Question

Is there any different between

NSString * str = @"123";

and

NSString * str = [[NSString alloc] initWithString:@"123"];

from compiler’s aspect?

Answer 1

Theoretically yes; in implementation detail, probably not.

In the first case, the compiler creates a constant string and assigns a pointer to it to the variable str. You do not own the string.

In the second case, the compiler creates a constant string (as before) but this time it is used by the run time as a parameter in initialising another string that you do own (because the second string was created using alloc).

That’s the end of the stuff you need to know.

However, there is a lot of optimisation that goes on. Because NSStrings are immutable, you’ll find that initWithString: actually just returns the parameter. Normally, it would retain the parameter before returning it to you (because you are expecting to own it) but literal strings have a special retainCount (INT_MAX I think) to stop the run time from ever trying to deallocate them. So in practice, your second line of code produces identical results to the first.

This incidentally, is why it is incorrect top say the string is autoreleased in the first case, because it isn’t. It’s just a constant string with a special retain count.

But you can and should safely ignore the implementation detail and just remember, you don’t own the string in the first case, but you do own it in the second case.