Objective C Memory Management Confusion

Question

I was reading the apple documentation for memory management, and came across something that I just don't understand. Basically, I don't understand why one does not need need to retain an instance variable through the "getter" method. I wrote this little program to see what would happen. I thought there would be a crash, but I am obviously missing something.

//  main.m
//  Test
//


#import <Foundation/Foundation.h>
#import "Test.h"

int main(int argc, char *argv[])
{
    NSAutoreleasePool *p = [[NSAutoreleasePool alloc] init];

    //Initialize the test object
    Test *t = [[Test alloc] init];

    //Set the value to 5
    [t setMyNum:[NSNumber numberWithInt:5]];

    //Save a temp number that points to the original number
    NSNumber *tempNum = [t myNum];

    //release old number and retain new
    [t setMyNum:[NSNumber numberWithInt:7]];

    //Shouldn't this crash because tempNum is pointing to a deallocated NSNumber???
    NSLog(@"the number is %@",tempNum);

    [p drain];
    return 0;
}

Doesn't tempNum point to a deallocated object??

All help is appreciated.

EDIT

This is the code in the getter and setter methods

#import "Test.h"


@implementation Test
- (void)setMyNum:(NSNumber *)newNum {
    [newNum retain];
    [myNum release];
    myNum = newNum;
}

-(NSNumber *)myNum {
    return myNum;
}
@end

As you can see I am calling release on the old object.

EDIT

It was suggested, and I thought rightfully so that the reason the tempNum is still around is because it hadn't been autoreleased from the pool yet. But even after moving the [pool drain] to right before the NSLog message, there is not crash??? Weird.

Answer 1

Since you are not explicitly releasing any objects, nothing is being deallocated until the autorelease pool is allowed to drain. Try inserting [p drain] before the last NSLog call. It should crash the NSLog call.

Additionally, if you are not retaining the NSNumber in your setMyNum: method, you will find that it will crash if you add [p drain] before tempNum is assigned.

To clarify the original question, calling a getter method doesn't (and shouldn't) necessarily imply that the caller wants to take ownership (i.e. retain) the variable. If that was the case, this code would leak:

NSLog("Number is %@", [t myNum]);

Also, it appears that NSNumber has an optimization whereby for small numbers, they cache the NSNumber objects, retain an extra copy, and return that version. So for small constants, [NSNumber numberWithInt: N] will return an object with 2 reference counts (available via [theNumber retainCount]). To explicitly see what happens, use a larger constant in the program, an NSNumber will retain a 'fresh' object with a reference count of 1 (that will also be autoreleased).