为什么是 [@"""class] != NSClassFromString(NSStringFromClass([@""class])) 在 OS X 上? [英] Why is [@"" class] != NSClassFromString(NSStringFromClass([@"" class])) on OS X?
问题描述
if([@"" class] == NSClassFromString(NSStringFromClass([@"" class])))
printf("foo");
else
printf("bar");
iOS 上的输出:foo
Output on iOS: foo
OS X 上的输出:条形
Output on OS X: bar
为什么这在 OS X 上表现得那么奇怪?
Why is this behaving that strange on OS X?
推荐答案
有趣的问题.以下指出评论中提到的一些事情:
Interesting issue. The following points out some things mentioned in the comments:
Class c1 = [@"" class];
Class c2 = NSClassFromString(NSStringFromClass([@"" class]));
// The names are the same under iOS and OS X
NSLog(@"c1: '%@', c2: '%@'", c1, c2);
// The pointers are the same under iOS but different under OS X
NSLog(@"*c1: '%p', *c2: '%p'", c1, c2);
if (c1 == c2) {
NSLog(@"== equal"); // iOS
} else {
NSLog(@"== not equal"); // OS X
}
if ([c1 isEqual:c2]) {
NSLog(@"isEqual: equal"); // iOS
} else {
NSLog(@"isEqual: not equal"); // OS X
}
const char *n1 = class_getName(c1);
const char *n2 = class_getName(c2);
if (strcmp(n1, n2) == 0) {
NSLog(@"name equal"); // Both iOS and OS X
} else {
NSLog(@"name not equal");
}
在 Mac (OS X 10.7.5) 上,这给出:
On a Mac (OS X 10.7.5) this gives:
2013-05-07 12:35:45.249 测试[27483:303] c1: '_NSCFConstantString', c2: '_NSCFConstantString'
2013-05-07 12:40:06.673 测试[27542:303] *c1: '0x7fff7d2bd740', *c2: '0x7fff7d28ae48'
2013-05-07 12:35:45.250 测试[27483:303] == 不等于
2013-05-07 12:35:45.251 测试[27483:303] isEqual: 不等于
2013-05-07 12:35:45.251 测试[27483:303] 名称相等
2013-05-07 12:35:45.249 Test[27483:303] c1: '_NSCFConstantString', c2: '_NSCFConstantString'
2013-05-07 12:40:06.673 Test[27542:303] *c1: '0x7fff7d2bd740', *c2: '0x7fff7d28ae48'
2013-05-07 12:35:45.250 Test[27483:303] == not equal
2013-05-07 12:35:45.251 Test[27483:303] isEqual: not equal
2013-05-07 12:35:45.251 Test[27483:303] name equal
在 iOS (6.1) 上,这给出:
On iOS (6.1) this gives:
2013-05-07 12:38:11.816 测试[27516:11303] c1: '_NSCFConstantString', c2: '_NSCFConstantString'
2013-05-07 12:41:01.319 测试[27557:11303] *c1: '0x1db88f8', *c2: '0x1db88f8'
2013-05-07 12:38:11.816 测试[27516:11303] == 相等
2013-05-07 12:38:11.816 测试[27516:11303] isEqual: equal
2013-05-07 12:38:11.816 测试[27516:11303] 名称相等
2013-05-07 12:38:11.816 Test[27516:11303] c1: '_NSCFConstantString', c2: '_NSCFConstantString'
2013-05-07 12:41:01.319 Test[27557:11303] *c1: '0x1db88f8', *c2: '0x1db88f8'
2013-05-07 12:38:11.816 Test[27516:11303] == equal
2013-05-07 12:38:11.816 Test[27516:11303] isEqual: equal
2013-05-07 12:38:11.816 Test[27516:11303] name equal
关键区别似乎在于,在 iOS 下,两个 Class
值是同一个对象,但在 OS X 下,它们是两个不同的对象.
The key difference seems to be that under iOS, the two Class
values are the same object but under OS X they are two different objects.
因此,至少在 OS X 下,使用 ==
或 isEqual:
比较两个 Class
值似乎是不安全的.我找不到任何函数来比较两个 Class
值,因此使用 class_getName
似乎是最好的选择.
So it seems it is not safe to compare two Class
values using ==
or isEqual:
, at least under OS X. I couldn't find any function to compare two Class
values so using class_getName
seems like the best alternative.
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