一个类中的extern NSString * const。

Hi I've got this header file:

嗨，我有这个头文件：

#import <Foundation/Foundation.h>

@interface PCConstants : NSObject
extern NSString *const kPCUserProfileKey;
extern NSString *const kPCUserProfileNameKey;
extern NSString *const kPCUserProfileFirstNameKey;
extern NSString *const kPCUserProfileLocationKey;
extern NSString *const kPCUserProfileGenderKey;
extern NSString *const kPCUserProfileBirthDayKey;
extern NSString *const kPCUserProfileInterestedInKey;
@end
#i