Question

I read about NSAutoReleasePool and understand that it makes memory management easier on iPhone. It is available on NSObject so autorelease message available for every object instance. Moreover, I shouldn’t use autorelease a lot as NSAutoReleasePool uses cache memory and you might runout of memory if there are plenty autoreleased objects. Is my understanding correct?

One thing I didn’t understand is what is the purpose of creating NSAutoreleasePool explicitly like it’s done in following method? What its purpose here? Is it like releasing imgData, img objects automatically? Because I could see that these objects are not released in this method.

- (void)loadImage {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
UIImage *img = [UIImage imageNamed: @"default_user.png"];
if(![[imgURL absoluteString] isEqualToString: @"0"]) {
    NSData *imgData = [NSData dataWithContentsOfURL: imgURL];
    img = [UIImage imageWithData: imgData];
}
if([target respondsToSelector: action])
    [target performSelectorOnMainThread: action withObject: img waitUntilDone: YES];
[pool release];
}

Thanks.

Answer 1

You can use local autorelease pools to help reduce peak memory
footprint. When your pool is drained, the temporary objects are
released, which typically results in their deallocation thereby
reducing the program’s memory footprint.

I can suggest this explicit autorelease pool was created for manage image loading. Probably that image has a large size (in Mb) and this pool can guarantee that memory will released asap. All autorelease pools organized in stack, so this inner pool will drain early than main pool.