objective c - Weak NSString variable is not nil after setting the only strong reference to nil -

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i have problem code :

__strong nsstring *yourstring = @"your string";  __weak nsstring *mystring = yourstring; yourstring = nil; __unsafe_unretained nsstring *theirstring = mystring; nslog(@"%p %@", yourstring, yourstring); nslog(@"%p %@", mystring, mystring); nslog(@"%p %@", theirstring, theirstring);

i'm expecting pointers nil @ time, not , don't understand why. first (strong) pointer nil other 2 not. why that?

tl; dr: problem string literal never gets released weak pointer still points it.

theory

strong variables retain value point to.

weak variables won't retain value , when value deallocated set pointer nil (to safe).

unsafe unretained values (as can read name) won't retain value , if gets deallocated nothing it, potentially pointing bad piece of memory

literals , constants

when create string using @"literal string" becomes string literal never change. if use same string in many places in application, same object. string literals doesn't go away. using [[nsstring alloc] initwithstring:@"literal string"] won't make difference. since becomes pointer literal string. worth noting [[nsstring alloc] initwithformat:@"literal string"]; works differently , release string object.

line line:

__strong nsstring *yourstring = @"your string";

you creating strong pointer string. ensure value not go away. in case it's little bit special since string string literal technically won't released.

__weak nsstring *mystring = yourstring;

you create weak pointer same thing strong pointer. if @ time strong pointer point else, value pointing deallocated, weak pointer change value points nil. still points same strong pointer. 

yourstring = nil;

your strong pointer points nil. nothing points old string should released if it wasn't fact literal string. if tried exact same thing other objects created yourself, weak variable change points nil. but, since string literal literal , doesn't go away. weak variable still point it. 

__unsafe_unretained nsstring *theirstring = mystring;

a new unretained pointer created, pointing weak pointer pointing string literal.

nslog(@"%p %@", yourstring, yourstring); nslog(@"%p %@", mystring, mystring); nslog(@"%p %@", theirstring, theirstring);

you print strings , confused why first value nil other 2 aren't.

related reading:

what's difference between string constant , string literal?


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