrumors 6502a

    Joined:
    Oct 29, 2006
    Location:
    Virginia
    #2
    Yes

    It may be because you are not releasing this controller properly. And so this controller is never dealloc'ed.

    Remember that AppKit will create an autorelease pool for each event cycle. So if you autorelease an object in that context, it will be released before the next event cycle.

    With a command line tool, you generally create and release your own autorelease pools. The template and most examples of this involve just one autorelease pool which is released right before the application terminates. This is probably why you think all autoreleases happen when any application terminates, which is not the case (especially when using AppKit)
     
  3. thread starter macrumors regular

    Joined:
    Feb 1, 2009
    #3
    Grate explanation. thank you.

    but the answer to question 2. i am still not quit understand. Why it didn't release properly. how can i improve it. and other example provided by the book mentioned above seems that don't invoke the dealloc at all.

    i also read:
    Note that when an application terminates, objects may not be sent a dealloc message since the process’s memory is automatically cleared on exit—it is more efficient simply to allow the operating system to clean up resources than to invoke all the memory management methods.--NSObject Class Reference
    i am puzzled
     
  4. macrumors 68030

    Catfish_Man

    Joined:
    Sep 13, 2001
    Location:
    Portland, OR
    #4
    If you need to do cleanup on application shutdown, use NSApplication's -applicationWillTerminate: delegate method rather than relying on dealloc. As an optimization, Cocoa won't always deallocate objects at shutdown since it knows they're about to go away anyway.
     
  5. thread starter macrumors regular

    Joined:
    Feb 1, 2009
    #5
    thank you. and your logo is pretty.
     
  6. macrumors member

    Joined:
    Aug 5, 2003
    Location:
    Scheßlitz, Oberfranken, Bavaria, Germany
    #6
    Gc?

    Hi,

    have you turned garbagge collection on or is it still off?

    Peter
     
  7. macrumors 6502a

    Joined:
    Oct 29, 2006
    Location:
    Virginia
    #7
    I may have read too much into your question. I assumed you were done with your controller and wondering why the dealloc was not being called.

    If you use your controller for the entire lifetime of your application then you may never get the dealloc call. Because when the application terminates, it doesn't go through and dealloc every object because that would be a waste of time. Once the application is terminated, the OS is going to reclaim all it's memory anyway.

    But, on the other hand, if your controller is not supposed to last the whole lifecycle of the application, then you can release it when it is not needed. Then when the release count is 0, your controller will get the dealloc message which in turn will release other objects.

    Hope that makes sense?
     
  8. thread starter macrumors regular

    Joined:
    Feb 1, 2009
    #8
    thank you so much. it really makes sense.:)
     

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