The right way of handling objects in loops with Xcode

Question

I was wondering can you please let me know about the following two scenarios which are almost alike but with little and bigger objects.

Scenario 1:

In the following code:

NSString *iAmAstring;
for(int i = 0; i < 100000;i++)
{
     NSLog(@"INT VALUE: %d", i);
     iAmAstring = [NSString stringWithFormat:@"%d", i];
     NSLog(@"STRING VALUE: %@", iAmAstring);
}

I have been told since I have ARC turned on there is no implications with doing the above massive loop and inside it for every loop run I am allocating and initialising a string pointer. Because I have been told its too small and won't matter, and ARC can handle it. But not to do it with bigger objects. I am originally from C# background and I am used to setting even my string at the end of loop to null, but here you cannot release it at the end of loop cause ARC will complain and I have been told if I set it to nil at the end of loop I would be creating more work for ARC and I am not really doing things right and am creating zombie code. Can some one clarify what they would do to the object they are allocating, initialising in a massive loop thats small in memory size (preferably the string I have used) with ARC turned ON, at the end of loop?

Scenario 2:

The same as above but assume now NSSTRING thats being used above is now another object that is big in size. Here I would obviously allocate & initialize it before the loop and then use it in the loop by setting it, but how would I again do some things to it at the end of the loop to kinda free it, so once again ARC turned ON?

Thanks in advance.

Sergey Kalinichenko · Accepted Answer · 2012-07-25 04:59:13Z

Under ARC you define your autorelease pools with @autoreleasepool {} blocks. These blocks create new pools of autoreleased objects, and drain them when the scope of the block ends.

NSString *iAmAstring;
for(int i = 0; i < 100000;i++)
{
    @autoreleasepool {
         NSLog(@"INT VALUE: %d", i);
        iAmAstring = [NSString stringWithFormat:@"%d", i];
        NSLog(@"STRING VALUE: %@", iAmAstring);
    }
}

This will drain autorelease pool and release all autoreleased objects in each iteration of the loop. This is a wasteful thing to do with tiny strings, but it may be of great help with larger objects.

A quick note on how autorelease works: all it does is giving your objects one additional call to release when the pool is drained. The default pool is drained in the event loop (i.e. some time after your method exits). The pools that you create manually are drained when the scope of @autoreleasepool ends.

I am originally from C# background and I am used to setting even my string at the end of loop to null

Unless you are re-using the variable later on, this adds perfectly useless lines of code to your program: the C# compiler has been smart enough for some time now to figure out the earliest point to make your objects available to garbage collection, so assignments to null are not helpful.

Jeff Laing · Accepted Answer · 2012-07-25 07:38:58Z

0

What I never understand is why people think they need to use an autorelease pool instead of just using alloc/init to create their objects and an explicit release to dispose of them.

There are a few objects that have side-effects requiring an autorelease pool (dates, I'm looking at you), but for the most part, doing

{
    NSString *s = [[NSString alloc] initWithFormat:...];
    ....
    [s release];
}

in your loop is going to be more efficient than relying on an autorelease.

With regard to scenario 2, sorry, NSString is immutable - you don't change their values.

answered Jul 25, 2012 at 7:38

Jeff Laing

9237 silver badges13 bronze badges

Collectives™ on Stack Overflow

The right way of handling objects in loops with Xcode

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related