Question

Why does this leak?

The arrayOfPerformances is a NSMutableArray, (nonatomic, retain) property that is synthesized.

The currentPerformanceObject is a Performance *, (nonatomic, retain) property that is synthesized.

Performance is a custom class

if(self.arrayOfPerformances == nil)
    {
        self.arrayOfPerformances = [[NSMutableArray alloc]init];
    }

    [self.arrayOfPerformances addObject:currentPerformanceObject];
    [currentPerformanceObject release];
    currentPerformanceObject = nil;

Answer 1

You are creating a new array and retaining it at the same time in this line, because you are invoking a (retain) property setter with the dot notation:

// Your property
@property (nonatomic, retain) NSMutableArray *arrayOfPerformances;

// The offending code
self.arrayOfPerformances = [[NSMutableArray alloc]init];

Because of that, the locally-created array is leaking because you don’t release it. You should autorelease that array, or create a temporary local var, assign, then release the local var, like so:

// Either this
self.arrayOfPerformances = [[[NSMutableArray alloc] init] autorelease];

// Or this (props Nick Forge, does the same as above)
self.arrayOfPerformances = [NSMutableArray array];

// Or this
NSMutableArray *newArray = [[NSMutableArray alloc] init];
self.arrayOfPerformances = newArray;
[newArray release];