每次都计算[NSString hash]吗？

Question

如果我有一个不可变的字符串,那么每次调用hash时都会运行哈希算法,或者它是否记住了值(假设字符串不能更改)？

Answer 1

它被重新计算.

- [NSString hash]实际上是对[NSCFString hash]的调用(由于免费桥接).

如果您创建一个程序,在同一个字符串上调用 - [NSString hash],并且您在调用之间中断并更改备份字符串的内存,则会获得重新计算的哈希值.这告诉我没有涉及缓存.

(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

请注意哈希值.

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是"isa"指针,0x00001fa2是指向"XXXXXX"字符串的指针.

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

将"XXXXXX"字符串更改为"YYYYXXXX",然后继续第二次哈希调用

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

注意,就ObjC而言,哈希值是不同的,知道不可变的字符串.

我(de)窃听的程序是:

#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];
}