Is [NSString hash] computed every time?

Question

If I have an immutable string is the hashing algorithm run every time I call hash, or does it remember the value (given that the string can\'t change)?

Answer 1

It is recomputed.

-[NSString hash] is in fact a call to -[NSCFString hash] (due to toll free bridging).

If you create a program that calls -[NSString hash] on the same string and you break in between the calls and alter the memory backing up the string, you get a re-computed hash value. This tells me there's no caching involved.

(gdb) b -[NSCFString hash]
Breakpoint 1 at 0x3b02fa3
(gdb) r
Breakpoint 1, 0x93652fa3 in -[NSCFString hash] ()
(gdb) c
Continuing.
2009-05-13 14:23:39.003 a.out[1754:813] Hash: -327163326

Note the hash value.

Breakpoint 1, 0x93652fa3 in -[NSCFString hash] ()
(gdb) bt          
#0  0x93652fa3 in -[NSCFString hash] ()
#1  0x00001f73 in main () at test.m:10
(gdb) fra 1
#1  0x00001f73 in main () at test.m:10
10      NSLog(@"Hash: %d", [m hash]);
(gdb) info locals
pool = (NSAutoreleasePool *) 0x109760
m = (NSString *) 0x2030
(gdb) x/20x 0x2030
0x2030 <dyld__mach_header+32>:  0xa06f54a0  0x000007c8  0x00001fa2  0x00000012

0xa06f54a0 is the "isa" pointer, 0x00001fa2 is a pointer to the "XXXXXX" string.

(gdb) set {int}0x1fa2 = 0x59595959

alter the "XXXXXX" string to "YYYYXXXX", then continue to the second hash call

(gdb) c
Continuing.
2009-05-13 14:24:35.884 a.out[1754:813] Hash: -246144954

Note the hash value that is different on the as far as ObjC knows immutable string.

The program I've (de)bugged is:

#import <Cocoa/Cocoa.h>

int main()
{
    NSAutoreleasePool * pool = [NSAutoreleasePool new];

    NSString * m = [NSString stringWithString:@"XXXXXXXXXXXXXXXXXX"];

    NSLog(@"Hash: %d", [m hash]);
    NSLog(@"Hash: %d", [m hash]);

    [pool release];
}