Objective-c 字符串比较的陷井

(lldb) po [success compare:@"yes"] 

$1 = 0 <nil>

(lldb) p [success compare:@"yes"] 

(int) $2 = 0

(lldb) p [success isEqualToString:@"yes"]  安全写法

(char) $3 = '\0'

(lldb) p success 

(NSString *) $4 = 0x00000000

(lldb) p [success compare:@"yes"] == 0 //NSOrderedSame

(bool) $5 = true