Why no retain is needed in my code but it works

I have a class

@implementation MyClass

- (void) foo    
{    
    ivar = [NSString stringWithString:@"ivar"];
}

- (void) bar    
{    
    NSLog(@"%@", ivar);
}
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And main.m

MyClass * m = [[MyClass alloc] init];
[m foo];
[m bar];

Why no retain is needed for stringWithString?

Can you show me an example where retain is needed?

Its because the autorelease pool had no time to drain its content. Here is a crashing example:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
MyClass *m = [[MyClass alloc] init];
[m foo];
[pool drain];
[m bar];

The autorelease pool that holds the string in your example belongs to 99% to the current runloop which creates a new pool at the begin of the event loop and then drains it at the end.

Why no retain is needed for stringWithString?

Because the autorelease pool is not being drained between line 2 and line 3 (as it would be in a Cocoa app as soon as your code returns control to the run loop).

You can start by reading Memory Management Programming Guide and look at this tutorial.

Have a look at Memory Management Rules from Apple. In your case, you did not alloc/retain/net the NSString so you don't "own" it and therefore you do not need to release it.

Internally, NSString would return you a autoreleased object. If you don't retain it then you'll lose reference to it if it gets dealloced by an autorelease pool.