Whats the correct way to alloc a UIImage to memory and release it iphone

Question

Using Instruments, I keep on getting pointed to a memory leak with a UIImage.
I think I'm assigning and releasing the memory correctly. The leaked object in instruments is described as NSConcreteData

Is the following the correct way to assign and release a UIImage?

UIImage* flagimg = [UIImage imageWithData: [NSData dataWithContentsOfURL:url2]];    
[flagimg release];
flagimg =nil;

Answer 1

[UIImage imageWithData:] returns an autoreleased object, which should not be released by you again. So this code snipped contains not a memory leak but the opposite, a double free (in the worst case).

Note that Instruments sometimes generates false positives and/or reports memory leaks in the Foundation itself (yep, they make mistakes too :-).

The fastest way to alloc/release an object is to avoid convenience initializers (like imageWithData:) and instead to something like

NSData* data = [[NSData alloc] initWithContentsOfURL:url]];
UIImage* img = [[UIImage alloc] initWithData:data];
[data release];
// use your image
[img release];

This will allocate and release your object right away and not wait until the autorelease pool is cleaned.

But please note too, that a memory leak is generally not memory that is not yet freed, but that is lost and cannot be freed anymore , so an object which will be deallocated by the autorelease pool is not considered a memory leak.

Answer 2

as a general rule you can say

if you create an object an theres a "init","copy" or "retain" in it, you have to release it. if not, you get an autoreleased object.

thats not always true, but in most cases

Answer 3

both imageWithData and dataWithContentsOfURL return autoreleased objects, so you should have no memory leaks in that code snippet.

Since flagimg is returned autoreleased, your [flagimg release]; call is not needed; you're over-releasing that object.