Memory Allocation Question with details

Hello everyone. I have a test program which just allocates one object and copies it. I get a leak though and I thought those with more experience might help me. This is all command line, BTW.

Here is the program, "simple.m":

Code:

[COLOR="Blue"]  1 //
  2 //
  3 #import <Foundation/Foundation.h>
  4 
  5 int main(int argc, char *argv[])
  6 {
  7     NSString *s1, *s2;
  8     s1=nil;
  9     s2=nil;
 10 
 11     NSLog(@"s1 retainCount: %u\n\n", [s1 retainCount]);
 12     NSLog(@"s2 retainCount: %u\n\n", [s2 retainCount]);
 13     //
 14     s1 = [[NSString alloc] initWithString:@"Hello There"];
 15     NSLog(@"s1: %@\n\n", s1);
 16     NSLog(@"s1 retainCount: %u\n\n", [s1 retainCount]);
 17     //
 18     s2 = [s1 stringByAppendingString:@" Computer"];
 19     NSLog(@"s2: %@\n\n", s2);
 20     NSLog(@"s1 retainCount: %u\n\n", [s1 retainCount]);
 21     NSLog(@"s2 retainCount: %u\n\n", [s2 retainCount]);
 22     [s1 release];
 23     [s2 release];
 24 
 25     return 0;
 26 }[/COLOR]

I get this output:

Code:

[COLOR="Red"]gcc -framework Foundation -o simple simple.m[/COLOR]
2010-05-18 00:56:45.769 simple[49262:903] s1 retainCount: 0

2010-05-18 00:56:45.771 simple[49262:903] s2 retainCount: 0

2010-05-18 00:56:45.771 simple[49262:903] s1: Hello There

2010-05-18 00:56:45.771 simple[49262:903] s1 retainCount: 4294967295

2010-05-18 00:56:45.772 simple[49262:903] ***
    __NSAutoreleaseNoPool(): Object 0x1001105d0 of class NSCFString
    autoreleased with no pool in place - just leaking
2010-05-18 00:56:45.772 simple[49262:903] s2: Hello There Computer

2010-05-18 00:56:45.772 simple[49262:903] s1 retainCount: 4294967295

2010-05-18 00:56:45.773 simple[49262:903] s2 retainCount: 1

So, my question is that it seems that there is some kind if inadvertent autorelease going on in line: 18

Why? How?

If this is a totally stupid question please forgive me.

 

Believe me I know this document (and memorized it) but it does not answer this particular question. Further, if I create a "NSAutoreleasePool" it will make the message go away but it will not tell me why there is an autorelease going on.

This might seem like a simple (and possibly stupid question) however for an iPhone application it has serious consequences. I mean isn't this a commonplace code fragment:

Code:

		NSString *s;


		s = [NSString stringWithString:@"First piece"];
		s = [s stringByAppendingString:@" -- second piece"];

What is wrong with that code? I can't figure it out.

 

admanimal: Thank you for the very informative answer. From your post and other info, it seems that this is what's going on:

In the original post, line 14 should be:

Code:

 14     s1 = [[[NSString alloc] initWithString:@"Hello There"] autorelease];

It doesn't have to be, actually the two versions of the line accomplish the same thing. However, getting to the warning message, line 18:

Code:

 18     s2 = [s1 stringByAppendingString:@" Computer"];

what going on here is: "stringByAppendingString" creates an autoreleased string. This the "autorelease" that the warning message was about.

Why worry about such things? Well one of the iPhone books I am reading:"Beginning iPhone Development. Exploring the iPhone SDK. Dave Mark, Jeff LaMarche", warns against relying on autoreleased objects. Specifically, on page 40:

Code:

The memory allocated for an autoreleased object will stay allocated for some
period of time after we’re done with it. On Mac OS X, with swap files and
relatively large amounts of physical memory, the cost of using autoreleased
objects is nominal, but on iPhone, these objects can have a detrimental
effect on your application’s memory footprint.

It is OK to use autorelease, but try to use it only when you really need to,
not just to save typing a line or two of code.

Actually the program, in my original post, was designed as a test of the autorelease mechanism and my attempt to understand exactly how these things work.

Now, this begs the question: If I am within a view and one of my custom methods uses heavy NSString manipulation, should I create a custom autorelease pool?

I don't see this as being used by iPhone programmers, except in threads.

Any thoughts?

 

chown33, thank you for your reply. The problem I have with the guide is that it tends to make sense after you completely understand it. You know what I mean?

My digital bookcase has no less than three iPhone Programing Books, two Objective-C books and about one hundred C books. And I also have Google.

A lot of what's going on, inside the iPhone, needs to be hacked at in order to become transparent. Reading the guide+books is a good starting point but a lot of the information is dry unless you use it and see it for yourself. At least that's how my mind works.

Also, for me, it is better when I hash out my thoughts in a forum, such as this one. It helps me think better when I am having a discussion.

As far as this particular problem is concerned, I was getting all sorts of info on what's appropriate for autorelease pools, on the iPhone.

The little program in my first post was only meant to be an test.

Your post and the post of admanimal hels me get a good view on what's going on. Thank you!

 

I was only being extremely verbose to show that the "autorelease" was there.

The reason, for my paranoid memory allocation issues, is that my project is getting big (close to 10k lines) and I was just going over the code for memory leaks. I ran the code with the Leaks Performance Tool and I was getting many leak indicators. Some were stupid mistakes but some are still puzzling.

So, I thought I'd start with some obvious examples, using NSString as a test, and take it from there. So this is all a part of preventive maintenance.

Thanks to this simple post, I know a lot more about string manipulation details, from you and admanimal. And for that, I thank you. Seriously!

How do you use the Leaks tool?

 

N-I-C-E TOOL!!! Thanks dejo. Running it now. Should be done by the morning, EST