Question

I realize this question may sound dumb, but just bear with me. I built an app to help new developers wrap their head around memory retention on the iPhone (no ARC yet). It is plain and simple, 4 buttons, init, access, retain, and release. Pretty self explanatory. I am displaying what the retain count for my string object that is the target of our poking and prodding. (Please no lectures on use of [myVar retainCount], I already know)

This stuff will never make it into actual apps, just toying with it for fun and hopefully help someone learn how memory works. My retain and release all work great. My question is that why does my retain count drop back to 1 if I call myString = [[NSMutableString alloc] init]; again. I can boost my retain count to 40, but after calling alloc/init I go back to zero. I am not leaking anywhere, just curious what happens to myString if/when alloc/init is called on it again.

Answer 1

Try this.

NSString *myString = [[NSMutableString alloc] init];
NSLog(@"%d", [myString retainCount]); // "1"
for (int i = 1; i < 40; i++)
    [myString retain];
NSLog(@"%d", [myString retainCount]); // "40"

NSString *backup = myString;
myString = [[NSMutableString alloc] init];
NSLog(@"%d", [myString retainCount]); // "1"
NSLog(@"%d", [backup retainCount]); // "40"

You see, you have a different object with a new retain count. Your original object still exists and still has the same retain count. Assignment changes the object a variable refers to. A variable doesn’t have a retain count, an object does.

myString = someOtherString;
NSLog(@"%d", [myString retainCount]); // who knows?

With retained property:

self.iString = backup;
NSLog(@"%d", [self.iString retainCount]); // "41" - 1 more because property retained
NSString *newString = [[NSMutableString alloc] init];
NSLog(@"%d", [newString retainCount]); // "1"
self.iString = newString;
// 1 for alloc 1 for retain (in real code you should release newString next)
NSLog(@"%d", [self.iString retainCount]); // "2"
NSLog(@"%d", [backup retainCount]); // "40" - self.iString released it