Question

So I just got asked this at an interview today and after some googling am still unable to figure out the answer (in fact I couldn’t even find any code at all which used the [NSString string] method).

What is the difference between

1. NSString *someString = [NSString string];
2. NSString *someString = [[NSString alloc] init];

Now my initial thoughts were that [NSString string] would return an object which would be autoreleased whereas using alloc and init would return an object which has been retained. However it seems that this answer was incorrect.

I’ve looked at the NSString class reference in the apple docs but all it says is

Returns an empty string.

+ (id)string 

Return Value
An empty string.

Could somebody explain to me exactly what the difference between these two are?

Answer 1

Now my initial thoughts were that [NSString string] would return an object which would be autoreleased

Technically, it’s a placeholder string that is constant, i.e., it lives throughout the entire program execution, never being released. It’s not an autoreleased string. Conceptually, and this is what I’d focus as an interviewer, it’s a string (an empty string) that is not owned by the caller, hence the caller shouldn’t release it.

whereas using alloc and init would return an object which has been retained

Technically, it’s a placeholder string that is constant, i.e., it lives throughout the entire program execution. In fact, it’s the same object as the one above, and it is not retained. Conceptually, and this is what I’d focus as an interviewer, it’s a string (an empty string) that is owned by the caller, hence the caller is responsible for releasing it when it’s not needed any longer.