ios – 类中的extern NSString * const.

#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

执行：

#import "PCConstants.h"

@implementation PCConstants

Nsstring *const kPCUserProfileKey                  = @"profile";
Nsstring *const kPCUserProfileNameKey              = @"name";
Nsstring *const kPCUserProfileFirstNameKey         = @"firstname";
Nsstring *const kPCUserProfileLocationKey          = @"location";
Nsstring *const kPCUserProfileGenderKey            = @"gender";
Nsstring *const kPCUserProfileBirthDayKey          = @"birthday";
Nsstring *const kPCUserProfileInterestedInKey      = @"interestedIn";

@end

当我在.pch文件中导入头文件时,我可以在任何地方访问常量.但我试着了解发生了什么.

我从不为此对象分配init,因此它们不能是实例常量.所以常量必须是“类对象常量对吗？但我认为类对象不能包含数据.

谁能解释一下？