Question

Here is the gist of some code I’m writing. I’m concerned that I am not properly addressing the retain/release issues with the array class method on NSMutableArray. Is the following actually leaking memory?

for(a while) { 
    // do stuff
    NSMutableArray *a = nil;
    // do stuff
    if (!a) {
        a = [NSMutableArray array];
    }
} // for(a while)

Answer 1

You wouldn’t leak memory in this code, and releasing the array yourself will cause a crash when the array is autoreleased at the end of the run loop.

Most Cocoa classes provide a couple of ways of making a new object, and are very consistent with this convention:

1. [[NSSomeObject alloc] init] : you are responsible for releasing the object (instance method).

2. [NSSomeObject someObject] : the object will be autoreleased for you, usually at the end of the run loop (class method). It’s roughly equivalent to [[[NSSomeObject alloc] init] autorelease].

The proper use of the instance method would be:

a = [[NSMutableArray alloc] init];
// do stuff
[a release];

The proper use of the class method would be:

a = [NSMutableArray array];
// do stuff, array is in the autorelease pool

Note that Apple has recommended you stay away from the convenience methods as much as possible to improve performance. This is controversial advice, may not save much processor time, and separates the alloc-init from the release on an object you may not actually care much about keeping.