#import <Foundation/Foundation.h>
#import <objc/runtime.h>

typedef NSString* MyStringRef;
typedef NSString MyString;

@interface ClassA : NSObject
@property (nonatomic,copy) MyStringRef stringA;
@property (nonatomic,copy) MyString *stringB;
@end

@implementation ClassA
@synthesize stringA = _stringA;
@synthesize stringB = _stringB;
@end

int main() {
    unsigned int count = 0;
    Ivar *ivars = class_copyIvarList([ClassA class],&count);
    for (unsigned int i = 0; i < count; i++) {
        Ivar thisIvar = ivars[i];
        NSLog(@"thisIvar = %s,%s",ivar_getName(thisIvar),ivar_getTypeEncoding(thisIvar));
    }

    ClassA *a = [[ClassA alloc] init];
    NSLog(@"Out: %@",[a valueForKey:@"stringA"]);
    NSLog(@"Out: %@",[a valueForKey:@"stringB"]);
}

这是输出：

$clang --version
Apple clang version 3.1 (tags/Apple/clang-318.0.58) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin11.4.0
Thread model: posix

$clang -o typedef -fobjc-arc -framework Foundation typedef.m && ./typedef
2012-06-06 20:14:15.881 typedef[37282:707] thisIvar = _stringA,@"NSString"
2012-06-06 20:14:15.884 typedef[37282:707] thisIvar = _stringB,^{NSString=#}
2012-06-06 20:14:15.885 typedef[37282:707] Out: (null)
2012-06-06 20:14:15.888 typedef[37282:707] *** Terminating app due to uncaught exception 'NSUnknownKeyException',reason: '[<ClassA 0x7fabe0501480> valueForUndefinedKey:]: this class is not key value coding-compliant for the key stringB.'
*** First throw call stack:
(
    0   CoreFoundation                      0x00007fff835fef56 __exceptionPreprocess + 198
    1   libobjc.A.dylib                     0x00007fff878e5d5e objc_exception_throw + 43
    2   CoreFoundation                      0x00007fff836891b9 -[NSException raise] + 9
    3   Foundation                          0x00007fff83e77703 -[NSObject(NSKeyValueCoding) valueForUndefinedKey:] + 240
    4   Foundation                          0x00007fff83dae38e _NSGetUsingKeyValueGetter + 108
    5   Foundation                          0x00007fff83dae315 -[NSObject(NSKeyValueCoding) valueForKey:] + 392
    6   typedef                             0x000000010e84bc6d main + 317
    7   typedef                             0x000000010e84b9c4 start + 52
)

我在这里遇到的问题是Objective-C是什么导致typedef NSString MyString有效地创建一个包含一个Class类型变量的结构,然后在我使用MyString的地方使用它.例如结构看起来像这样(在咨询this之后)：

struct NSString {
    Class a;
};

它有意义,但导致valueForKey：失败,大概是因为它现在是一个结构,所以它不能以与对象相同的方式返回它.或者更确切地说,它属于搜索顺序described in the docs的“抛出异常”部分.

我只想了解导致这种情况发生的语言是什么,以及为什么它不能以同样的方式处理我的2 typedef.