是[NSString hash]计算每次？ [英] Is [NSString hash] computed every time?

问题描述

如果我有一个不可变的字符串是哈希算法运行每次我调用哈希，或者记住该值（假定字符串不能更改）？

- [NSString hash]实际上是一个调用 - [NSCFString hash]（由于自由桥接）。

如果你创建一个程序在同一个字符串上调用 - [NSString hash]，并且打断了调用和更改内存备份字符串，你得到一个重新计算的哈希值。

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

注意哈希值。

 断点1，0x93652fa3  -  [NSCFString hash]（）
 （gdb）bt 
＃0 0x93652fa3 in  -  [NSCFString hash]（）
＃1 0x00001f73在main.m（）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   

0xa06f54a0是isa指针，0x00001fa2是指向XXXXXX字符串。

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

将XXXXXX字符串更改为YYYYXXXX，然后继续第二个哈希调用

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

请注意，在ObjC中知道不可变字符串的哈希值是不同的。

我已经编写的程序是： p>

  #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]; 
} 
  

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)?

解决方案

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];
}

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